Electronics > Metrology

Cheap GPSDO Buying Help 2024?

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Dang -- I didn't even know that was a potentiometer!

Any idea what it might do?


It is used to adjust the discrimination voltage of the amplifier. When it's off level, the 10 MHz outputs get stuck at logical 0 or 1.


That mini trimpot is very twitchy and subject to being "bumped." I rotated it through its range, then gave it a drop of fader lube, then rotated it some more, and then I gave it a taste of the hot air station.

After cooling down I was able to find a fragile setting that results in this output from the square and sine wave ports. When doing so you can see what Lady Heather says.

For most of the pot's rotational range there is no coherent output on the scope. Shown here is the "best" setting, but the square wave isn't quite square, and the sine wave is a bit goofy, although the oscillator stops/locks at 10.0000 MHz (after drifting upward for a few hertz after every cold start or reset), which is the decimal precision limit of the counter in my old Rigol scope (never calibrated by me).

For various "nearby" pot settings the waveforms get weird double humps (lower side) and look really goofy, with the counter displaying freqs between 5 and 11 MHz (I assume because of the weird wave shapes or zero/trigger crossings?).

Left as shown, the waveforms have remained locked for a day or so.

On the MAX4016 opamp: When there's no coherent output on the scope, pin 8 (dc in) and pin 3 (non-inverting input) show 5 VDC. When the trim pot is rotated to produce the display as shown, and there is coherent output on pin 1, pin 3 shows 2.5 VDC -- half of the dc input. I think this is normal.

My problem all along probably was related to the trim pot, which seems to measure 500 ohms from top to bottom, although I don't know if any other resistances are involved. The coherent output is produced near the halfway point on the trimpot.

The pot is probably bad...and replacing it seems VERY difficult. I can barely see the three obscured solder pads. I could barely get the SOICs replaced. :)

What if I supply my own (separate) dc voltage to pin 3 with a real, large trim pot (through a 10k resistor to limit current)?

Can the existing waveforms be used to discipline my HP 5328A freq counter if I adjust the reference signal for proper input strength?

What if I connect the outputs to a distribution amp with adjustable gain?

I am learning a lot through this torturous process, which is good. But I'd really like to come through this with a working GPSDO. :)

Thanks again for your help,


The blue output waveform indicates a higher resistance termination, if you are using a 2 m cable between generator and scope. If you terminate the cable correctly at the scope end, the waveform will look clean. Amplitude will be about 2 V - half of the generator voltage.
If the HP 5328A external timebase input includes correct termination it will work.

Regards, Dieter


You are indeed an officer AND a gentleman! And perhaps a psychic!

New pic shows the dual-trace scope with 50-ohm terminating resistors "teed" onto each input with the requisite 2-meter-long BNC cables.

This looks usable! Sorry Mr. Bodnar -- no cash flow for you (just yet)!  Without replacing the SMD trimpot, I don't expect that this unit will work forever...

With help from many on this list I have learned a lot throughout this repair process. I learned how to replace SMD chips without killing the chips or the DUT. That's something that's been on my list for about 20 years. I learned that SMD trimpots exist -- who knew? I learned the basics of my DSO, and about potential termination issues. And more.

After learning about the discriminator level trimpot late in the game, it's my chips probably didn't need replacing, but it was for a good cause.

I will connect this to the HP 5328A later tonight, where I expect that things will work as normal.

If not, I'll be back.  :)

Thanks, everyone, for all the help. In the modern era most mentoring is distributed via the interwebs.




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