I assume by circuit description, you mean this from
"To invert the sideband between USB and LSB, the second oscillator is switched between 33 MHz and 57 MHz."
I'd say that's just a casual statement, not meant to be read by someone with a law degree.
(But should be corrected. This has come up before.)
The second oscillator is normally at 45-12 = 33mhz for LSB, and 45+12=57mhz for USB.
The VFO is normally on the high side, at 45 + operating_freq, putting it on the other side of 45mhz
would flip the sidebands (and create lots of new birdies, not recommended).
The stock firmware holds the BFO at slightly below the crystal filter, perhaps 11996500 hz.
Moving the BFO to 11999500 hz would flip the sidebands.
I'm assuming the 12mhz filter passband is roughly 11997000 to 11999000 hz,
that will vary depending on which sorting bin your crystals were taken from.
The BFO was made low side to avoid audio tones resulting from harmonics of the BFO
beating with harmonics of the 16mhz clock frequency of the Nano.
However, the 16mhz ceramic resonator of the Nano is not very accurate, could be off by 100khz.
I doubt it makes much difference if the BFO is high or low for most rigs.
Here's a more complete description of what's going on in the stock firmware
along with an attempt at explaining why the sidebands flip.
On Mon, Jul 30, 2018 at 01:53 PM, dan magers wrote:
I've measured the second LO, clk1 at test point 15 and found it to be 33 and 57 MHz, LSB and USB respectively. Opposite of what the circuit description gives. So, I assume the sidebands are reversed.