[time-nuts] Injection locking

Kit Scally kScally at BYTECAN.com.au
Wed Feb 10 23:55:10 UTC 2010


As others have already noted, there's a paucity of circuits or down to
earth advice on this topic - unless your poison is "millimetre wave"
I set out to GPS-lock my Icom 706 transceiver some while ago.  This uses
a (single) hi-stability master 30MHz xtal oscillator.  With support from
Murray (ZL1BPU), I decided to IL rather than generate an external
GPS-locked 30MHz source.

The only reference other than that by Uzunoglu to IL oscillator design I
managed to find was in Jessop, 4th Ed, 1994 (p9.50) which stated:

P1/P2 = [2Q*deltaF/F]^2

P1   = pwr of injected signal
P2   = Oscillator (to be controlled) power
Q    = loaded Q of cavity (ie: xtal osc)
F    = oscillator frequency
delta F = locking range

Working the formula backwards together with lab testing confirmed that
the greater the injection level, the greater the lock-in range and
In practice, I coupled a +10dBm 10MHz square wave via a ~15dB attenuator
into the 30MHz xtal osc's "pulling inductor" via a 2 turn loop. This
allowed the IC-706 to lock-up within 10" of turning on from cold and
then remain in lock.

Theory suggests that lock sensitivity could be improved in this instance
if the 10MHz duty cycle was changed from 1:1 to 6 or 7:1, the idea being
that the narrower pulse should be <0.5 period of the frequency of the
oscillator to be locked.  I didn't pursue this avenue as the additional
stray GPSDO 10MHz signal present within the Icom didn't interfere too
greatly with WWV reception on 10MHz.  Spectran's display, however,
showed another story.

Measuring the lock BW with varying injection levels was an interesting
exercise !

On a related topic, I found some while ago - and promptly lost - a
graph/chart showing harmonic level variations with varying duty-cycle of
an input waveform.  This was to some degree a graphical representation
of the Wenzel document referenced by Bruce recently.  Has anyone got a
link to this document please ?



> Joop,
> you may also search for "synchronous oscillator". You will find for
> example:
> http://www.amalgamate2000.com/radio-hobbies/radio/synchronous_oscillat
> or.htm
> Best regards
> Ulrich

>> Are there references to some practical circuits? That would be great.
>> Cheers,
>> Joop


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