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Thanks for the good advice from both you and Dave Platt. I'll try
lowering the volume and see if that has any effect. I'm going to bet that this will probably help matters a lot. I took a look inside the radio and it's all discrete components. The audio even sounds pretty good (to my ears). And I can transmit packets from my Yaesu VX-2R to the Icoms from within my apartment with good decoding. If you can gain short-term access to an oscilloscope, it'd be beneficial to simply watch the waveform coming out of the audio-out jack. Tune to a busy packet frequency (144.390 will usually get you boatloads of APRS packets), slowly turn up the volume control, and find the point at which the audio waveform stops increasing in amplitude and begins exhibiting a flattening-off of the tops and bottoms. At this point, your signal is clipping - you definitely want the volume control turned down below this point. Further reductions may be beneficial if you're accidentally overdriving the TNC's audio input circuitry. Can I build a simple filter that removes everything except 1200/2200Hz tones? It's possible, but it's not necessarily trivial. It can be done fairly easily with active electronics (i.e. one op amp and a dozen or so passive parts). Take a look at www.tnc-x.com and pull up the schematic - the receive-audio filter is implemented by U4 and the parts around it. It can also be done with a strictly passive circuit, requiring no power, but that's a bit more involved, and will probably require some fairly large audio inductors. What you'll want, in either case, is a bandpass filter which passes roughly 1000 - 2500 Hz, and rolls off both the low and high frequencies on either side of that passband. A well-designed TNC should already have this sort of bandpass filtering built into its input circuit, I think - adding an outboard filter between radio and TNC ought not to be necessary. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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