Through experimentation I've determined that the QRP Labs VFO with a 1pps GPS can be used to provide a reasonably stable and accurate crystal substitute for the uBitx raduino (many thanks to everyone's suggestions on this and other threads). The impetus was to see if I could use the two units together to get decent results for the hamsci.org Eclipse Festival happening today.
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The 25 MHz crystal is unsoldered from the raduino (using good flux and braid to remove the solder on the two pins), then two ground pads scraped on the backside ground plane to reveal the copper, and a SMA connector soldered.
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Before soldering the SMA connector to the board, I created an assembly of the connector with a 47 ohm resistor load input (parallel), followed by a 150 pF cap in series for DC blocking. The output of the cap wire goes through the former crystal hole to the si5351 pin 2 (XA) and excess wire cut (and almost forgot to solder it).
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Experiments with an additional PI (C-L-C) LPF right before the 47 ohm resistor load worked (to reduce harmonics), but proved unnecessary, so I left it out.
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The QRP Labs VFO is configured to be GPS disciplined (in my case an old Fastrax 1pps GPS).
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It is then configured to generate 25 MHz out which is also set into the first preset so it runs 25 MHz out on power up.
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I'm using the CEC firmware with Nextion on my v5 uBitx. I set uBitx calibration to 0 because the VFO is generating 25 MHz exactly (within a couple of hertz). This was confirmed by setting the uBitx VFO to 9.999 Mhz and monitoring the carrier of WWV as a 1kHZ audio output tone and the SpectrumLab software FFT measured it to within a couple of hertz of 1000 Hz. One thing I didn't do is change the BFO but I don't think that is necessary (might have to check that at some point anyway).?
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The documentation for the VFO recommends a 50 Hz delta (correction threshold) before it resyncs to the GPS time to avoid audible glitches, however, I'm finding that the minimum of 1 Hz seems ideal for my application where I'm not actually listening to the audio but I need the highest possible frequency accuracy due to crystal temperature drifts (not unusual for the 27 MHz crystal to change 20 Hz over the course of a day indoors in the spring.
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So far the graphs and results look good for such a simple time standard (the Fastrax accuracy is only rated 50 ns RMS in the 1pps output). I do see a 1 or so Hz frequency jumps occasionally when the QRP Labs VFO recalibrates its output 25 Mhz. On the VFO 16x2 LCD I display the output frequency, the 1pps heartbeat, the lock (No/2D/3D), the number of satellites, and the reference frequency (somewhere around 27.005 MHz, as the QRP Labs VFO uses a 27 MHz crystal intended for WSPR applications).
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I've run into two issues:
1. The power up sequence is manual. I first turn on the VFO and then the uBitx to ensure the raduino's si5351 has oscillator present when it powers up.
2. If the GPS signal deteriorates for whatever reason and the unit goes in and out of lock, the heartbeat "skips" a beat or two and under just the right circumstances VFO's pps detector gets confused (possibly due to a bug or implementation limitation) which then causes the VFO to do a severe recalibration throwing the output frequency off hundreds of hertz until it gets a steady lock again. Must get a better GPS.
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I still need to build a shielded box for the VFO - I find that the GPS loses signal when the VFO is outputting 25 MHz but is fine when it outputs 5 MHz or less (thereabouts). This is a problem because I have the GPS module next to a window so it only sees half the sky. My temporary solution is to connect the GPS with a 1 meter cable to move it away from the VFO and put a 20 cm x 40 cm ground plane about 5 cm under it (these numbers are arbitrary but seems to increase the number of satellites seen by 1 or 2)
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Hope this is useful info for those looking for a simple hardware solution.
Roman
