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[SOLVED] Need help to transmit a PPM signal

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Jan 16, 2013
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Hi Friends,

I have built a PPM encoder and decoder circuit and I am planning to use it to control a home built model aircraft. Basically the PPM encoder generates a PPM signal that depends on the position of 4 POTs. So its basically a four channel PPM.

I have tested the circuit and its able to change the position of the four servos that are connected to the decoder in accordance to the position of the corresponding POTs shaft. The only thing I now need to add to this circuit is an RF transmitter and receiver.

I have read elsewhere on the web that the RF receiver's AGC will add noise to the incoming PPM frame during the time when the PPM signal goes low or high for longer than a few milliseconds. Since the RC PPM frame pauses for nearly 10 milliseconds between each frame, I am worried that I might encounter the said AGC problem? kindly help me out with the following two questions. Thank you :smile:

1) Can I use readily available off the shelf RF modules after inverting the PPM frame or are they unsuitable for this task?

2) Do you think inverting the PPM frame will help to keep the AGC from adding noise by keeping the RF carrier on for most of the time?

Keeping the carrier on and using the cut off of the carrier to mark the positions is good practice xD. If a micrel OOK RF demodulator is used the AGC time constant can be adjusted to avoid the gain ramping up too fast during off keying. By the description, it reads like the discussion is about AM modulation. FM modulation gives more noise immunity.
Just the kind of information I needed. Thanks a lot cntube.

I now understand that its possible to address the AGC issue by adjusting the AGC time constant.

I have one more question. Its about the data slicer. Will the 10 millisecond pause cause the data slicer in the receiver to loose track of the logic levels? Or is there no need for me to worry about the data slicer while using an OOK RF module? Please let me know. Thanks once again.

Could you suggest an FM module that I can use for this purpose.

During the 10 millisecond frame separator can the transmitter remain on? or do you want to transmit in burst?

If burst mode then before each PPM data frame a training/sync preamble would tell the radio front end and baseband logic that this is real data, not noise impulses and so.
I don't mind if the transmitter remains on during the 10 ms frame separator. Its just that the batteries will run out sooner. But I do like the sound of the burst mode you have mentioned here.

Could you tell me how the usual Radio Control Tx and Rx'es manage to send a PPM signal overcoming the AGC and Data Slicer issues during the PPM idle state? I have been checking all around the web and so far I have not been able to get this particular information anywhere.

Hobby Tx are on continuously and then off-keyed for short ~50uS of pulses, according to old ARRL Handbook chapter about PPM. The frame separator or blanking time is longer circa 50 ms and since any interference would add to the strength there would be no glitches, even though the 50microsecond off pulses could be interfered causing missing pulses, for which the PPM modulator should be designed to tolerate, i.e. designed to reset if idle interval longer than the "blanking time" comes even if pulses are missing. That way a good set of PPM data next cycle can be received.

FM systems do not have the carrier power drop out, the frequency just shifts a bit to signal a mark. SNR is better since there is still carrier power and a interference source is still many dB below the carrier at least is the near range of the transmitter.

The PPM rxeris still robust because of the PPM will be repeated and the blanking time reset will bring the PPM back in sync in the event of an upset.

FM transmitters use xtal frequency synths and the vco is pulled to modulate. Rxer are narrow band FM receivers, single conversion from RF, IF limiters, and quad demodulators to get baseband.

So if the transmitter is sending continuous control data, then burst mode seems overcomplicated. WiFi and other packet radios deal with the issues of channel training, sync prambles because of the scattered bursty data and channel sharing issues.
Thanks for the detailed explanation cntube. You are quite knowledgeable in this field and your responses were to the point. During this weekend I purchased a pair of OOK Tx/Rx'es and tested sending PPM over to the Rx. The signal was received without much distortion. I believe feeding the OOK Tx with the inverted PPM contributed to the successful reception of the signal. So I hooked up the servos to the PPM decoder and they responded as expected. Problem solved. Thanks again :-D
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