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Don Wilhelm wrote:
> Ron,
>
> The problem is that your crystals have a Q which is high and they are
> more difficult than the average to pull down in frequency.
>
> Your best recourse is to pad the crystals down in frequency a bit. You
> can first try a small capacitor from the junction of the crystals and
> C174 (U11 pin 6) to ground. My rule is that the capacitor should not
> exceed 20 pf. Too much capacity here can cause the BFO oscillation to
> act 'strangely'.
>
> If 20 pf will not lower the low frequency enough, then add a padder
> capacitor to both C174 and C173. 47 pf across C174 and 120 pf across
> C173 should do the job nicely and preserve the ratio of the two
> capacitors - the ratio is important to control the feedback for the
> oscillator circuit.
>
> 73,
> Don W3FPR
>
Don,
Updated data below. No real progress with the padding. :-(
A B 10pf 20pf 47/122pf
BFO Hi 4917.62 4917.63 4917.11 4916.8 4917.39
BFO Lo 4913.59 4913.49 4913.3 4913.16 4913.02
Range 4.03 4.14 3.81 3.64 4.37
A is the original set of BFO crystals
B is the replacement set of crystals
Try 1: 10pf from pin 3 - 6 of U11
Try 2: 20pf from pin 3 - 6 of U11
Try 3: 47pf across C174, 122pf across C173
As you can see by the data padding the junction of the crystals only
reduced the upper range.
So I then tried the recommended padding across C173 and C174, and, as
you can see, it still fails to bring the lower range into spec.
Where to go from here?
Thanks a mil for all the help.
Ron
- --
- -=-------------------------o---------------------------=-
Ron Hahn, EI2JP | Grid Square: IO62TG
Rose Hill | Fists: 10883
Rosslare Strand | QRP-ARCI: 12584
County Wexford, IRELAND | K2: 4922
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