I've been looking for information on varactor-tuned front end/preselectors
and really have not found much in the way of practical/DIY implementations.

I've seen the commercial offerings from (e.g.) www.polezero.com and vague
references to others.

Can any of you here recommend a text/app notes/practical circuits that can
be studied and/or emulated? I'm interested in VHF applications that could be
uP controlled (via D/A, etc.)

Thanks,

Bill

I think part of the reason for little information or enthusiam is the
diodes tendancy for IMD on strong signals. This increases the
complexity of the circuit. You might check with the ARRL and see if any
information is available from past QEX articles. I am researching the
same thing for a HF preselector. I am going to experiment with solid
state relays. Just a suggestion. Try the Vishay site
as(http://www.vishay.com/solid-state-relays/). I have found little info
on varactor tuned pre-selectors

wrote:

I think part of the reason for little information or enthusiam is the
diodes tendancy for IMD on strong signals. This increases the
complexity of the circuit. You might check with the ARRL and see if any
information is available from past QEX articles. I am researching the
same thing for a HF preselector. I am going to experiment with solid
state relays. Just a suggestion. Try the Vishay site
as(http://www.vishay.com/solid-state-relays/). I have found little info
on varactor tuned pre-selectors

Are those things specified for RF? I wouldn't touch them with a 10-foot
pole if they aren't. If you're planning on tuning by switching elements
in and out have you considered PIN diodes? If you have them either all
the way on or all the way off they should be less susceptable to IMD
than varactors. Furthermore, I understand that if you're cheap you can
use the old 1N400x series of power diodes at HF (I haven't tried this,
though).

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Hi Tim,

I use 1N4007's or 1N4004's as a drive control in 3 homebrew
transceivers but never for switching. I have used PIN diodes for the
same application I am considering the SSR for. I looked at the the
MosFET SSR's because I have usedIRF510's and IRF740B's to cathode key
my tube transmitters. I was expecting that a MosFet relay would likely
work in a similar manner to switch xtal converters for bandswitching. I
chose the IRF740B for keying my TX due to the VDS of 400 but most due
to low RDS 1 ohm. The low price SSR have avg of 22 ohms but with low
current that may not be a big minus. For a $1 to $3 investment each its
worth an experiment.

Some are specified for RF up to Ghz but most SSR's for sensing and
power control. And they are pricey.

I am really just looking for something different to switch convertors
for 20 and 15 M on homebrew tube rx. I didn't comment correctly above
in my reply. After I read it again he was looking for a way to tune a
band pass filter and i am trying to switch a complete circuit. But I
did look at the varactors too but that was when I considered premixing
for the LO and tuning the front end. Gotta admit I have been building
with tubes a long time.
I just didn't want to use plug in coils.

I looked at your web site and I know Worsester, MA. I was born in
Newton and my dad went to MIT.

Don

Dittos. Also most electrically tuned circuits suffer from lower Q
and strays. It is hard enough to build hi Q cavities let alone try
to emulate one. It might be more worth while to build a motorized
cavity or electrical band switching of helicals, but who has room for
all that? Many manufacturers have concentrated on making RF stages
that can withstand very high levels without intermod. Of course none
of this will help you if you are at a congested site with mixes in
transmitters without circulators.

At HF it is practical to build a bandswitched set of bandpass filters
much like the ones found in some radios. Toroids can make them small
enough, but again you need to be careful not to overload them.

wrote in message
oups.com...
I think part of the reason for little information or enthusiam is
the
diodes tendancy for IMD on strong signals. This increases the
complexity of the circuit. You might check with the ARRL and see if
any
information is available from past QEX articles. I am researching
the
same thing for a HF preselector. I am going to experiment with
solid
state relays. Just a suggestion. Try the Vishay site
as(http://www.vishay.com/solid-state-relays/). I have found little
info
on varactor tuned pre-selectors