the 45 mhz if amplifier is distorting and producing a second harmonic at 90 mhz. this mixes with the local oscillator to produce a spur at 90 mhz - local oscillator.
here is 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 is 73Mhz.
X - 73 = 17, 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.
what is th cure?
there are three solutions
1. decrease the distortion of the if amp by modifying or replacing the IF amp (the tx side at least)
2. add a low pass or bandpass filter between the final mixer and the if amp.
3. move to 60 mhz or 70 mhz IF insteead of 45 mhz. a double or triple bandpass should do it.
this is a result of joint investigation between Raj and I.