"Paul Keinanen" wrote in message
...
On Sat, 30 Oct 2004 09:59:15 -0700, "Joel Kolstad"
wrote:


I'm not quite following this (I don't know what a 'varactor varacitor
is'?).
Something like:

signal -- | | -- || -- gnd

Where | | is the 5pF capacitor and || is the varactor with anode facing
'left'? I'm not following how the 'back to back' configuration works.

Something like this (fixed font):

Vt
+-+
| |
| R
| |
+----||------+---||---+
| | |
| R |
| | |
+----||---+------||---+
| |
| |
+--LLLLLLLL+LLLLLLLLL--+
|
---
Gnd

The cent er of the inductance L is grounded to get ground reference
for the tuning voltage Vt. With a small series capacitance and a large
varactor capacitance, most of the RF voltage is over the fixed
capacitor. The anti-parallel structure should reduce the distortion.
You need to bring the tuning voltages Vt to the varactors through
separate resistors R (or through inductor resistor combinations).

Paul OH3LWR


Paul,
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)
I don't know the advantages of either config. as the RF cycle effects each
diode the same way "on opposite half cycles" in either.

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.

Steve K9DCI