In article , "W3JDR"
writes:

Steve,
I think the main reason for back-to-back diodes is to prevent rectification
of the RF. Rectification can cause several bad things, including pushing DC
current back out the tuning voltage line, instability, and increased phase
noise. Having said that, I tried back-to-back diodes a couple times and I
don't ever recall ending up with it in the final design, so it must not have
added all that much value. On the negative side, it halves the capacitance.
One technique to improve the phase noise in wide-band VCO's was shown by
Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF
current in each diode, reportedly lowering losses and improving noise. I
never tried it myself.

Variable-capacitance diodes exhibit that phenomenon only in
reverse-DC-bias connections. In forward-DC-bias connection they
behave generally like ordinary diodes, conducting very little current
until about 0.6 V across the junction.

A single variable-capacitance diode across an RF circuit will conduct
- and thus have an effect on the RF tuned circuit - when the
combination of DC tuning bias and RF voltages are above the
forward-conduction breakpoint.

The purpose of "back-to-back" connection is to keep the (now two)
variable-capacitance diodes always in reverse-conduction...the RF
voltage (peak-to-peak) is not supposed to exceed either the break-
down voltage of the diodes or cause either of them to be forward-
biased during any part of the RF cycle.

With no forward conduction, the variable capacitance diodes remain
just that - variable capacitances.

When forward conduction occurs, it adds more non-linearity to the
RF circuit and tends to decrease the action of the variable capacitance.

When used in low-level RF stages of a receiver input, the RF voltage
hardly ever exceeds 1 V peak-to-peak and thus the variable
capacitance diode never goes into forward conduction. Single diodes
can be used there without doubling-up.

Len Anderson
retired (from regular hours) electronic engineer person