Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed >oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
On 2/23/24 16:42, John Larkin wrote:
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
I read somewhere that to get the lowest possible phase noise,
the gain element should be cut off for most of the cycle while
the tank freewheels. Once per period, it should give a little
kick at the peak of the cycle, where that doesn't affect the
phase. I've also seen arguments that say this is false.
I haven't tried it and I haven't analyzed it in detail. YMMV.
Touch-tone oscillators are interesting. They oscillate on two
frequencies at the same time. Squegging is interesting too:
It's put to good use in super-regenerative receivers.
Jeroen Belleman
On Fri, 23 Feb 2024 07:42:12 -0800, John Larkin <jl@997PotHill.com>
wrote:
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
A violin uses a negative resistance source (the bow) and a resonator
with multiple modes.
So does a human voice. I recall that Joan Baez could make almost
perfect sine waves.
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
On Fri, 23 Feb 2024 18:29:07 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 2/23/24 16:42, John Larkin wrote:
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
I read somewhere that to get the lowest possible phase noise,
the gain element should be cut off for most of the cycle while
the tank freewheels. Once per period, it should give a little
kick at the peak of the cycle, where that doesn't affect the
phase. I've also seen arguments that say this is false.
I haven't tried it and I haven't analyzed it in detail. YMMV.
Touch-tone oscillators are interesting. They oscillate on two
frequencies at the same time. Squegging is interesting too:
It's put to good use in super-regenerative receivers.
Jeroen Belleman
https://www.dropbox.com/s/br6namqyxas1f02/Bell_DTMF.jpg?raw=1
I think the original touch-tone (maybe this is it) used an expensive point-contact germanium transistor to provide a negative resistance to
two LC resonators in series. Each LC had its own varistor to limit
amplitude.
On Fri, 23 Feb 2024 18:29:07 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 2/23/24 16:42, John Larkin wrote:
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
I read somewhere that to get the lowest possible phase noise,
the gain element should be cut off for most of the cycle while
the tank freewheels. Once per period, it should give a little
kick at the peak of the cycle, where that doesn't affect the
phase. I've also seen arguments that say this is false.
I haven't tried it and I haven't analyzed it in detail. YMMV.
Touch-tone oscillators are interesting. They oscillate on two
frequencies at the same time. Squegging is interesting too:
It's put to good use in super-regenerative receivers.
Jeroen Belleman
<https://www.dropbox.com/s/br6namqyxas1f02/Bell_DTMF.jpg?raw=1>
I think the original touch-tone (maybe this is it) used an expensive >point-contact germanium transistor to provide a negative resistance to
two LC resonators in series. Each LC had its own varistor to limit
amplitude.
John Larkin <jl@997PotHill.com> wrote:
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
In an audio intermodulation meter I designed, the two tone sources had
to be as noise-free as possible. The problem was that any noise from
the gain-determining element of the high-frequency oscillator
intermodulated with the oscillator signal and appeared as if it were >amplitude variations. That set a limit to the maximum sensitivity of
the measurements.
After trying various configurations, I found that the quietest results
came from a Wien Bridge type of circuit where the gain was accurately
set to just over 3 and the op-amp was allowed to clip. The wavform and >frequency stability weren't brilliant, but that didn't matter.
As a noise-free way of controlling the amplitude, I generated
symmetrical low-noise power supply rails from op-amps driving >emitter-follower buffers, cntrolled by the voltage from a 'set level'
control on the front panel. Both oscillators were controlled by this
method, so their amplitudes tracked together.
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscillates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed oscillation mode is sometimes called squegging.
John Larkin <jl@997PotHill.com> wrote:
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
In an audio intermodulation meter I designed, the two tone sources had
to be as noise-free as possible. The problem was that any noise from
the gain-determining element of the high-frequency oscillator
intermodulated with the oscillator signal and appeared as if it were >amplitude variations. That set a limit to the maximum sensitivity of
the measurements.
After trying various configurations, I found that the quietest results
came from a Wien Bridge type of circuit where the gain was accurately
set to just over 3 and the op-amp was allowed to clip. The wavform and >frequency stability weren't brilliant, but that didn't matter.
As a noise-free way of controlling the amplitude, I generated
symmetrical low-noise power supply rails from op-amps driving >emitter-follower buffers, cntrolled by the voltage from a 'set level'
control on the front panel. Both oscillators were controlled by this
method, so their amplitudes tracked together.
On a sunny day (Fri, 23 Feb 2024 18:33:28 +0000) it happened >liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in ><1qpe61w.1sb87hi1sw23zcN%liz@poppyrecords.invalid.invalid>:
John Larkin <jl@997PotHill.com> wrote:
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
In an audio intermodulation meter I designed, the two tone sources had
to be as noise-free as possible. The problem was that any noise from
the gain-determining element of the high-frequency oscillator >>intermodulated with the oscillator signal and appeared as if it were >>amplitude variations. That set a limit to the maximum sensitivity of
the measurements.
After trying various configurations, I found that the quietest results
came from a Wien Bridge type of circuit where the gain was accurately
set to just over 3 and the op-amp was allowed to clip. The wavform and >>frequency stability weren't brilliant, but that didn't matter.
As a noise-free way of controlling the amplitude, I generated
symmetrical low-noise power supply rails from op-amps driving >>emitter-follower buffers, cntrolled by the voltage from a 'set level' >>control on the front panel. Both oscillators were controlled by this >>method, so their amplitudes tracked together.
In the late seventies I build an audio signal generator using
a 4046 voltage controlled oscillator feeding into 4040 binary counter >connected to an EPROM with sine lookup table feeding into an DAC.
You could make that as precise (as many bits) as you want.
8 bits was good enough for me :-)
It could frequency sweep audio circuits using a pot or some other source
on the 4046 VCO input.
Some lowpass against the switching noise..
Its easy....
On a sunny day (Fri, 23 Feb 2024 18:33:28 +0000) it happened liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in <1qpe61w.1sb87hi1sw23zcN%liz@poppyrecords.invalid.invalid>:
John Larkin <jl@997PotHill.com> wrote:
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
In an audio intermodulation meter I designed, the two tone sources had
to be as noise-free as possible. The problem was that any noise from
the gain-determining element of the high-frequency oscillator >intermodulated with the oscillator signal and appeared as if it were >amplitude variations. That set a limit to the maximum sensitivity of
the measurements.
After trying various configurations, I found that the quietest results
came from a Wien Bridge type of circuit where the gain was accurately
set to just over 3 and the op-amp was allowed to clip. The wavform and >frequency stability weren't brilliant, but that didn't matter.
As a noise-free way of controlling the amplitude, I generated
symmetrical low-noise power supply rails from op-amps driving >emitter-follower buffers, cntrolled by the voltage from a 'set level' >control on the front panel. Both oscillators were controlled by this >method, so their amplitudes tracked together.
In the late seventies I build an audio signal generator using
a 4046 voltage controlled oscillator feeding into 4040 binary counter connected to an EPROM with sine lookup table feeding into an DAC.
You could make that as precise (as many bits) as you want.
8 bits was good enough for me :-)
It could frequency sweep audio circuits using a pot or some other source
on the 4046 VCO input.
Some lowpass against the switching noise..
Its easy....
On a sunny day (Sat, 24 Feb 2024 11:41:28 +0000) it happened liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in <1qpfk35.1yewcwu1d58680N%liz@poppyrecords.invalid.invalid>:
Jan Panteltje <alien@comet.invalid> wrote:
On a sunny day (Fri, 23 Feb 2024 18:33:28 +0000) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1qpe61w.1sb87hi1sw23zcN%liz@poppyrecords.invalid.invalid>:
John Larkin <jl@997PotHill.com> wrote:
Among other parasitic functions, I'm the local LC oscillator designer. >> >>
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a >> >> SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a >> >> Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending >> >> on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
In an audio intermodulation meter I designed, the two tone sources had
to be as noise-free as possible. The problem was that any noise from
the gain-determining element of the high-frequency oscillator
intermodulated with the oscillator signal and appeared as if it were
amplitude variations. That set a limit to the maximum sensitivity of
the measurements.
After trying various configurations, I found that the quietest results
came from a Wien Bridge type of circuit where the gain was accurately
set to just over 3 and the op-amp was allowed to clip. The wavform and >> >frequency stability weren't brilliant, but that didn't matter.
As a noise-free way of controlling the amplitude, I generated
symmetrical low-noise power supply rails from op-amps driving
emitter-follower buffers, cntrolled by the voltage from a 'set level'
control on the front panel. Both oscillators were controlled by this
method, so their amplitudes tracked together.
In the late seventies I build an audio signal generator using
a 4046 voltage controlled oscillator feeding into 4040 binary counter
connected to an EPROM with sine lookup table feeding into an DAC.
You could make that as precise (as many bits) as you want.
8 bits was good enough for me :-)
It could frequency sweep audio circuits using a pot or some other source >> on the 4046 VCO input.
Some lowpass against the switching noise..
Its easy....
What was the amplitude variation and noise after filtering? I needed at >least 96dB below signal and a few stages of filtering could actually >generate more noise than it removes.
The amplitude variation over the audio range was next to zero (DAC
output), the filtering was just an RC with -3 dB point above the maximum audio I used (20 kHz IIRC). 'noise' is the number of steps I think, so for
an 8 bit sinewave you had 256 steps If you use 12 bits you get more
smaller steps. You could make a RC filter with opamps, then anything is possible. I build that because I needed to test some systems, quick
hack...
An other way to make a changing audio frequency is mix 2 RF signals, one
from a fixed crystal oscillator, the other maybe an LC oscillator if you
need a big range or also an xtal oscillator if you need a narrow range.
Most xtal oscillators are tunable over some range, I use that all the
time, Say if you mix 10 MHz with 10.1 MHz you get 100 kHz Filtering out
the 10+ MHz is then easy. Tuning is also easy, varicap, or some transistor used as varicap. 2 RF signals, 1 MHz and 1.01 MHz on your AM radio will demonstrate that (10 kHz out). 96 dB is a lot, best of luck!
Jan Panteltje <alien@comet.invalid> wrote:
On a sunny day (Fri, 23 Feb 2024 18:33:28 +0000) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1qpe61w.1sb87hi1sw23zcN%liz@poppyrecords.invalid.invalid>:
John Larkin <jl@997PotHill.com> wrote:
Among other parasitic functions, I'm the local LC oscillator designer.
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a
SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a
Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending
on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
In an audio intermodulation meter I designed, the two tone sources had
to be as noise-free as possible. The problem was that any noise from
the gain-determining element of the high-frequency oscillator
intermodulated with the oscillator signal and appeared as if it were
amplitude variations. That set a limit to the maximum sensitivity of
the measurements.
After trying various configurations, I found that the quietest results
came from a Wien Bridge type of circuit where the gain was accurately
set to just over 3 and the op-amp was allowed to clip. The wavform and
frequency stability weren't brilliant, but that didn't matter.
As a noise-free way of controlling the amplitude, I generated
symmetrical low-noise power supply rails from op-amps driving
emitter-follower buffers, cntrolled by the voltage from a 'set level'
control on the front panel. Both oscillators were controlled by this
method, so their amplitudes tracked together.
In the late seventies I build an audio signal generator using
a 4046 voltage controlled oscillator feeding into 4040 binary counter
connected to an EPROM with sine lookup table feeding into an DAC.
You could make that as precise (as many bits) as you want.
8 bits was good enough for me :-)
It could frequency sweep audio circuits using a pot or some other source
on the 4046 VCO input.
Some lowpass against the switching noise..
Its easy....
What was the amplitude variation and noise after filtering? I needed at >least 96dB below signal and a few stages of filtering could actually
generate more noise than it removes.
On a sunny day (Sat, 24 Feb 2024 11:41:28 +0000) it happened >liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in ><1qpfk35.1yewcwu1d58680N%liz@poppyrecords.invalid.invalid>:
Jan Panteltje <alien@comet.invalid> wrote:
On a sunny day (Fri, 23 Feb 2024 18:33:28 +0000) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1qpe61w.1sb87hi1sw23zcN%liz@poppyrecords.invalid.invalid>:
John Larkin <jl@997PotHill.com> wrote:
Among other parasitic functions, I'm the local LC oscillator designer. >>> >>
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a >>> >> SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz
parasitic oscillation. A couple of things fix that, including using a >>> >> Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending >>> >> on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at
one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a
diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed
oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
In an audio intermodulation meter I designed, the two tone sources had
to be as noise-free as possible. The problem was that any noise from
the gain-determining element of the high-frequency oscillator
intermodulated with the oscillator signal and appeared as if it were
amplitude variations. That set a limit to the maximum sensitivity of
the measurements.
After trying various configurations, I found that the quietest results
came from a Wien Bridge type of circuit where the gain was accurately
set to just over 3 and the op-amp was allowed to clip. The wavform and >>> >frequency stability weren't brilliant, but that didn't matter.
As a noise-free way of controlling the amplitude, I generated
symmetrical low-noise power supply rails from op-amps driving
emitter-follower buffers, cntrolled by the voltage from a 'set level'
control on the front panel. Both oscillators were controlled by this
method, so their amplitudes tracked together.
In the late seventies I build an audio signal generator using
a 4046 voltage controlled oscillator feeding into 4040 binary counter
connected to an EPROM with sine lookup table feeding into an DAC.
You could make that as precise (as many bits) as you want.
8 bits was good enough for me :-)
It could frequency sweep audio circuits using a pot or some other source >>> on the 4046 VCO input.
Some lowpass against the switching noise..
Its easy....
What was the amplitude variation and noise after filtering? I needed at >>least 96dB below signal and a few stages of filtering could actually >>generate more noise than it removes.
The amplitude variation over the audio range was next to zero (DAC output), >the filtering was just an RC with -3 dB point above the maximum audio I used (20 kHz IIRC).
'noise' is the number of steps I think, so for an 8 bit sinewave you had 256 steps
If you use 12 bits you get more smaller steps.
You could make a RC filter with opamps, then anything is possible.
I build that because I needed to test some systems, quick hack...
An other way to make a changing audio frequency is mix 2 RF signals,
one from a fixed crystal oscillator, the other maybe an LC oscillator if you need a big range
or also an xtal oscillator if you need a narrow range.
Most xtal oscillators are tunable over some range, I use that all the time, >Say if you mix 10 MHz with 10.1 MHz you get 100 kHz
Filtering out the 10+ MHz is then easy.
Tuning is also easy, varicap, or some transistor used as varicap.
2 RF signals, 1 MHz and 1.01 MHz on your AM radio will demonstrate that (10 kHz out).
96 dB is a lot, best of luck!
On 2/24/24 16:46, John Larkin wrote:
On Sat, 24 Feb 2024 12:18:11 GMT, Jan Panteltje <alien@comet.invalid>
wrote:
On a sunny day (Sat, 24 Feb 2024 11:41:28 +0000) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1qpfk35.1yewcwu1d58680N%liz@poppyrecords.invalid.invalid>:
Jan Panteltje <alien@comet.invalid> wrote:
On a sunny day (Fri, 23 Feb 2024 18:33:28 +0000) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1qpe61w.1sb87hi1sw23zcN%liz@poppyrecords.invalid.invalid>:
John Larkin <jl@997PotHill.com> wrote:
Among other parasitic functions, I'm the local LC oscillator designer. >>>>>>>
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a >>>>>>> SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz >>>>>>> parasitic oscillation. A couple of things fix that, including using a >>>>>>> Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending >>>>>>> on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at >>>>>>> one frequency, where there's just enough gain to drive one mode. >>>>>>>
But an oscillator may amplitude limit by some hard clipping, like a >>>>>>> diode clamp. When it's not clamping, there's full gain, basically >>>>>>> negative resistance, to excite any parasitic resonators. The mixed >>>>>>> oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
In an audio intermodulation meter I designed, the two tone sources had >>>>>> to be as noise-free as possible. The problem was that any noise from >>>>>> the gain-determining element of the high-frequency oscillator
intermodulated with the oscillator signal and appeared as if it were >>>>>> amplitude variations. That set a limit to the maximum sensitivity of >>>>>> the measurements.
After trying various configurations, I found that the quietest results >>>>>> came from a Wien Bridge type of circuit where the gain was accurately >>>>>> set to just over 3 and the op-amp was allowed to clip. The wavform and >>>>>> frequency stability weren't brilliant, but that didn't matter.
As a noise-free way of controlling the amplitude, I generated
symmetrical low-noise power supply rails from op-amps driving
emitter-follower buffers, cntrolled by the voltage from a 'set level' >>>>>> control on the front panel. Both oscillators were controlled by this >>>>>> method, so their amplitudes tracked together.
In the late seventies I build an audio signal generator using
a 4046 voltage controlled oscillator feeding into 4040 binary counter >>>>> connected to an EPROM with sine lookup table feeding into an DAC.
You could make that as precise (as many bits) as you want.
8 bits was good enough for me :-)
It could frequency sweep audio circuits using a pot or some other source >>>>> on the 4046 VCO input.
Some lowpass against the switching noise..
Its easy....
What was the amplitude variation and noise after filtering? I needed at >>>> least 96dB below signal and a few stages of filtering could actually
generate more noise than it removes.
The amplitude variation over the audio range was next to zero (DAC output), >>> the filtering was just an RC with -3 dB point above the maximum audio I used (20 kHz IIRC).
'noise' is the number of steps I think, so for an 8 bit sinewave you had 256 steps
If you use 12 bits you get more smaller steps.
You could make a RC filter with opamps, then anything is possible.
I build that because I needed to test some systems, quick hack...
An other way to make a changing audio frequency is mix 2 RF signals,
one from a fixed crystal oscillator, the other maybe an LC oscillator if you need a big range
or also an xtal oscillator if you need a narrow range.
Most xtal oscillators are tunable over some range, I use that all the time, >>> Say if you mix 10 MHz with 10.1 MHz you get 100 kHz
Filtering out the 10+ MHz is then easy.
Tuning is also easy, varicap, or some transistor used as varicap.
2 RF signals, 1 MHz and 1.01 MHz on your AM radio will demonstrate that (10 kHz out).
96 dB is a lot, best of luck!
In your circuit, every cycle has the exact same DAC steps. In a DDS
system, the dac steps squirm from cycle to cycle. Noise-wise, DDS is
about as good, provided it has an ideal lowpass filter after the DAC.
With a decent ratio of clock to sine frequency, say 20:1, the lowpass
isn't terribly hard. But the DDS will always be noisier than the
sychronous scheme.
A combo would be best: make a DDS frequency synthesizer, a phase
accumulator, sine lookup, DAC, lowpass filter, comparator. Then use
that to clock a counter, sine lookup, DAC, filter. That just takes a
few more parts and some code. Share the sine lookup.
Possibly eliminate the first DAC, or make a 2-bit DAC with resistors.
I was sort-of shocked when I came across a project with a local
oscillator for a spectrum/network analyzer where the designer
had used a DDS to provide the reference frequency for a PLL.
The DDS was used just to cover the interval between two
successive steps of the PLL. Quite over the top, I'd say.
Then again, frequency synthesizers with small steps can get
complicated.
Jeroen Belleman
the PLL.
On Sat, 24 Feb 2024 12:18:11 GMT, Jan Panteltje <alien@comet.invalid>
wrote:
On a sunny day (Sat, 24 Feb 2024 11:41:28 +0000) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1qpfk35.1yewcwu1d58680N%liz@poppyrecords.invalid.invalid>:
Jan Panteltje <alien@comet.invalid> wrote:
On a sunny day (Fri, 23 Feb 2024 18:33:28 +0000) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1qpe61w.1sb87hi1sw23zcN%liz@poppyrecords.invalid.invalid>:
John Larkin <jl@997PotHill.com> wrote:
Among other parasitic functions, I'm the local LC oscillator designer. >>>>>>
A new board has a triggered 50 MHz oscillator, a Colpitts that uses a >>>>>> SAV541 phemt. It has excess, erratic jitter that traces to a 6 GHz >>>>>> parasitic oscillation. A couple of things fix that, including using a >>>>>> Murata bead in the gate, thanks to a gift from Phil Hobbs.
I was thinking that there are two classes of LC oscillator, depending >>>>>> on the amplitude limiting mechanism.
Many are AGC types (like the original HP audio oscillator) or a
circuit that self-biases off as amplitude goes up. That oscilates at >>>>>> one frequency, where there's just enough gain to drive one mode.
But an oscillator may amplitude limit by some hard clipping, like a >>>>>> diode clamp. When it's not clamping, there's full gain, basically
negative resistance, to excite any parasitic resonators. The mixed >>>>>> oscillation mode is sometimes called squegging.
Interesting cases are HP, Walt Disney, and the first touch-tone
phones.
In an audio intermodulation meter I designed, the two tone sources had >>>>> to be as noise-free as possible. The problem was that any noise from >>>>> the gain-determining element of the high-frequency oscillator
intermodulated with the oscillator signal and appeared as if it were >>>>> amplitude variations. That set a limit to the maximum sensitivity of >>>>> the measurements.
After trying various configurations, I found that the quietest results >>>>> came from a Wien Bridge type of circuit where the gain was accurately >>>>> set to just over 3 and the op-amp was allowed to clip. The wavform and >>>>> frequency stability weren't brilliant, but that didn't matter.
As a noise-free way of controlling the amplitude, I generated
symmetrical low-noise power supply rails from op-amps driving
emitter-follower buffers, cntrolled by the voltage from a 'set level' >>>>> control on the front panel. Both oscillators were controlled by this >>>>> method, so their amplitudes tracked together.
In the late seventies I build an audio signal generator using
a 4046 voltage controlled oscillator feeding into 4040 binary counter
connected to an EPROM with sine lookup table feeding into an DAC.
You could make that as precise (as many bits) as you want.
8 bits was good enough for me :-)
It could frequency sweep audio circuits using a pot or some other source >>>> on the 4046 VCO input.
Some lowpass against the switching noise..
Its easy....
What was the amplitude variation and noise after filtering? I needed at >>> least 96dB below signal and a few stages of filtering could actually
generate more noise than it removes.
The amplitude variation over the audio range was next to zero (DAC output), >> the filtering was just an RC with -3 dB point above the maximum audio I used (20 kHz IIRC).
'noise' is the number of steps I think, so for an 8 bit sinewave you had 256 steps
If you use 12 bits you get more smaller steps.
You could make a RC filter with opamps, then anything is possible.
I build that because I needed to test some systems, quick hack...
An other way to make a changing audio frequency is mix 2 RF signals,
one from a fixed crystal oscillator, the other maybe an LC oscillator if you need a big range
or also an xtal oscillator if you need a narrow range.
Most xtal oscillators are tunable over some range, I use that all the time, >> Say if you mix 10 MHz with 10.1 MHz you get 100 kHz
Filtering out the 10+ MHz is then easy.
Tuning is also easy, varicap, or some transistor used as varicap.
2 RF signals, 1 MHz and 1.01 MHz on your AM radio will demonstrate that (10 kHz out).
96 dB is a lot, best of luck!
In your circuit, every cycle has the exact same DAC steps. In a DDS
system, the dac steps squirm from cycle to cycle. Noise-wise, DDS is
about as good, provided it has an ideal lowpass filter after the DAC.
With a decent ratio of clock to sine frequency, say 20:1, the lowpass
isn't terribly hard. But the DDS will always be noisier than the
sychronous scheme.
A combo would be best: make a DDS frequency synthesizer, a phase
accumulator, sine lookup, DAC, lowpass filter, comparator. Then use
that to clock a counter, sine lookup, DAC, filter. That just takes a
few more parts and some code. Share the sine lookup.
Possibly eliminate the first DAC, or make a 2-bit DAC with resistors.
Nowadays one can do a DDS phase accumulator and a sine lookup, in an
FPGA or all software in a uP. Frequency and amplitude and even phases
and sweeps can be precisely programmed.
It does need a pretty good lowpass filter after the DAC to take out
the jaggies.
96 dB is a lot, best of luck!
Jan Panteltje wrote:
[...]
96 dB is a lot, best of luck!
I've put some more information, including circuit diagrams at: http://www.poppyrecords.co.uk/other/DistortionMeter/intermodmeter.htm
My momentary muse happens to be fiddling with a crystal radio. Jan suggested using a linear power supply for it. And now your excellent intermodulation meter documentation serendipitously shows a suitable schematic:
<http://www.poppyrecords.co.uk/other/DistortionMeter/images/PSU.gif>
I'll probably use some sort of linear regulator from the bone pile in
place of the transistors.
Jan Panteltje <alien@comet.invalid> wrote:
[...]
96 dB is a lot, best of luck!
I've put some more information, including circuit diagrams at: >http://www.poppyrecords.co.uk/other/DistortionMeter/intermodmeter.htm
On a sunny day (Sun, 25 Feb 2024 15:56:37 +0000) it happened liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in <1qphqsp.10h3p6m1u718xsN%liz@poppyrecords.invalid.invalid>:
Jan Panteltje <alien@comet.invalid> wrote:
[...]
96 dB is a lot, best of luck!
I've put some more information, including circuit diagrams at: >http://www.poppyrecords.co.uk/other/DistortionMeter/intermodmeter.htm
Nice interesting method.. In the old days we did measure distortion in the Eurovision network audio channel by having the remote end (some other country) send a 1 kHz tone, we then measured it and measured all the harmonics 2, 3, 4, 5 kHz etc with a selective volt meter. And then did the math, distortion (harmonics) had to be below a specific level. But nowhere near like -96 dB.. :-)
These days with PC sound card as generator, for example 'sox' in Linux
will generate tones and sweeps either as output or as file, ( have some 1
kHz files and sweep files over the audio range made with it) All 16 bit 48 kHz sampling... see: https://www.audiosciencereview.com/forum/index.php?threads/howto-sox-audio -tool-as-a-signal-generator.4242/
There is also an audio spectrum analyzer program somewhere... So any harmonics will be shown. I really do not know where the audiophiles are as
to noise level these days, or even if they are into bat frequencies.
Usually the environmental noise here when listening to audio is orders of magnitude higher ..
Don wrote:
[...]
My momentary muse happens to be fiddling with a crystal radio. Jan
suggested using a linear power supply for it. And now your excellent
intermodulation meter documentation serendipitously shows a suitable
schematic:
<http://www.poppyrecords.co.uk/other/DistortionMeter/images/PSU.gif>
I'll probably use some sort of linear regulator from the bone pile in
place of the transistors.
The voltage on the smoothing capacitors is perilously close to the 30v maximum input voltage of most commonly-available voltage stabilisers,
that was why I used a descrete component pre-stabiliser instead of a
chip.
Most stabilisers have rejection factor of about 60dB, so interaction
between circuits on the same power supply rails isn't usually a
problem, but I was trying to measure interactions 90dB down, so each
section had its own stabiliser running off pre-stabilised lines that
were common to all of them. Two steps of 60dB are much easier to
achieve than one step of 120dB.
| Sysop: | Keyop |
|---|---|
| Location: | Huddersfield, West Yorkshire, UK |
| Users: | 300 |
| Nodes: | 16 (2 / 14) |
| Uptime: | 89:12:17 |
| Calls: | 6,717 |
| Calls today: | 1 |
| Files: | 12,252 |
| Messages: | 5,358,663 |
| Posted today: | 1 |
