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 .
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