It appears that Connor-Winfield have been experiencing some production issues with their D75 series TCXOs. A couple of years ago I used a 27MHz unit to replace the crystal in an Octagon OTLSO and the performance when compared to the standard crystal was transformed. Earlier this year I bought a batch of the same TCXOs from Digikey. On this occasion I was disappointed to find that several of the units had very poor short term stability.
The problem appeared to be a control loop issue where, at near constant temperature, the output would ‘hunt,’ moving up and down over a period of a few minutes. Technically, the TCXOs were still well within their 500ppb tolerance and so returning them as ‘out of specification’ to Digikey was questionable.
Fortunately, the Conner-Winfield TCXOs are a premium product and I contacted technical support at their HQ. After sending a lengthy description, a few graphs and returning a few unusable TCXOs they performed a failure analysis, acknowledged the fault and a couple of weeks later replaced the entire batch with some straight off the production line. – date code 2619
As a test, you can assess your TCXO in about 10 minutes by looking at the CW beacon on QO-100.
If you use an SDR on 739MHz, set up the system and let it all ‘warm up’. This is important as there is a TCXO in the LNB but another in the SDR which will also contribute to drift.
1) Set the demodulator window to about 2kHz as a visual reference and the bandwidth across the screen to about 10 – 20kHz.
2) Slow the waterfall down so it takes about 10 minutes to fill the available space. Add time markers to the plot if you can.
Below I have shown 2 plots for comparison. The first is the plot from a faulty TCXO which over 10 minutes moves around its ideal frequency apparently unhappy to stabilise. The second plot shows the performance of a normal TCXO. In both cases a small amount of this drift will be due to the SDR’s oscillator at 739, but the majority of the drift will be from the 9750MHz VCO in the LNB.