Re: BFO Clarification?

Ashhar Farhan <farhan@...>
 

Mark,

USB requires setting the carrier to the lower edge of the filter's
passband and LSB requires setting the carrier to the upper edge.

given that we are sourcing crystals from varied places, it probable that
you will have to pull the crysal frequency down for USB and pull it up for
LSB.

adding a small series capacitance to the crystal pulls the frequency up
(good for LSB). to pull the frequency down you need inductive capacitance
in series with the crystal. it is often difficult to estimate just how
much (capactiance/inductance) will be required to position the bfo
correctly. in case of LSB, a 22pf trimmer in series with the crystal can
be tweaked to get the proper results.

in case of USB, i have found about 5uH to be ideal for the crystals that i
have used (3 builds so far). this may not work for you. so you can either
play with the number of turns on L3, or use the inductor with a
capactiance in series to vary the net reactance to set the frequency
properly.

- farhan

On Fri, 25 Jun 2004, Mark wrote:

All

I'm happy soldering here, but have a question:

In the text for the BITX20 it says "If you are using this for 14MHz
and above, the BFO will need a coil in series with the crystal".
This is what I have done - seems to work OK (see G0MGX folder in
pictures on reflector).

I just saw the alternative coil data that has been posted on the
reflector and that says "L3 was used in series between the crystal
and the 22pf trimmer cap plus I added 7pf Mica fixed cap across the
trimmer. Needed for USB"

I am therefore confused!

Can anyone shed any light on this?

Mark. G0MGX





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