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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