Spurs fix (See update below)
Ashhar Farhan VU2ESE, the designer of the µBITx has been experimenting with solutions to the spurs on SSB mode above 18MHz. He has posted to the list that one spur fix that is now consistently working for him, and that is easy to apply, is to fix the distortion in the 45 MHz amp.
He inserts a 0.3µH inductor in series with a 10 pf capacitor across TP13 on the v4 board. This is on the output of the 45 MHz IF amp going to the front end of the mixer. The inductor comprises 8 turns on a T30-6 toroid (to give 0.3µH or 300nH).
UPDATE But wait … he proposes a better spurs fix a day or two later
1. Change the resistors R26 and R46 to 220 ohms (from 470 ohms). this increases the darlington pair’s standing current and decreases the Harmonic distortions.
2. The 90 MHz trap was difficult to tune unless you had a spectrum analyzer. Instead, we can use a Low Pass Filter instead. A ‘T’, rather than a Pi is used. This helps improve isolation on both the RX/TX path.
3. The LPF consists of an L-C-L of 300nH, 47pf, 300nH.
Removing the BFO leak into the local oscillator
Ashhar also found that there is cross talk inside the Si5351, whereby the 12 MHz output from the BFO leaks into the local oscillator.
Ashhar used a 5µH inductor in series with a 47 pF capacitor placed across C202 of the LPF suppresses this. The inductor comprises 35 turns on a T30-6 toroid (to give 5µH). However, another constructor found this blanked out the 30m band, and calculations reveal that there may be an error with the calculation of inductance or capacitance that is waiting to be confirmed. The filter should be tuned to 12MHz suggesting a slightly small capacitance value of around 35pF
uBITx.net will be looking for verification from constructors that these mods represent a definitive fix for the spurs and carrier leak problems.