Si5351 Quadrature Issues

 Since I enjoy the higher HF bands more 40m and below, I've found that using the dual D-type (74xxx74) flip flop method for generating a quadrature LO to be squirrely.  A 5351 can easily generate the 4x LO frequency signal needed, but 74AC74's seem marginal for 15m and 10m where they have to operate at 84 to 112 MHz - I've observed high current draw and low output.  No issues at 20m and down.  I even tried the notorious Potato Semiconductor version of the 7474 that's supposed to work well at high clock frequencies.  Results were mixed at best.
In order to cover these two bands, I've used "set phase" capability of the 5351 to directly generate a quadrature LO.  Code to accomplish this abounds on-line.  I happened to use the ZL2CTM variant.  It's listed in one of my previous posts.  This method seemed to work pretty well on receive (phasing DCR), although I noticed that opposite sideband suppression was intermittently and variably lousy at times.  When this occurred during transmit, the resulting signal was full of garbage.  I eventually tracked this behavior down to the fact that the two 5351 outputs that were supposed have 90 deg phase difference, randomly did not.  This occurred over relatively small frequency spans that did not require a divider value change and "pll reset" command.
 
Here's what it should look like on the scope:
 
Once in a while though, the output looked like this:
This anomalous phase difference varied, with no apparent pattern to either the amount of shift away from the intended 90 deg or the frequency at which it occurred.  Simply executing a "pll reset" command always fixed it.  I have yet to find clear cause for this behavior.  As a practical workaround, I simply added a polled, front-panel switch to the phasing TRX, that when pressed, had the code execute a "pll reset".  Crude and a bit of a pain, but fortunately, doesn't require frequent use. 




 



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