On Tue, 2 Nov 2004 11:58:17 -0600, "Steve Nosko"
wrote:


Why both? It is the total series cap which is of concern. One cap
would simply be half the value of each of the two. 2 x 5pf = 1 x 2.5pf,
no?

Yes, it is doable with one fixed capacitor, however, the voltages
across the stray capacitances would be different and their effect
would be harder to predict.


Of course, the series resistor will reduce the tuning range.

I don't see this. It would be just like the one shown. 100k or 1M. It
is for DC and is large enough to be neglegible at RF. (strays acknowledged)

I meant to say series _capacitance_ (not resistance), sorry for the
confusion.


THough I don't think it is necessary, if you require symmetry.
Another option:
+----+ Control voltage
| |
R R
| |
+----||-+-||-+--||---+
| 2.5pf |
| |
| |
| |
| |
| |
+--LLLLLLLL+LLLLLLLLL--+

This would be an elegant solution.


Another option:
+-----------+ Control voltage
| |
R R
| |
+-||-+-||-+-||-+--||-+
| 5pf | 5pf |
| | |
| gnd |
| |
| |
| |
+--LLLLLLLL+LLLLLLLLL--+

As well as this.


To reduce the Z in a resonator, you will have to reduce both the
inductive Xl and capacitive Xc reactance by reducing the inductance
and increasing the capacitance (e.g. by multiple varactors).

So now the varactor must be _more_ of the overall capacitance to get the
desired tuning range, no. If you want minimal side effects, then the
varactors need to be just barely inthe circuit, so to speak, which means
that they will have small voltages.

For a given total capacitance, putting more varactors in parallel
means that the capacitance each varactor must produce is reduced, thus
the tuning voltage must be increased, which is a good thing, since
this improves the ratio between the tuning voltage and RF voltage.
Dropping the impedance levels and hence increasing the capacitance
required also means that even more varactors can be connected in
parallel.

However, I think that the OP should also study of making a shortened
1/4 (stripline or microstrip) resonator, with very wide resonators

clip

Something you should not reject is a "ranged" system. Tune a smaller
range with the varactors and switch in/out other caps for larger shifts.

The OP clearly had something similar in mind, since he originally
asked for a 2:1 tuning range, but now he is asking for the 30-500 MHz
range. I do not think that such huge range can be handled by just
switching base capacitors. With such large frequency range, it would
make more sense, to build at least three completely independent
filters, say 30-90 MHz, 90-270 MHz and 270-500 MHz, while each filter
could switch in base capacitances. The two lower filters could be
lumped LC filters, while the last range could be limited to less than
one octave, so that strip line filters could be used.

Paul OH3LWR