Re: Selecting the correct BFO frequency

Jerry Gaffke

It definitely matters if the BFO is above or below the crystal filter passband.

I was talking both transmit and receive in that last post, but the math is the same regardless of which direction the signals travel.
If you want a BFO slightly lower in frequency than the crystal filter, the 10mhz crystal filter will be receiving a 10mhz USB signal.
So to compensate, must run the VFO at 17mhz, on the high side of 10mhz crystal filter, which flips things so that you will be receiving 7mhz LSB signals.
A 17mhz analog VFO is possible, but will be tricky to get stable enough.

The parallel resonant frequency of a crystal is a few khz above the series resonant frequency, that may be the key to using
a 3mhz analog VFO with your 10mhz crystal filter to receive a 7mhz LSB signal.
It is the series resonant frequency that is used by your crystal filter, and by many oscillator circuits.
So I'd fiddle around with the circuit in figure 18 of the link you posted to get a BFO above your crystal filter.
And/or look this over:


On Mon, Jan 1, 2018 at 02:54 pm, Kerr Smith wrote:
Just to clarify - will it matter if I decide to put the BFO above or below the center frequency of the crystal filter - this is just going to be a receiver for 40m, there will be no transmitting circuits?

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