Re: Selecting the correct BFO frequency

Jerry Gaffke
 

I assume this is a single conversion receiver, like the Bitx40.
With the BFO set above the crystal filter, you get a lower sideband signal at around 10mhz.

Assume you wish to transmit at 7mhz.
You could put the VFO at 3mhz,  stuff at 10mhz would get converted to 10-3 = 7.0mhz,
and stuff at 9.999mhz would get converted to 9.999 - 3 = 6.999mhz.
This has the advantage of a low frequency VFO which is likely to be more stable.
But you need the BFO on the high side of the crystal filter.

Or you could put the VFO at 17mhz, stuff at 10mhz would get converted to 17-10 = 7,.0mhz,
and stuff at 9.999mhz would get converted to 17-9.999 = 7.001mhz, so now an upper sideband signal.
To compensate, you want the BFO on the low side of the crystal filter go generate 10mhz USB
which gets flipped to 7mhz LSB.  The advantage is fewer birdies because mixer products
of the VFO haromonics are all well above the operating frequency, but the VFO won't be so stable.

VFO stability is not such an issue if you go to the $1 si5351 instead of building a traditional VFO
using analog techniques.
 
Jerry, KE7ER


On Mon, Jan 1, 2018 at 11:00 am, Kerr Smith wrote:
From what I have read I think the current BFO frequency is a little low and it should be around (9.998887MHz + 2.5kHz) so 10.001387MHz.
Would it also work if I set the frequency to (9.998887 - 2.5kHz) so 9.996387MHz, I think it may be easier to lower the BFO frequency rather than raise it.

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