Re: Bitx40 75KHz Spurious, How to solve it. #bitx40

Jerry Gaffke
 

Recommendation:  Use Allard's v2 firmware and associated hardware mods:
    https://github.com/amunters/bitx40-raduino-v2
Moving to a high side vfo using Allard's firmware should fix this spur.

When operating at 7.2 mhz with a low side vfo,
that vfo is at 12-7.2 = 4.8 mhz.
The fifth harmonic of the vfo mixes with the second harmonic
of the 12mhz bfo:    4.8*5 = 24  = 2*12
PA4Q (Cor) figured this one out:  https://groups.io/g/BITX20/message/21996

The bfo and vfo have no reason to mix, except that this rig is not shielded.
Also, both are being generated inside the si5351, there may be crosstalk there.

Going to a high side VFO should work, the vfo is now at 12+7.2 = 19.2
To operate lower sideband as is customary on 40m,
the bfo must now be moved down a few khz to the other side of the 12mhz crystal filter.
The easiest way to do all of this is to use Allard's v2 code for the Bitx40,
he uses the si5351's clk0 to drive the bfo instead of the 12mhz crystal oscillator.

Curiously enough, 19.2*5 = 96 = 12*8
So their might still be a slight birdie when operating at 7.2 mhz with a high side vfo
but much reduced from the stock low side vfo.

Jerry, KE7ER


On Sun, Jul 1, 2018 at 10:53 pm, Raj vu2zap wrote:
OK! connected up the bitx40 with Raduino.

Yes, there are spurious as reported. The spurious distance from carrier varies with frequency.

All the spurs all come together and disappear at near 7.2 Mhz. This is mixing of harmonics but
cant remember the maths.. Jerry and others had worked it out.

I will try out a filter that I had wired on some board but donno where I kept it!

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