Add-On BFO Project

From: Mudplant 
Date: 8 Feb 1997 01:15:46 GMT
Newsgroups: rec.radio.shortwave

Carl  wrote:

> >Does anyone know how I can build a simple BFO to externally couple with
> >an AM SW radio to resolve SSB.

I found what you are looking for on the WWW one day. I haven't built it
but I plan to - Michael

************************************************************************
Subject: Project 10: A BFO for your $40 SW rcvr

It seems there are quite a few cheapie shortwave receivers out there that
have everything (synthesizer tuning, digital readout, fairly good specs)
but lack the all important BFO to copy SSB and CW. Well, let's remedy that
problem right now. The heart of this project is a 455 kHz IF transformer:
go to your closest junked broadcast radio receiver and steal one from
there.

If you are reading this on rec.radio.shortwave you may skip this paragraph;
if you are reading this on rec.radio.amateur.homebrew or on the QRP mailgroup
here's a neat idea: get one of these cheapie SW rcvrs and after installing
this BFO you'll have a great companion to any of the simple transmitters
I've been posting! Let's face it, xmtrs are easy to build, but rcvrs
certainly require more care; for $40 you can pick up one of these
synthesized digital readout pups and have a complete backpack
transmitter-receiver pair.


PARTS LIST

Q1    2N544 transistor
C1    15 pfd variable capacitor
C2    .001 mfd
C3,4  .01 mfd
R1    7.5K resistor
R2    5.1K
B1    1.5 VDC battery
T1    Miller #9-C2 IF transformer
SW1   SPST switch


-------------------------------------                            ANT
|                                   |                             |
|                                   |                             |
R1                                  |                             |
|                                   |                             |
|                                   |                             |
*------*------B-Q1-C-----*----------)-------------*------C2--------
|      |        E        |          |             |
|      |        |        |          |             |
R2     |        |        C1         |             *-----
|      |       GND       |          |             |    )
|      |                 |          |             |    )
GND    |                GND         |             C    )  -------O
       |                            |             |    )  (
       |                            *-------------)----)  (     N.C.
       |                            |             |    )  (
       |                            |             |    )  -------O
       |                            |             |    )
       --------------C3-------------)-------------*-----
                                    |
                                    |
                                    *-------SW1-----B1+----GND
                                    |
                                   C4
                                    |
                                   GND

Recall: Q1 is marked B=base, C=collector, E=emitter; the case of Q1
MUST be grounded; GND=ground; note the polarity of B1.
Now, the unmarked capacitor, C, and its associated transformer, are
inside the can of the IF transformer; if you remove the can you'll
see where to make the connections. N.C. means not connected - while
you have the can off you'll have to cut those leads. Also, screw the
tuning screw slug all the way out. After you've put everything together
set C1 about midway. Adjust the transformer slug until it resonates
at 455 kHz, which can be accomplished using a receiver or GDO. AS
you tune the slug (using the receiver) you should hear the heterodyne note
of the BFO lowering in pitch until it eventually zero-beats. Stop tuning
at this point. The BFO antenna can be any piece of hookup wire wrapped
around the antenna wire near (or even inside of) the receiver. Whenever
you want to copy CW or SSB, merely flick on SW1.

Jeff NH6IL

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