Message-ID: <41E58B5D.6070700@freenet.de>
Date: Wed, 12 Jan 2005 21:41:01 +0100
From: Wolf DL4YHF <dl4yhf@freenet.de>
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To: RSGB LF Group <rsgb_lf_group@blacksheep.org>
Subject: LF: Simple image-cancelling exciter for digimodes
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Hello Alberto, Eberhard, John, and all,

I made some tests with an image-cancelling mixer for the 136 kHz lately, 
using the "SSB phasing method" as described in the ARRL handbook. The 
only tricky part, the 90° audio phase shifter,  is realized via software.

Only standard parts are required: A 4.433619 MHz-crystal, a 74HC161, a 
74HC107, a 74HC4066 (or something "better"), a TL084, and a few passive 
components. No inductors, no transformers !

For CW transmission without the need for a PC, you will need an audio 
frequency generator with I/Q output .

A short description and some ugly hand-drawn schematics of my first 
prototype can be found here  :

http://people.freenet.de/dl4yhf/phasing_lf_exciter/index.html

The rejection of the carrier (here: 138.5 kHz) and unwanted sideband 
when tested with Jason (beta) and Spectrum Lab was surprisingly good:
"Wanted" signal: S9+20 dB on a TS 850, all "unwanted" signals within +/- 
100 kHz: no S-meter reading .
 (ok, an S-meter is not a measuring tool.. but good enough for this 
inital test )
Sounds adequate for a "spare" transmitter for your next DXpedition, 
using digital and "slow" modes ;-)

At the moment, there are at least two programs available which can 
generate audio signals with I/Q output using a "stereo" soundcard. More 
on that, hopefully, from the software author(s) soon !

Any suggestions and comments are welcome.

Cheers,
Wolf DL4YHF .