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I'm looking for a modem that can hook up to the input port on a portable ham radio and an ardunio board serial connection. Not really interested in APRS, it's too high level for my needs (long distance remote control).

  • Just need to be able to send 10 characters maybe 3 times a second (>1200 baud)
  • low power if possible
  • distance of like 40-50m max

Seems like that will all be taken care of by the radio, just wondering if there is a good hardware modem for this kinda of thing.

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  • \$\begingroup\$ What kind of data rate and bandwidth do you need? Last I recall, there were different legalities on HF (<30 MHz) than above, so what is your target? What is your relative priority for size, power consumption, cost? \$\endgroup\$ – Chris Stratton Aug 5 '11 at 20:14
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As you probably know, often one has a choice of implementing a function is either hardware or software. I understand that this is a hardware rather than a software forum, but you may want to consider the fact that this can be done completely in software.

Here is full documentation on the project: https://sites.google.com/site/ki4mcw/Home/arduino-tnc

In response to the comment I will add some background. Generally modems, either telephone or radio, use tones of various frequencies to transmit data. On the transmitting end one needs to generate tones, and the receiver needs to detect them. Timing and accurate frequency are important to successful operation.

To generate tones of a particular frequency, one can use purely analog circuits employing precision inductors and capacitors. Think of touch-tone telephones from the 1960s. Alternatively one can use a digital to analog converter (DAC) and a micro-controller to generate accurate tones, generally at far less cost.

On the receiving end one can also use a purely analog tone detector, once again with precision inductors and capacitors, to detect tones of a preset frequency. A key drawback of this is that a separate tuned circuit is required for each tone frequency to be detected. The digital alternative is to use a analog to digital converter (ADC) to convert the incoming signal to digital format and pass it to a micro-controller. In the processor one generally runs a fast fourier transform (FFT) to detect the tone frequencies contained in the signal.

A further refinement is that many micro-controllers designed for digital signal processing will have on-board ADCs and DACs.

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  • \$\begingroup\$ Could you give us here a brief summary of the setup? Links are appreciated, but they should only be used for information adding to what is explained here. \$\endgroup\$ – stevenvh Aug 6 '11 at 17:57
  • \$\begingroup\$ This is a forum about electronics design - some software is quite acceptable and necessary to do stuff like this, as you've noted. Don't be afraid to post some code! \$\endgroup\$ – Kevin Vermeer Oct 8 '11 at 1:13
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Microchip has this free dsPIC soft modem library. It includes V.21/Bell 103, V.22, V.22bis, V.23, V.32, V.32bis modems.

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Frequency shift keying is a pretty simple means of digital signaling that can work with either a single sideband or FM transceiver. You can implement it with a few ops amps or at low data rates (75 baud?) possibly even in software... maybe or maybe not on an atmega, but certainly on other small chips. An advantage over on/off keying is that both symbols are positively detected - you can basically say "which is louder, the high frequency or the low frequency" More primitively after a limiter you can put both on the sloping skirt of a filter to get some frequency-driven change in voltage and then threshold that voltage, ie, use an FM slope detector.

DTMF is commonly used to control VHF/UHF FM repeaters, and single-chip detectors and generators are available. This might work on HF SSB under good conditions (and is presumably used on 10m FM) but I don't know if it's allowed on HF in general.

Sub-audible tones have also been long used to open squelch on VHF FM land mobile and amateur service.

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