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signal tap

Old 03-31-2004, 04:04 PM
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buggo
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Default signal tap

Hello,

Does anyone know the best way to tap in to the reciever signal in order to determine when it becomes weak. This is needed to activate a home built fail safe device during signal failure.
Old 03-31-2004, 04:08 PM
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mr.rc-cam
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Default RE: signal tap

Which FM demod IC is used in your Rx?
Old 03-31-2004, 04:11 PM
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buggo
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Default RE: signal tap

no idea, i am using JR radio if thats any help?
Old 03-31-2004, 04:19 PM
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Mac_Man_UK
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Default RE: signal tap

If you are using JR receivers, there is a monitor point at the battery connector pins. The top pin ( orange ) is either the DSC in or Monitor out.


[8D]
Old 03-31-2004, 04:28 PM
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XJet
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Default RE: signal tap

I assume you're talking about a PPM receiver.

If so, a possibly better solution would be to simply monitor the pulses coming from one of the decoder output pins and use a PIC processor to count the number of pulses which fall outside the legal range (roughly 1mS-2mS).

You should also check to make sure that the inter-pulse gap isn't too long or too short either.

By doing this, you can determine when the receiver is either out of range of the transmitter *OR* is receiving bad interference that is over-riding the transmitter signal. Your plan to monitor the signal level won't catch all forms of intereferece because they may present a very strong signal to the receiver so this alternative solution is going to be more versatile.

This checking of the decoder pulses is basically the way that some PPM receivers with failsafe implement that failsafe functionality.

A cool aspect is that you can then build your failsafe box in such a way that it sits between the receiver and the servo(s), thus requiring no modifications whatsoever to the receiver.

When a bad set of pulses is detected, the failsafe unit can then simply output the pre-programmed pulses associated with that failsafe setting.

I've almost considered building such a device for my own use -- but then again, it's cheaper, quicker and easier to buy an off-the-shelf PPM receiver with failsafe or PCM set so there's little point in reinventing the wheel unless you enjoy the challenge.
Old 03-31-2004, 04:51 PM
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buggo
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Default RE: signal tap

Yes, i am planning on using a PPM receiver, i need to know as part of a degree project and using an off the shelf product would be cheating.

Ill have to hook my receiver up to an oscilloscope to find the correct pulse timing, but it sounds pretty simple. I am using a microcontroller for the rest of the fail safe system so i should be able to use a spare input pin to handle the signal detection part.

Is there any documentation on the internet about how to do this?
Old 03-31-2004, 06:37 PM
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mr.rc-cam
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Default RE: signal tap

Looking at the PPM pulse will not tell you signal strength. The encoded signal is substantially conditioned at that point and you will only be able to determine when the signal is corrupt or missing. This is the equiv of detecting that you are low on fuel after you run out of gas.

If you want to know RF signal strength then that is another matter. Some FM Demod IC's have an RSSI output for this purpose. You will need to look in the Rx and post the part number to the FM Demod IC. If you do this then either I or someone else will be able to offer some help.
Old 03-31-2004, 06:39 PM
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Default RE: signal tap

the pulse-width is pretty much fixed at 1.5mS +- 400uS (although I'd tend to allow +-600uS to be safe.

The pulse/frame-rate varies somewhat, depending on the transmitter. Just measure the rate on your transmitter and see what it is -- but expect somewhere between 25 and 50 pulses per second.

Note that the pulse/frame rate may vary as the individual control pules are varied in length -- this (again) tends to be transmitter dependent but is something you may have to take into account.
Old 03-31-2004, 06:58 PM
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mr.rc-cam
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Default RE: signal tap

Ahh, I misunderstood your app. You're looking for a way to switch on the failsafe. Looking for valid PPM pulses is the best way to do that, so follow the other advice that was given.

Knowing RF signal strength is great for reporting back in a telemetry applications. But that is not what you are doing here.

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