[time-nuts] injection locking crystal oscillator

[Neil]

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
>