[aprssig] CNSP-18 now 30m HF APRS transmitter
chrism at crosscountrywireless.net
Wed Dec 5 02:23:14 CST 2012
About 4 years ago I made a 10W transmitter for 30m with AX.25 frequency
shift keying of a crystal oscillator.
It worked really well.
I'd forgotten about it until I spotted your posts. I'll take look at it
again. It would be relatively easy to add it to an APRS TNC Digi Tracker
to make a self-contained HF APRS tracker with it's own GPS receiver.
Cross Country Wireless
7 Thirlmere Grove, Bolton, Lancs, BL4 0QB, UK
Tel/fax: +44(0)1204 410626
Mobile: +44(0)7752 391908
Yahoo group: http://uk.groups.yahoo.com/group/cross_country_wireless/
On 05/12/2012 06:21, Stephen H. Smith wrote:
> On 12/5/2012 12:12 AM, Bob Poortinga wrote:
>> Lynn KJ4ERJ writes:
>>> If anyone knows of a cheap SSB HF 30m transmitter, or better yet
>>> transceiver, I'm interested!
>> I wonder if a Norcal 40A can be adapted to 30M.
>> 73 de
> I've just been carrying on an off-list discussion on this very issue.
> After playing around with the various APRS Messenger digimodes on 30
> meters, I think there is a way to do this easily. You don't need a
> full-blown 30M SSB transmitter with a separate TNC converting data to
> audio tones.
> Classic AX.25 packet, GMSK and MFSK16 are all single-tone-at-a-time FSK
> modes. Two throughts I had were to either have a DSP chip directly
> generate the RF signal at 10 MHz, given that DSPs can operate on signals
> at 50 MHz and beyond.
> These three modes are constant-power FSK (i.e. like key-down CW) between
> two RF frequencies 200 Hz apart in the case of FSK, several tones in the
> case of GMSK, or between 16 discrete RF freqs at only a 2-3 Hz rate over
> a spread of about 500 Hz in the case of MFSK16.
> Since they are not a complex two-tone simultaneous signal like PSK63:
> a) you aren't concerned with the shape of the waveform, just it's
> frequency. It shouldn't be much of a challenge for a DSP to directly
> synthesize a simple sine wave (or reasonable approximation thereof) at
> 10 Mhz.
> b) with a single tone at a time involved, there is no issue with
> amplifier linearity or intermodulation distortion. You stick a couple of
> MOSFET power amp stages directly onto the output pin of the DSP to jack
> the output up to half a watt or a watt or so. Saturate them and drive
> them into highly-efficient class-C operation.
> After my experiences on multiple cross-country road trips running both
> classic AX.25 300-baud FSK and the APRS Messenger PSK63-GMSK-MFSK16
> modes, I would vote for MFSK16. Any of the digimodes is vastly
> superior to classic 300-baud FSK for weak signal work, especially in the
> presence of multipath and noise, thanks to their low symbol rates, but
> MFSK16 is waaay beyond any of the others in the signal-to-noise ratio
> required to work successfully. It will provide solid copy on signals so
> weak, you can't even hear them in the speaker.
> MKSK16 has about a 10 dB advantage over PSK63 in t;erms of the minimum
> SNR required. You can cut the transmit power in half (losing 3 dB on
> TX) and still be 5-6 dB ahead at the RX end. And the TX can use a
> saturated efficient class-C PA which PSK63 can't!
> An alternative approsch we discussed would be to hang a varactor diode
> across a simple crystal oscillator running directly at 10.149.70 MHz.
> I.e the basic crystal-controlled "rockhound"-type 1-watt QRP CW
> A single-chip low-end microcontroller could easily output a 0/5 volt DC
> logic-level waveform to apply across the varactor (rather than audio
> tones) to directly FSK the crystal on classic AX.25. Or an output
> voltage at 16 stepped values could be generated by the on-chip a-to-d to
> modulate the varactor for MFSK16 operation. One could even use a table
> to generate 16 UNevenly spaced voltages to compensate for the
> non-linearity of voltage vs freq in the varactor/crystal system. Such
> a device would have the cheap, essentially-disposable, minimalism of a
> TinyTrack III.
> The microcontroller and QRP crystal transmitter could be combined on one
> small board to yield the 30-meter equivalent of a MicroTrack AIO.
> Stephen H. Smith wa8lmf (at) aol.com
> Skype: WA8LMF
> Home Page: http://wa8lmf.net
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