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[aprssig] New program - HF APRS SDR

Chris Moulding chrism at crosscountrywireless.net
Tue Jun 15 19:41:45 UTC 2010

I've just released a freeware SDR program specifically designed for HF 
APRS use.

It has tight DSP filters for 300 bd HF AX.25 and APRS over PSK-63. On 
start it uses a 900 Hz filter that covers the frequencies used by both 
HF APRS modes.
There are five filter settings with optimum filter bandwidths for APRS 
modes and
KAM and PK-232 TNC audio tone sets.

How to use it...

You need a SDR receiver with a crystal oscillator or synthesizer in the band
10.12770 to 10.147600 MHz. The 30m SDR receiver we make has a crystal
oscillator around 10.140000 MHz.

The program works on Windows 2000 and XP. Vista uses a different set of 
drivers so it won't work on Vista.

The program uses DirectX so a dxwebsetup program is included in the zip 
file to
download or update DirectX from the Microsoft website if it isn't already on
your PC.

On program start select the sound card. If you have two of them select 
your best
one for the SDR program leaving your default card for APRS Messenger. 
the output of the sound card used for HF APRS SDR to the input of the 
sound card
used for APRS Messenger. Adjust sound card levels so that APRS Messenger 
over driven. The audio output can also be used to feed a 300bd AX.25 TNC 

Enter the SDR receiver crystal or synthesizer frequency in Hz. The 
program sets
the near zero IF frequency so that the receiver and program is tuned to 
MHz. The audio frequency outputs are therefore 1600 / 1800 Hz for 300bd 
and 2100 Hz for APSK63.

The settings are saved on program close so that it can be used on Igate 
PC's that
are set up for automatic unattended re-boot after power outages.

Does it work...

Yes. The receive filters are the big advantage of SDR. I've been using 
this SDR program
for some time now on my HF Igate and it removes all the PACTOR, PSK125 
and over the
horizon radar near to the 300bd AX.25 and APSK63 frequencies. The AGC 
isn't hammered by large adjacent
signals any more and I can leave the SDR receiver running 24/7 without 
using a lot
of power running a transceiver. It's a nice way to run a HF APRS Igate.

Why use this when I can use PowerSDR etc...

You can but this program uses far less computer resources as it isn't 
running a
FFT display and waterfall. You can still see the traffic on the APRS 
PSK window.


The program can be downloaded from 


Chris, G4HYG

Chris Moulding
Cross Country Wireless
7 Thirlmere Grove, Bolton, Lancs, BL4 0QB, UK
Tel/fax: +44(0)1204 410626
Mobile:  +44(0)7752 391908
Website: http://www.crosscountrywireless.net

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