Regarding the Bitx40 spur, I previously wrote:
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> I'd say the best solution is to use Allard's v2 firmware and mods,
> stay with a high side VFO up around 19.2mhz instead of down at 4.8mhz.
It's possible that a low pass filter on the VFO is sufficient, allowing through 4.8mhz
but not the harmonic at 19.2 mhz. However, the diode ring mixer is non-linear,
might somehow recreate that 4'th harmonic for us.
On Wed, Dec 5, 2018 at 09:56 AM, Jerry Gaffke wrote:
There is a way in which the Bitx40 can produce a spur, as Brian reports.
Look over the last few posts in this thread:
The thread has to do with images received when using a low side VFO on the Bitx40.
When receiving 7.1mhz, the VFO is at 12 - 7.1 = 4.9mhz.
The 4'th harmonic of the VFO is at 4 * 4.9 = 19.6mhz, effectively a high side VFO.
We will receive an image if there is a shortwave broadcaster at 19.6 - 12 = 7.6mhz
Not stated in that particular thread, but this can happen on transmit as well.
When transmitting at 7.1mhz, we will have a faint spur at 7.6mhz.
When transmitting at 7.21mhz, we will have a faint spur at 4*(12 - 7.21) - 12 = 7.16mhz
Note that this transmitted spur can easily pass through the 7mhz bandpass filter
and the transmit low pass filter..
The only factor that keeps the spur in check is that the 4'th harmonic of the VFO
is much weaker than the fundamental of the VFO, and this may vary from board to board,
the VFO drive level programmed into the si5351, and how the VFO wires get routed.
The curious thing is that when transmitting at 7.2mhz, the spur is at 7.2 mhz.
The spur is a valid SSB signal, but on the opposite sideband.
So there's a vague connection with the 7.2mhz Bitx40 birdie,
but the math behind the two is quite different.
I'd say the best solution is to use Allard's v2 firmware and mods,
stay with a high side VFO up around 19.2mhz instead of down at 4.8mhz.
As Brian points out, this cures the spur, but when transmitting an LSB signal
we have the not-so-sharp skirt of the 12mhz crystal filter facing the carrier and opposite sideband.
So we will have additional carrier leakage, and the opposite sideband is not suppressed quite as well.
However, that's a perfectly legal signal anywhere AM phone is allowed.
Especially if only transmitting 5 or 10 watts.
I am of the opinion that if transmitting at 5 or 10 watts, the spur with a stock low side VFO
is likely down far enough to be more or less acceptable.
But to fully meet regulations with a Bitx40, use Allard's v2 firmware and mods, and a high side VFO.