Simple DiSEqC monitor and signal analyzer

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DiSEqC™ is a protocol developed by Eutelsat. It is widely used in satellite receiving equipment to control devices over the same coaxial cable that carries RF signal. DiSEqC can be used to select a local oscillator of the LNB, to operate a LNB switch or to point a motorized dish.

But when a device communication fault arises, the source of the problem may be difficult to find. The coaxial cable that goes from receiver to accessories and finally to LNB carries multiple signals: the radio frequency signal from LNB to receiver, the power voltage from receiver to connected devices and LNB and the DiSEqC signal. Faults can be caused by receiver, cable, accessories (switch, positioner, motor) or LNB.

The simple computer adapter that will be described allows you to view and analyze DiSEqC waveform using the soundcard input and Audacity software. This is possible because the protocol is modulated over a 22 kHz carrier. The same carrier (when sent continuously, at a smaller amplitude) tells the LNB to switch on the high local oscillator.

Simple DiSEqC monitor and signal analyzer
DiSEqC protocol encodes bits as follows:
  • 1 ms of 22 kHz burst followed by 0.5 ms pause means bit 0;
  • 0.5 ms of 22 kHz burst followed by 1 ms pause means bit 1.
A byte is sent using 9 bits (first 8 make the actual byte and the last is the parity bit). If you want to know more, see protocol specification at Eutelsat web site.

The schematic of the adapter is very simple: just a low pass filter and a DC decoupling capacitor. According to the specification, the DiSEqC signal amplitude is 0.65 ± 0.25 V so it can be fed directly into the sound card.

Simple DiSEqC monitor and signal analyzer schematic
DiSEqC adapter schematic
It is highly recommended to build this in a metal case or on a PCB to ensure proper RF shielding and to avoid any accidental short-circuits. Remember that the coaxial cable carries 13-18 V DC depending on LNB polarity setting. Most receivers claim to be short circuit protected, yet make sure to avoid this kind of situation.

When you connect this adapter, turn off completely the receiver. Plug it into Line in, not into microphone input! Launch Audacity, select the proper audio input and click on Record button. Usually any sample rate higher than 44.1 kHz should produce good results. Below are some samples.

DiSEqC commands in Audacity
DiSEqC commands
DiSEqC commands in Audacity - zoom 1
Zoomed in command
Diseqc in audacity - bit level zoom
Zoomed in at bit level
Diseqc in audacity with 22 khz tone
DiSEqC commands and 22 kHz tone used to select LNB L.O. High
Diseqc frequency 22 khz
Spectrum analysis: see the 22.3 kHz peak
I used Audacity because it can record the signal. But basically you can use other soundcard signal visualization tools in a similar way to IR protocol analysis.

This is a cheap and simple way to troubleshoot DiSEqC communication issues. By inserting this adapter into the satellite receiving installation you will know whether the receiver actually sends DiSEqC commands.

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