Q10 is the BFO oscillator and Q11 is the buffer between the oscillator and the load.
The buffer interfaces the oscillator and the load so that the oscillator is not "pulled"
by the load.
Being a crystal oscillator, Q10 produces a complete AM signal, i.e., a carrier and
two sidebands, USB and LSB. This oscillator (The BFO) is tuned to the IF frequency.
The load is a mixer. Its function is to suppress the carrier and inject audio modulation on transmit
and to convert the IF by demodulating (recovering) the audio signal on receive.
It is very like the main mixer but acts on only one frequency (the IF) instead of many (RF)
The nominal IF frequency is that of the crystal -- 12 MHz. In actuality, it is slightly lower
because we want to isolateand use the LSB. If we wanted USB, it would be several KHz higher.
The crystal filter receives the IF with both sidebands (Remember, only the carrier was
suppressed) and removes the USB(which has no information), in this case, passing
on the LSB to the final mixer. How it does this is achieved by tuning the IF to the exact
frequency needed to reflect the LSB side of the filter.
The filter is sort of neutral; it will also pass the USB if the IF is tuned properly, but is very
selective in that it will reject the unselected one because it is so narrow in its "passband".
What this means to a DDS is that one must generate 2 if signals, one for USB and one for
LSB, and select which one to send through the filter depending on the mode one wants.
Since a DDS is the source, the IF oscillator (Q10) can be eliminated. Q11 may or may not
be necessary; it depends on how well the DDS drives the mixer. The mixer might have to
be tuned a bit as well.
For example, let's say that the center frequency of your filter is 12,000,300 Hz. It doesn't
have to be, and may be considerably different in a given rig. Every rig will be a little different from
the others. Then the IF necessary to carry the LSB would be about 2KHz below that and the IF
for the USB would be about 2 KHz above that. What it actually is will require experimentation and measurement.
That is your challenge as a builder.
john
AD5YE
