QRP Rcvr Front Ends

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From: Bob Gobrick WA6ERB (70466.1405@CompuServe.COM)
Date: Tue May 17 1994 - 16:20:29 EDT


TO: >INTERNET:qrp@think.com

QRP Project Receiver Front Ends

I've built a number of QRP kits (both club and commercial) over
the last year or so and I'm now getting a little more refined (?)
in my choice of "new" projects. I've been happy with the
development work done my many QRPers on stable analog vfo's
(although the new S&S ARK-4 digital vfo has caught my eye),
decent transmitters, and decent relayless QSK circuits. The
receiver portion of the rigs though has me looking for something
more.

There's been an influx of great club kit designs (NN1G, NORCAL 40
& Sierra, NE-QRP 40-40) based on the inexpensive NE-602. Some
designs are better than others depending on the amount of front
end protection to prevent overload. Are there better designs?
We all know there are. But can they beat the NE-602 for
simplicity and cost? Although I have not seen the schematic for
the Oak Hills Research Spirit or Classic the specs say that the
front end has an RF amp and diode ring mixer (probably a
Minicircuits SBL-1 or newer TUF-1). Is this a better performing
front end and at what cost over the NE-602, especially on 20
meters were the front end noise figure and gain distribution need
to be considered.

I've compiled a little list of front end designs that I know
about and some that I don't know about. By the way I'd like to
just stop at this point and commend the designers (like NN1G and
N6KR) who verbally walk-through their circuits and the reviewers
who describe the circuit of the unit they are evaluating. I know
these descriptions help me understand the unit a lot better than
a bunch of slick photographs. I used to criticize the technical
review policy of CQ magazine when they spent pages describing the
circuitry of a complex rig to boredom, but now I really do miss a
circuit description especially in a simple QRP rig which I know
has to have some compromises. Anyway back to my list:

NE-602 or similar gilbert cell mixer front ends: NN1G Superhet,
ARCI - W1CFI surface mount SL6440 DC xcvr, NORCAL-40, NORCAL
Sierra, NE-QRP 40-40, MXM Simple RX/TX Superhet, A&A K9AY MC3362
xcvr, G-QRP Sudden Receiver, Neophyte Receiver, Ten-Tec kits

Diode Ring Mixer: Oak Hills Research Sprint DC Xcvr, OHR Spirit
and Classic Dual Bander - rf amp and mixer, OHR Explorer (?), S&S
ARK-40 - rf amp (?) and mixer, ARK-4 (?), Tejas RF Technology
Backpacker II DC xcvr.

Others: Index Labs QRP Plus (does anyone know the front end of
this rig and filtering (xtals plus SCAF?)

Has anyone compared the front performance of these rigs and tried
to evaluate a cost benefit in complexity of design and power
budget? I'd be curious to hear what design approach the Index
Labs QRP Plus took to get all that performance with such a small
power budget.

Another area of review is the gain distribution and audio stages
of simple QRP rigs. I noticed a good article by Stan AK0B in the
North West QRP newsletter on the use of a more expensive (how
much more ?) MC34119P audio chip over the standard noisy LM-380
and LM-386 chips. Anyway use this chip? How does it perform?

If you would like, keep this as an open forum for comments since
it may be good material for some of the QRP newsletters
represented here on the Internet QRP List.

Thanks 73, 72 Bob VO1DRB/WA6ERB


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