GPSDO – finally, maybe, finally

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Posted by w0ep on February 4, 2018 at 6:55 pm

[Yesterday: 10 miles; today: 3.5 miles]

I built this GPSDO using an oscillator and other parts that I took from an ebay HP frequency counter. It has worked pretty well.  I had some problems at the beginning with the oscillator not adjusting like it should in response to the control voltage.  I did some work on it and that seemed to help.

Then recently it has been acting up again.

In the mean time, I had acquired another HP frequency counter of the same type with another of this kind of oscillator inside.  I got that at the Jackson, MS hamfest 3-4 years ago.  So I took the osc. out of that one and put it in the GPSDO.  Lo and behold, it actually worked very well.  I could manually run the control voltage to max positive and max negative and see a small but distinct change in the output frequency.  That is the way it is supposed to work.

So, with that as my backup I dug into the first oscillator (we’ll call it “A” and the 2nd one “B”).  The circuit inside the oscillator that does this is pretty small.  There is a manual with schematic online, so I could see that there were only about 5 parts doing this control, the main one is a varacter diode which changes capacitance in response to changing voltage.  So I ordered a new one of those.  Eventually it came in the mail and I installed it.  Osc “A” goes back into the GPSDO and it still isn’t responding to the control voltage.  So swap back in “B” and “A” goes back on the bench.

Today I did some close examination of the two related fixed-value capacitors.  Long story short, the one called “C2” in the manual was supposed to be a 15 pf capacitor.  In my unit it was a 150 pf capacitor.  That would definitely make a difference, 150 being 10 times larger than 15.  So I put in a 15 pf and put it all back together.

Now I have “A” installed in the GPSDO again, and now this thing seems to be working in the expected way.  It did work for quite awhile in it’s own style; I’m not sure if this is the only issue. But I feel like I’ve found an original error and it will help to have it corrected.

I should explain that the HP frequency counters these were in are not the only use for this oscillator module. And, the counter application does not use the control voltage.  So if they had units coming out of manufacturing which did not respond to control voltage, putting one of those units in a counter would be perfectly acceptable. It could well be that this is a manufacturing defect and they just sent the unit to counter duty.  That’s a theory anyway.

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