I can comment on the filter change.............the 10.7MHz ceramic filters
that are being used right now have an IN/OUT Z of 330 ohms.................a
crystal filter will have an IN/OUT Z of 3000 to 6000 ohms. It will work, but
you will need to match the impedances. If the crystal filter is not properly
matched, you will experience a deep nose in the response in the center of
the passband, and your insertion loss could rise sharply. If you are
interested, e-mail me directly and I can send you an Excel spreadsheet that
will calculate the values needed for a series L, shunt C type of matching
network.

Pete

"Netgeek" wrote in message
...
As a follow-up - I bought and assembled the really simple kit sold
by Ramsey - and described in detail he

www.sentex.ca/~mec1995/aviarx/avrxkitc.html

I wasn't expecting much - but was really surprised! It works really well
and has provided a platform that I can hack to death without feeling
bad about destroying it if it comes to that...

I'm about 8 miles from Washington National (DCA) with alot of "stuff"
in between. With a lousy 21" wire, I have no problems picking up both
Potomac Departure and Potomac Approach (with the first I heard being
Air Force One coming in to Andrews AFB).

The only modifications I've made are to increase the supply to 12V (from
9V - per Jim Weir's comments) and to replace the worthless 10k varactor
tuning pot with a 3-turn widget salvaged from some old Tektronix gear.

The thing works great!

Now I'm looking for suggestions on how to "improve" it. My list includes:

Replace NE 602 with SA612A mixer and add "real" tuning via National PLL
and Minicircuits VCO.

Replace 10.7 Mhz. ceramic filter with 4-pole Xtal-based filter.

Replace MC1350 with MC1349 (per Len Anderson) and rework AGC.

Add serious LP audio filter to output of AM detector diode.

Throw away or rework "squelch" circuit - as it is makes lots of noise and
is annoying (loud pops when switching in and out)...

Get a decent (e.g. dipole 1/2 wave antenna).

It seems to me that after attempting to improve the RF section the biggest
gains are to be had in tweaking the audio stages - so that's what I'm
focusing on. Just for fun - alot of filtering, noise gate, compression,
limiting, etc. seem to be areas where I can really play with the thing and
improve the performance. I've been looking at both Analog Devices
and THAT components as a step short of going to DSP. I think some
real improvements can be made. The basic signal is there (certainly at
levels which are "good enough") even with such a simple receiver. I'd
like to see what can actually be done (keeping in mind that this is a
"learning exercise"..)....

Any suggestions appreciated! (And, yes, there are plans to tweak the
IF per previous suggestions!)...

Thanks,
Bill

wrote in message
oups.com...
From: "Pete KE9OA" on Fri, Mar 18 2005 6:36 am

I would go directly from 45MHz to 455kHz..............this is typical
in HF
receivers. An NE602 mixer can be used for the 2nd mixer and a
44.545MHz
crystal used with the on-board oscillator allows you to derive your
2nd I.F.
Alternatively, you can use a TDA1572 as the 2nd mixer/I.F./
demodulator.
This is also a good system and it will give you fairly good strong
signal
handling capability.

Pete

"Netgeek" wrote in message
...
Parts are on the way...

First IF = 45 MHz
Second IF = 10.7 MHz

Any point in going further to 455 KHz for a third IF or just stick
with the 10.7 - (MC1350 plus IF transformer) scheme?

Bill

As a suggestion (too late if parts are incoming), a
single conversion to a 21.4 MHz IF is quite suitable.
Using a monolithic quartz crystal filter (available
from DigiKey for about $15 a pair of two) between
the mixer and first IF could eliminate ALL tuned
circuits in the IF following the 1st IF amplifier.
Those are available in 12.5 KHz bandwidth which will
be fine for a VOR signal. [DigiKey has a link to
download specs for the ECS-made monolithics]

This is now common in mobile radios, both new and
retrofit of older ones.

If a single-conversion scheme with 21.4 MHz is done,
the LO can be 86.6 to 96.6 MHz with an image at
65.2 to 75.2 MHz. That is a low enough frequency
to allow a simple L-C "top coupled resonator" fixed
bandpass filter for the front end at 108 to 118 MHz
(8.9 % bandwidth).

Doing double conversion with a first IF of 45 MHz
is, by itself, no problem. However the 2nd IF image
is a bit too close to the nominal bandwidth of any
45 MHz 1st IF tuning (it's only 0.91 MHz away). With
the second's image (on either side depending on 2nd
LO above or below 45 MHz), there's still a chance to
pick up part of the FM BC band locally. To avoid
that, the 2nd LO should be on the high side of 45.
Second IF image would then fall into the low end of
the 118 to 137 MHz comm band (also AM) and those
transmitters are much lower powered ones than BC
stations.

With a 10.7 MHz 2nd IF, its image would be 21.4 MHz
away and rather easy to attenuate in the 45 MHz 1st
IF. There's only a slight problem using stock
10.7 MHz IF components: Bandwidth of the whole 2nd
IF might be around 160 KHz; less discrimination to
nearby VORs and Localizers. A study of FAA sectional
charts might be called for to check on potential
interfering stations although those are assigned in
regards geographic locations to minimize normal
interference.