I was unable to get 20 KHz tuning range using varicaps. So I tried a variable capacitor instead (not a trimer). The tuning range increased to 14 KHz but that is not enough.Then I added a coil in serie with the varco. No luck, 16 Khz was maximum.
Finally I tried the super VXO configuration (2 xtals parallel and a coil in serie).
That did the trick. The tuning range is 25 KHz.
see http://www.qsl.net/7n3wvm/supervxo.html for information about super VXO's.
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