On Mon, 1 Nov 2004 17:24:10 -0600, "Steve Nosko"
wrote:


The "back to back" configuration I am familiar with is like this:

+ Control voltage
|
R
|
+----||-----+---||---+
| |
| |
+--LLLLLLLL+LLLLLLLLL--+

There could also be a series cap in this configuration--except a resistor or
choke would be required on one of the varactors. (no different than the one
to the control line)

You would need fixed capacitors on _both_ anodes and also resistors to
ground from both anodes to get the DC bias. The small series
capacitors are essential, since they take the most part of the RF
voltage. Of course, the series resistor will reduce the tuning range.

You could get away with the series capacitor and put multiple (maybe
10) varactors in series. Thus, the RF voltage across each varactor
would be low. You will need a high tuning voltage, perhaps 100 V.

The worst problem is how to get the DC voltage distributed over the
varactors. The simple solution would be to put resistors across each
varactor to form a voltage divider. Apart from possible thermal noise
problems (if weak signals are also involved), the nasty thing is the
resistors will have a parasitic capacitance across the ends of the
resistor. This capacitance is in parallel with the varactor, forming a
significant base capacitance. Also the losses (and hence Q) of these
parallel parasitic capacitances may degrade the total Q of the
resonant circuit. Putting multiple varactors in series also increase
the total inductance, which would not be so nice in this case, since
the inductance levels are already low.

However, if the parasitic capacitance/diode is much less than the
varactor minimum capacitance, quite large tuning ranges could be
obtained.

I also don't get this talk about the filter Z. Since you'd need to Z match
in/out of the filter, it seems to me the varactor voltages will be the same
for any Zin/out since this will be determined by how "tightly" they are
coupled into the resonant circuit and not the Zin/Zout, no? The Z match
will just change the Vin/out.

Think about two resonant circuits coupled by a small capacitance at
the top. Connect the signal from the input line to the first resonator
using inductive coupling (transformer with untuned primary). By
selecting the number of turns on the primary, you can get any
impedance transformation ratio, so you can match the 50 ohm line to
any low impedance resonator. On the output side on the other
resonator, you can do the opposite with the other transformer and
restore the impedance to 50 ohms for the output line.

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).

Rhode wrote an article in QST a few years ago about running the HF
varactor tuning front end at a lower impedance level to avoid the high
RF voltages on the varactors.

In VHF/UHF reducing the inductance to enable larger capacitances and
thus lower impedance and RF voltages is problematic, since the
inductance is already extremely small.

In principle it should be possible to connect several "coils" in
parallel (actual wire loops across the capacitor) and this is how many
text books explain how the cavity resonators are formed by adding
further and further wire loops surrounding the capacitor. However, I
have never seen parallel "coils" in any practical circuit, apparently
there are some parasitic capacitance problems.

However, I think that the OP should also study of making a shortened
1/4 (stripline or microstrip) resonator, with very wide resonators
(and thus low impedance levels) and do the impedance transformation at
the input and output coupling. If multiple stage filtering is needed,
look for interdigital filters and again design for low resonator
impedances to reduce the RF voltage across the tuning capacitors.
These might be more practical for the intended frequencies than
ordinary LC filters.

If the tuning speed is not very large, look for some mechanical tuning
at the end of the stripline resonator, such as moving the grounding
electrode closer to the resonator hot end by a piezoelectric crystal
etc., thus increasing the capacitance.

Paul OH3LWR