Some information from Jerry, KE7ER from post
Assume for now that we are transmitting an AM signal at 12.000mhz
with a 1khz tone from the microphone. The two diode modulator get's a
12.000mhz carrier from the BFO and mixes that with the 0.001mhz audio
from the microphone. The modulator gives us a signal consisting of the
carrier, the carrier plus the audio or 12.001mhz and the carrier minus
the audio or 11.999mhz. If we send that straight out to the IRF510,
that's AM phone on 12mhz.
For LSB, all we want is the 11.999mhz
signal. We can get that by having very sharp bandpass filter (the
crystal filter) that allows through the 11.999mhz modulator product but
not the 12.000mhz or 12.001mhz components. Sending that on to the
IRF510 would give an LSB signal at 12.000 MHz. (The number you read on
the dial is the frequency of the suppressed carrier.)
For USB, all
we want is the 12.001mhz signal. We can get that by having very
sharp bandpass filter that allows through the 12.001mhz modulator
product but not the 12.000mhz or 11.999mhz components. Sending that on
to the IRF510 would give a USB signal at 12.000 MHz.
hard to move the crystal filter. Easier to build the crystal filter for
only 12.000mhz, and place the BFO at either 11.999mhz (for USB) or
12.001mhz (for LSB).
If we speak into the microphone, most of us
give more than just a 1khz tone, there's a range of audio frequencies of
interest for intelligible speech from around 500hz to 3000hz. So
that crystal filter must allow a band of frequencies about 2500hz wide
through, not just a single frequency. The Bitx40 has a balanced
modulator, so the carrier is mostly suppressed. It's good enough if the
crystal filter does a good enough job of knocking out the far sideband,
which starts at 500+500=1000hz away from the low audio frequencies that
we are trying to transmit.
We want to transmit at 7mhz, not
12mhz. The diode ring mixer combines the above 12mhz sideband signal
with the VFO. If the VFO is at 5mhz, then the 7mhz bandpass filter (the
three torroids L1,L2,L3) will allow through the 7mhz difference signal
from 12mhz-5mhz. If the VFO is at 19mhz, then the bandpass filter will
allow through the 7mhz difference of 19mhz-12mhz. Note that 19-12.001 =
6.999mhz, so in the 19mhz case what was 12mhz USB has become 7mhz LSB.
sketch has been written. Several here are successfully using a 19mhz
VFO to transmit 40m USB. Ask them. Only physical change is to remove
C91 and C92, the Raduino doesn't need them for 5mhz or 19mhz. Pete is
successfully using the alternate method of moving the BFO from the
More to be added soon, 73 de Noel, ZL1NC.