Paul Burridge wrote:

An inductor in series with the varactors, then another one in parallel
with the series combo can get you a very wide range of impedance from a
decent varactor.


Thanks, is this the kind of thing you mean?



+-------+
| |
| |
| |
C| |
L1 C| |
C| |
| |
| |
V |
D1 - |
| C|
Applied DC control voltage | C| L2
Line --------------------+ C|
| |
| |
D2 - |
^ |
| |
| |
| |
+-------+

View in FP font.

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Right. I might make the series inductors symmetrical, if I needed the
tap point to be near signal ground (e.g. with a centertapped coil or a
differential pair driving it). Last time I used this trick was in a
160-MHz phase shifter. The two inductors will generally be about the
same size for best results with a hyperabrupt varactor--5 minutes with a
math program will give you the right values. Generally you need to keep
the reactance capacitive if you're resonating a crystal against this
combination--there are multiple operating frequencies otherwise, since
the resonator will look capacitive almost everywhere.


Cheers,

Phil Hobbs