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How can I improve the decoding of signals with my Keysight 8920?

Here are some ideas on why tones are sometimes lost in a given decoded sequence:

1) Trigger level: tone sequences are often sent over the air with relatively low deviations. This requires careful setting of the trigger level parameter of the DECODER screen. Even though you may have the trigger level set such that the start of the tone sequence triggers the decoder, it must be set such that it also recognizes each of the following tones (in other words, it can be set high enough that it will trigger initially, but not decode all of the tones). If the radio pre-emphasizes the tones, they will come through at different "levels." You can verify this easily using the scope. Compensate by adjusting the trigger level or using the de-emphasis function.

2) Audio filtering: because of low tone deviation, you may end up with a fairly low input for your trigger level. If the noise level at the input of the decoder is high enough, you will get false triggering. Changing the audio bandwidth using the 300 Hz HPF and 3 kHz LPF often cures this (the default bandwidth is 50 Hz to 15 kHz).

3) Gate time: if you're losing the last few tones of a sequence, it's often because the gate time is too short. A good approximation for an entry for gate time is: [number of tones x (on + off duration for a single tone)]+20% The default (490 ms) for ZVEI formats should easily cover a sequence of 6 tones.

These are just some ideas; there certainly could be many other causes for not getting all tones in a sequence. Make sure to use the troubleshooting tools (such as the scope) to help isolate these causes. Also, because there are so many steps in setting up the decoder, TX TEST, RX TEST, and other screens for signal decoding the SAVE register function can save you a lot of time the next chance you have use decoding.

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