Re: [time-nuts] injection locking crystal oscillator
I have five systems using injection locking. There are a few issues to
watch. If you inject at too high a level, any noise on the reference
will appear in the oscillator output. I use a 56 ohm resistor to
terminate the reference signal coax input, then a 100pF cap and a series
resistor connected to on leg of the crystal. The resistor value needs
to be selected so I can get a solid pull-in and lock over an
acceptably-wide range. In most cases, I am multiplying the crystal osc
up to 3.3, 5.6 or 10.2 GHz and using a PLL chip driven from an Rb as the
reference to generate 0dBm at the correct locking frequency around 117
MHz for example.
If the crystal free-runs close to the lock frequency, say within 200ppb,
the series resistor can be 5k or so, and there is almost no effect on
close-in noise, and no sign of spurs. If I have to pull the crystal
more than about +-500ppb, the resistor needs to be a few hundred ohms,
and the synth noise sidebands start to be seen in the osc output. With
a 70 ohm series resistor, the noise of the osc is only about 10dB down
on the noise of the synth, but the lock-in range is around +-1200 ppb,
slightly more on the LF side.
When the osc drifts too far away from the reference, or the level is too
low, you get a spread of frequencies out of the oscillator as it tries
to pull into lock, but doesn't make it. As the lock level rises, it
pulls closer in, but still with a spread of frequencies until it finally
jumps into lock. There is considerable hysteresis, so check thoroughly
that it will pull in under all likely conditions of voltage and temperature.
Remember that the coax lead is going to have a major influence on the
oscillator, so keep it short and watch for mechanical vibration or
ringing or temperature variation effects on the cable. Make certain the
connectors are torqued well. If there is a trimmer on the osc, remember
to tune it to the correct frequency with the cable and distribution amp
connected, but not delivering a signal, as it will probably be pulled a
little by the cable capacitance and any reflections from the far end.
Keep that series resistor high to reduce those effects. Also, make sure
the resistor and cap are solidly fixed so you don't get microphony
effects. A little hot-melt glue seems to work well to keep the
components from moving and causing wobbles during the
tap-it-with-a-screwdriver stability tests.
The modified Butler overtone circuit from a G4DDK Anglian 144MHz
transverter running at 116MHz seems to give the best locking range
versus noise performance. The single-transistor circuit used in the
Kuhne G2 transverters is much tougher to drive. I managed to get a solid
lock with a 47k series resistor on the Anglian.
There is a rule of thumb saying that if you inject into the side of the
crystal furthest from the output of the oscillator, the crystal acts as
a bandpass filter and makes it cleaner. Not sure I'd subscribe to that,
it depends on your oscillator circuit.
If your GPSDO is clean and quiet, and you aren't multiplying it up by a
large factor, then just pick a series resistor that allows you to lock
over the desired range with a 6dB attenuator between the GPSDO and the
locking input, and then ditch the attenuator and you'll have plenty of
headroom.
Good luck
Neil
On 28/02/2019 23:43, Thomas S. Knutsen wrote:
Hello.
I have a device that consists of a PLL, that has as its reference a
10MHz crystal.
What I would like to do, is to inject this with 10MHz from a GPSDO,
when that is available, and to use the internal crystal when that is
not available.
Would it be feasible to just connect it to one leg of the crystal
oscillator with a small capacitor, and with that get injection
locking?
The crystal oscillator is on chip, there is a couple capacitors that
allow for frequecy adjustments, other than that, I know nothing about
what is on the chip. The PLL is SP5769.
Br.
Thomas
In the June 1946 issue of "Proceedings of the I.R.E.", Robert Adler published "A Study of Locking Phenomena in Oscillators*. I believe this is the first full study of injection locking. This paper was so important that it was republished in the October 1973 issue of "Proceedings of the IEEE". This paper give the required condition (under a small signal approximation) for injection synchronization as:
(Einj/E) > (2 Q) | delta w / w | (equation 13b)
I can't accurately reproduce this equation in plaintext, but it states that the ratio of the injected voltage to natural oscillator voltage must be greater than twice the product of the circuit Q and the absolute value of the fractional frequency error between the injection frequency and natural oscillator frequency. From this equation it would appear that for a large Q (such as 100,000) the lock range of the injection frequency would be much less than +/- 5 ppm, since the injection voltage would normally be much less than the natural oscillator voltage. For lower Q circuits a larger lock range would be available as long as the injection voltage wasn't too weak.
Access to the Adler paper from either of the publication dates requires IEEE membership or related credentials, but the principles laid out there are extended in the freely available papers below:
** "A Study of Injection Locking and Pulling in Oscillators" (Behzad Razavi in IEEE Journal of Solid-State Circuits, September 2004) :
http://www.seas.ucla.edu/brweb/papers/Journals/RSep04.pdf
This paper derives in a different manner "Adler's equation" [ equation (28) in the paper ], which describes the behavior of LC oscillators under injection. This should also be applicable to crystal (and other resonator) oscillators.
Section III (Injection Pulling) C (Quasi-lock) describes the behavior when the injection frequency is outside the lock range.
Section IV (Requisite Oscillator Nonlinearity) shows that nonlinear behavior in the oscillator is necessary for injection locking to work.
Section V (Phase Noise) describes the reduction of the phase noise of an oscillator by a low-noise injection source.
Section VI describes the effect of injection pulling on a PLL.
** "Gen-Adler: The Generalized Adler’s Equation for Injection Locking Analysis in Oscillators" (Bhansali and Roychowdhury in IEEE 2009 Asia and South Pacific Design Automation Conference):
http://potol.eecs.berkeley.edu/~jr/research/PDFs/2009-01-ASPDAC-Bhansali-Roychowdhury-GenAdler.pdf
The second paper listed above uses "Perturbation Projection Vector (PPV)" analysis, which I don't understand. The authors derive a Generalized Adler's equation which is valid for any type of oscillator. Ring oscillators are discussed, and oscillator waveforms are shown when the injection has a sine, square wave, or exponential waveform.
I post this in case anyone wants to use an analytical approach to investigating injection locking.
--
Bill Byrom N5BB
On Fri, Mar 1, 2019, at 9:00 AM, Neil wrote:
I have five systems using injection locking. There are a few issues to
watch. If you inject at too high a level, any noise on the reference
will appear in the oscillator output. I use a 56 ohm resistor to
terminate the reference signal coax input, then a 100pF cap and a series
resistor connected to on leg of the crystal. The resistor value needs
to be selected so I can get a solid pull-in and lock over an
acceptably-wide range. In most cases, I am multiplying the crystal osc
up to 3.3, 5.6 or 10.2 GHz and using a PLL chip driven from an Rb as the
reference to generate 0dBm at the correct locking frequency around 117
MHz for example.
If the crystal free-runs close to the lock frequency, say within 200ppb,
the series resistor can be 5k or so, and there is almost no effect on
close-in noise, and no sign of spurs. If I have to pull the crystal
more than about +-500ppb, the resistor needs to be a few hundred ohms,
and the synth noise sidebands start to be seen in the osc output. With
a 70 ohm series resistor, the noise of the osc is only about 10dB down
on the noise of the synth, but the lock-in range is around +-1200 ppb,
slightly more on the LF side.
When the osc drifts too far away from the reference, or the level is too
low, you get a spread of frequencies out of the oscillator as it tries
to pull into lock, but doesn't make it. As the lock level rises, it
pulls closer in, but still with a spread of frequencies until it finally
jumps into lock. There is considerable hysteresis, so check thoroughly
that it will pull in under all likely conditions of voltage and temperature.
Remember that the coax lead is going to have a major influence on the
oscillator, so keep it short and watch for mechanical vibration or
ringing or temperature variation effects on the cable. Make certain the
connectors are torqued well. If there is a trimmer on the osc, remember
to tune it to the correct frequency with the cable and distribution amp
connected, but not delivering a signal, as it will probably be pulled a
little by the cable capacitance and any reflections from the far end.
Keep that series resistor high to reduce those effects. Also, make sure
the resistor and cap are solidly fixed so you don't get microphony
effects. A little hot-melt glue seems to work well to keep the
components from moving and causing wobbles during the
tap-it-with-a-screwdriver stability tests.
The modified Butler overtone circuit from a G4DDK Anglian 144MHz
transverter running at 116MHz seems to give the best locking range
versus noise performance. The single-transistor circuit used in the
Kuhne G2 transverters is much tougher to drive. I managed to get a solid
lock with a 47k series resistor on the Anglian.
There is a rule of thumb saying that if you inject into the side of the
crystal furthest from the output of the oscillator, the crystal acts as
a bandpass filter and makes it cleaner. Not sure I'd subscribe to that,
it depends on your oscillator circuit.
If your GPSDO is clean and quiet, and you aren't multiplying it up by a
large factor, then just pick a series resistor that allows you to lock
over the desired range with a 6dB attenuator between the GPSDO and the
locking input, and then ditch the attenuator and you'll have plenty of
headroom.
Good luck
Neil
On 28/02/2019 23:43, Thomas S. Knutsen wrote:
Hello.
I have a device that consists of a PLL, that has as its reference a
10MHz crystal.
What I would like to do, is to inject this with 10MHz from a GPSDO,
when that is available, and to use the internal crystal when that is
not available.
Would it be feasible to just connect it to one leg of the crystal
oscillator with a small capacitor, and with that get injection
locking?
The crystal oscillator is on chip, there is a couple capacitors that
allow for frequecy adjustments, other than that, I know nothing about
what is on the chip. The PLL is SP5769.
Br.
Thomas
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Hi
Where it gets nasty is when you realize that Q is just an approximation for the phase slope of the
oscillator at the operating point…….
The Q of a crystal does not change as a function of frequency. The rate of impedance change vs
frequency most definitely does. As you approach parallel resonance it gets quite high. This impacts
your ability to tune the oscillator. It also impacts your ability to injection lock it. It is the source of the
commonly heard comment “easier to injection lock on the low side ….”.
Bob
On Mar 2, 2019, at 12:37 AM, Bill Byrom time@radio.sent.com wrote:
In the June 1946 issue of "Proceedings of the I.R.E.", Robert Adler published "A Study of Locking Phenomena in Oscillators*. I believe this is the first full study of injection locking. This paper was so important that it was republished in the October 1973 issue of "Proceedings of the IEEE". This paper give the required condition (under a small signal approximation) for injection synchronization as:
(Einj/E) > (2 Q) | delta w / w | (equation 13b)
I can't accurately reproduce this equation in plaintext, but it states that the ratio of the injected voltage to natural oscillator voltage must be greater than twice the product of the circuit Q and the absolute value of the fractional frequency error between the injection frequency and natural oscillator frequency. From this equation it would appear that for a large Q (such as 100,000) the lock range of the injection frequency would be much less than +/- 5 ppm, since the injection voltage would normally be much less than the natural oscillator voltage. For lower Q circuits a larger lock range would be available as long as the injection voltage wasn't too weak.
Access to the Adler paper from either of the publication dates requires IEEE membership or related credentials, but the principles laid out there are extended in the freely available papers below:
** "A Study of Injection Locking and Pulling in Oscillators" (Behzad Razavi in IEEE Journal of Solid-State Circuits, September 2004) :
http://www.seas.ucla.edu/brweb/papers/Journals/RSep04.pdf
This paper derives in a different manner "Adler's equation" [ equation (28) in the paper ], which describes the behavior of LC oscillators under injection. This should also be applicable to crystal (and other resonator) oscillators.
Section III (Injection Pulling) C (Quasi-lock) describes the behavior when the injection frequency is outside the lock range.
Section IV (Requisite Oscillator Nonlinearity) shows that nonlinear behavior in the oscillator is necessary for injection locking to work.
Section V (Phase Noise) describes the reduction of the phase noise of an oscillator by a low-noise injection source.
Section VI describes the effect of injection pulling on a PLL.
** "Gen-Adler: The Generalized Adler’s Equation for Injection Locking Analysis in Oscillators" (Bhansali and Roychowdhury in IEEE 2009 Asia and South Pacific Design Automation Conference):
http://potol.eecs.berkeley.edu/~jr/research/PDFs/2009-01-ASPDAC-Bhansali-Roychowdhury-GenAdler.pdf
The second paper listed above uses "Perturbation Projection Vector (PPV)" analysis, which I don't understand. The authors derive a Generalized Adler's equation which is valid for any type of oscillator. Ring oscillators are discussed, and oscillator waveforms are shown when the injection has a sine, square wave, or exponential waveform.
I post this in case anyone wants to use an analytical approach to investigating injection locking.
--
Bill Byrom N5BB
On Fri, Mar 1, 2019, at 9:00 AM, Neil wrote:
I have five systems using injection locking. There are a few issues to
watch. If you inject at too high a level, any noise on the reference
will appear in the oscillator output. I use a 56 ohm resistor to
terminate the reference signal coax input, then a 100pF cap and a series
resistor connected to on leg of the crystal. The resistor value needs
to be selected so I can get a solid pull-in and lock over an
acceptably-wide range. In most cases, I am multiplying the crystal osc
up to 3.3, 5.6 or 10.2 GHz and using a PLL chip driven from an Rb as the
reference to generate 0dBm at the correct locking frequency around 117
MHz for example.
If the crystal free-runs close to the lock frequency, say within 200ppb,
the series resistor can be 5k or so, and there is almost no effect on
close-in noise, and no sign of spurs. If I have to pull the crystal
more than about +-500ppb, the resistor needs to be a few hundred ohms,
and the synth noise sidebands start to be seen in the osc output. With
a 70 ohm series resistor, the noise of the osc is only about 10dB down
on the noise of the synth, but the lock-in range is around +-1200 ppb,
slightly more on the LF side.
When the osc drifts too far away from the reference, or the level is too
low, you get a spread of frequencies out of the oscillator as it tries
to pull into lock, but doesn't make it. As the lock level rises, it
pulls closer in, but still with a spread of frequencies until it finally
jumps into lock. There is considerable hysteresis, so check thoroughly
that it will pull in under all likely conditions of voltage and temperature.
Remember that the coax lead is going to have a major influence on the
oscillator, so keep it short and watch for mechanical vibration or
ringing or temperature variation effects on the cable. Make certain the
connectors are torqued well. If there is a trimmer on the osc, remember
to tune it to the correct frequency with the cable and distribution amp
connected, but not delivering a signal, as it will probably be pulled a
little by the cable capacitance and any reflections from the far end.
Keep that series resistor high to reduce those effects. Also, make sure
the resistor and cap are solidly fixed so you don't get microphony
effects. A little hot-melt glue seems to work well to keep the
components from moving and causing wobbles during the
tap-it-with-a-screwdriver stability tests.
The modified Butler overtone circuit from a G4DDK Anglian 144MHz
transverter running at 116MHz seems to give the best locking range
versus noise performance. The single-transistor circuit used in the
Kuhne G2 transverters is much tougher to drive. I managed to get a solid
lock with a 47k series resistor on the Anglian.
There is a rule of thumb saying that if you inject into the side of the
crystal furthest from the output of the oscillator, the crystal acts as
a bandpass filter and makes it cleaner. Not sure I'd subscribe to that,
it depends on your oscillator circuit.
If your GPSDO is clean and quiet, and you aren't multiplying it up by a
large factor, then just pick a series resistor that allows you to lock
over the desired range with a 6dB attenuator between the GPSDO and the
locking input, and then ditch the attenuator and you'll have plenty of
headroom.
Good luck
Neil
On 28/02/2019 23:43, Thomas S. Knutsen wrote:
Hello.
I have a device that consists of a PLL, that has as its reference a
10MHz crystal.
What I would like to do, is to inject this with 10MHz from a GPSDO,
when that is available, and to use the internal crystal when that is
not available.
Would it be feasible to just connect it to one leg of the crystal
oscillator with a small capacitor, and with that get injection
locking?
The crystal oscillator is on chip, there is a couple capacitors that
allow for frequecy adjustments, other than that, I know nothing about
what is on the chip. The PLL is SP5769.
Br.
Thomas
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On 3/1/19 9:37 PM, Bill Byrom wrote:
In the June 1946 issue of "Proceedings of the I.R.E.", Robert Adler published "A Study of Locking Phenomena in Oscillators*. I believe this is the first full study of injection locking. This paper was so important that it was republished in the October 1973 issue of "Proceedings of the IEEE". This paper give the required condition (under a small signal approximation) for injection synchronization as:
(Einj/E) > (2 Q) | delta w / w | (equation 13b)
Also, since injection locking is a case of coupled oscillators.. you
might be interested in this (freely downloadable):
https://descanso.jpl.nasa.gov/monograph/series11_chapter.html
Coupled-Oscillator Based Active-Array Antennas
- Ronald J. Pogorzelski
- Apostolos Georgiadis
Am 02.03.19 um 17:02 schrieb jimlux:
Also, since injection locking is a case of coupled oscillators.. you
might be interested in this (freely downloadable):
Ulrich has a discussion of n promiscously coupled oscillators in [1].
In real life probably a debugging nightmare.
I'd like to couple a bunch of MTI-260 oscillators slooowly to a common
incoming
reference and then Wilkinson the outputs together. A somewhat more
"disciplined" approach. I see them shiver with anticipation in the drawer.
Need more free time.
:-) Gerhard
[1] Rohde, Poddar, Böck: The Design of Modern Microwave Oscillators For
Microwave Applications, Wiley