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.