W3JDR wrote:
Paul,
The amount of tuning range is a function of the ratio of Cmax/Cmin. If you
parallel varactors, Cmax will double, but so will Cmin. The ratio hasn't
changed.

If you're not already using a "hyper-abrupt" type of varactor, you should
look into one. They offer a wider capacitance range.

What type of varactor are you using, and what's the frequency of the
resonator? What's the application...linear frequency modulation like FM or
data keying like FSK???

Joe
W3JDR


"Paul Burridge" wrote in message
...

Hi,

I'm currently working on this VCXO that achieves frequency shift by
applying DC bias to two varactor diodes connected cathode to cathode
(bias applied to the junction between them). If I can't get enough
shift with the available bias voltage, is there any problem with just
putting another pair of the same diodes in parallel with the existing
ones?
This is a ceramic resonator oscillator, BTW, so will stand a lot more
'pulling' than a xtal would, so don't worry about that aspect of it.

p.
--

"I expect history will be kind to me, since I intend to write it."
-

Winston Churchill



If he puts two diodes in parallel he will double the capacitance and
will have to reduce the amount of inductance to have the same
min. frequency. Im not sure that the high end won't be greater than
before, even though the capacitance ratio is the same, since the
fixed inductance is lower.

Another idea would be to put the two varicaps in parallel, but switch
one of them out as you approach the upper frequency.