While building an SSB transceiver we need to take correct reading of the crystals used in a typical home brew crystal filter, as these readings will determine the correct frequency for the BFO.
What i noticed that many Bitx builders were adjusting the BFO freq as mentioned in the manual, now wait this frequency may not be the bfo freq of your set. as the builder had taken readings of his crystals and arrived at his BFO freq USB for 20mt.
You need to take readings of your crystals and find out the freq for the BFO.
Below we have mentioned how this can be done.
Many professional transceivers and transmitters used this scheme, as it simplified the generation of ISB, Independent SideBand.
Also, the Drake TR-3/TR-4/T-4 used this approach.
In amateur transceivers, it was more common that a single SSB filter was used, and the carrier frequency was shifted between USB and LSB.
Assuming a filter centre frequency of 10000.0 kHz, the USB carrier frequency would be 9998.6 kHz and the LSB 10001.4 kHz.
The assigned frequencies for both LSB and USB would be the same, 10000.0 kHz.
If the frequency display was to show carrier or reference frequency, it would be necessary to add or subtract 1.4 kHz to the centre frequency reading. Also keep in mind that some mixing schemes interchange the sideband positions relative to the reference frequency.
So take the example above the Xtal frequency is 10000.0 khz now if we subtract 1.4 khz from this as below for USB freq we get results.
Calculating a BFO. LSB.USB freq for a 10MHZ crystal filter
Minus 1.4 khz
freq 9998.6 khz is our USB freq
So now we need to take our LSB Freq which has to be above Xtal
filter freq 1.4 khz above 10.000 khz
PLUS 1.4 khz
freq 10001.4 kHz. is our LSB freq
So we have found out that by using xtal filter at 10.000 khz
our USB freq is 9998.6 khz and LSB freq is 10001.4 kHz.
The set above used 1.4khz bandwidth but standard is 1.5 khz .
So follow the above procedure and calculate your Xtal freq LSB and USB from the actual freq you get
from your Bitx VFO.