Re: Experiments with #Progrock Follow-up #progrock

Peter Mulhare

Hello Mike and Allison,


I am also a member of the QRSS fraternity, and run a Grabber Station in Northern New Zealand.  I have in the past looked for a solution to keep off the shelf receivers (in particular) from drifting too much.  My Station receiver is an Icom IC-R75, and I wanted to be able to dial set the receive frequency to better than 1e-9 (+1 part in 10-9), and for it to remain there over extended periods of time..   Like you Mike, I have a home brewed VE2ZAZ GPSDO, which outputs a 10MHz signal with an accuracy of a further 2 orders of 10 better than what I wanted.  I had noted that the IC R75 (plus the IC706, and a number of other rigs), source all their internal frequencies like, BFO/IF Mixer LO’s/etc, from the Rig’s own onboard Reference Oscillator.


I already knew that it is possible to phase lock crystal oscillators relatively easily, by injecting a suitable stable signal into an oscillator appropriately, usually in series with the crystal current.  I did this using the 10.0MHz output from my ‘ZAZ GPSDO, injecting it in series with the crystal current of the IC-R75’s Reference Oscillator which runs at 30.0MHz, to my amazement the IC-R75 locked up immediately to the GPSDO!  I simply wound 5 turns of small insulated hookup wire (from ground) over an RF Choke in the Reference Oscillator’s circuit, which was in series with the Reference Crystal, and took this out via small diameter coax to an SMA connector on the rear apron of the receiver, then connected it via a coax fly-lead directly to the GPSDO output.  The RF Choke looked like a small 1/8th watt resistor, and was through hole soldered to the PC board in a vertical orientation. 


This worked because the ‘R75 receiver’s Reference Oscillator runs at 30.0MHz (the 3rd Harmonic of the GPSDO output), so the reference oscillator gets a kick every third cycle to keep it in phase step with the GPSDO.  I have had this arrangement working now for around 4 years with the R75 remaining locked, 24/7 over that period.  This method can absolutely tame the rogue ICOM rigs that drift too much, simply and effectively!  There is no need to do fan modifications, as in the case of the IC706 or IC756 series, or even to go to using fancy TCXO’s.  The next one I do, will be my IC756 Pro3, however I will need to use a phased locked ProgRock (already purchased) for this, as the IC756’s Reference oscillator runs at 33.16666MHz (from memory).  What I like is that immediately from switch on the rig is exactly on frequency (as long as the GPSDO is never switched off!) and will never drift from its intended frequency, over time.


This will work with other manufacturers Rigs too, however the given is that all frequencies within the rig must be derived from the set’s internal Reference Oscillator.




Pete Mulhare





New Zealand



From: [] On Behalf Of ajparent1/KB1GMX
Sent: Tuesday, 21 May 2019 10:43
Subject: Re: [QRPLabs] Experiments with #Progrock Follow-up


Its why I picked on Icom as I've worked people using them on 6 and 2 and they drift.
Chasing them up and down the band is no fun.  The FT817 has issues too as well 
but adding a TCXO helps.  there are many other that wander about on 2M.

The IC706 series is notable for many things including phase noise so bad that banning it from
any multi radio event is considered required.  However the fan can be modded to run slowly
all the time and its a good mod to help with cooling.


Join to automatically receive all group messages.