The cause of spurs

Ashhar VU2ESE and Raj VU2ZAP have nailed the cause of the spurs.  The 45 MHz  IF amplifier is distorting and producing a second harmonic at 90 MHz,  which mixes with the local oscillator to produce a spur at 90 MHz.
Here is an example of how it happens :
  • the spur moves down as you tune up.
  • when the radio is tuned to 28 MHz, the spur is at 17 MHz.  When the uBITx is tuned to 28.150 MHz, the spur is on 16.850 MHz.
  • At 28 MHz, the local oscillator  is at 73 MHz.
  • There is a signal X such that :

X – local oscillator = 17 mhz

Local oscillator = 73Mhz.

X – 73 = 17,  so X must be 90 MHz.

After checking the spur on a number of frequencies between 15 MHz and 30 MHz, it was confirmed that the above formula works consistently to predict the spur.
Now to confirm a simple solution …  if the problem is in the 45MHz IF, then it may also be possible to address the IMD at the same time.
Reference

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