Thursday, September 30, 2010

Discriminator out of MC3361


Now that the VHF FM receiver part is almost done, nothing like giving it a little "professional" touch...said that, on to make a "packet" dedicated output, also know as discriminator out...

Basically this output is a low pass filter from the recovered audio of the MC3361. The same signal goes to the speaker amplifier, so it's just a matter of filtering and preserving the signal in it's more pure form.
I add an amplifier because the sound card wasn't liking the very low output of the MC3361. The "filter" part is just the 5k6 resistor and the 4.7nF capacitor.


Before making the amplifier I tested for some time with the my PIC aprs decoder but not even one packet was decoded (it's working/well connected since I get the boot message and the blinking led...)

Then I proceeded to "multimon" software, now with the amplifier working, had limited success, about 20% of all packets were decoded (remember, I am using a peace of wire as antenna inside the shack). By this time the TM-D700 was decoding around 95% (I was beaten by a kitchen electronics manufacture).

Bellow, "multimom" decoding:


Also tested with "soundmodem" software but no success. I didn't proceeded with further testing since I installed and tested "multimon" and "soundmodem" today for the first time. Could be my mistake in the configurations.
Hope to get better results in the future with the outside antenna.
I also tweaked the IF frequency a little bit and the quad coil to try improving reception but not much success.

Incidentally I found a PIC aprs decoder project with an LCD that migh work a litle bit better since is based on the MX614 chip, unfortunately is very hard to find the MX614, so for now I will put on standby this project. Here's one of the links:

http://www.ke4nyv.com/peekpack.htm


Further reading:

* http://www.xastir.org/wiki/HowTo:SoundModem

not in documentation but port on "Xastir" should be configured at 4800 and probably you need to "chmod" a+rw /dev/pts/"whateveryours"

* http://www.baycom.org/~tom/ham/linux/multimon.html

Download link for "multimon" is at the bottom!

* http://www.discriminator.nl/

No comments: