+ve and -ve shift PPM selectable RX

[charles0198]

Does anyone made a home-made RX that can select the PPM shift? If so does it work fine with both JR and Futaba TXs?

My point for asking this question is, does the JR and Futaba have the same exact 455KHz IF frequency? I am sure about Futaba RXs that they have 455KHz but what about JR?

[slipstick]

All current RC receivers I've seen use 455KHz IF. Airtronics/Sanwa used to use 460KHz for some reason but that was years ago.

Don't know about shift select because it isn't needed on any frequency band I use.

Steve

[Bruce Abbott]

Shift select and IF frequency are unrelated. JR receivers use 455kHz, but in any case it doesn't matter what the IF frequency is, so long as a compatible receiver cystal is installed.

Shift can be changed by adding an invertor to the demodulated PPM signal stream, swapping inputs on the data slicer comparator, or with software inside a DSP based decoder.

[Acetronics]

;) Hi, Bruce

I Begin to think Charles is looking for the universal receiver ...w/auto detection of the modulation side ...

Good luck to him ...

Alain

[charles0198]

Alain

Yes I have made a synthesized RX with a mechanical shift select switch that can select between the shifts by an inverter. The problem that I am facing is that the RX is not working correctly with both JR and Futaba TXs.
If it is tuned to work with Futaba, it works ok but with JR TX and shift changed there is slight cross mixing between adjacent channels!

[vintage1]

There is one way to make an autoselection receiver.

Use an IF of zero.

Both +ve and -Ve shifts will show up as equvalent audio frequency tones.

Use TX xtal in the receiver, and post the mixer, use an audio amp with a low pass filter in it, and an audio detector.

[charles0198]

vintage1

This seems interesting... would you please give some more info about IF of zero!

[chaitanya]

Actually the shift problem has nothing to do with what IF you use, the crystal type, etc... it depends on how you convert the raw pulses comming from the receiver IC to square/rectangular shaped pulses. When using a schmitt-trigger there is no problem with the positive shift pulses but with negative shift pulses the sync pulse goes high around the center instead of being low throught the sync duration atleast this is what i observed.
my 2 penny.
-chaitanya

[vintage1]

vintage1

This seems interesting... would you please give some more info about IF of zero!

Yeah - well it happend in the DIY FM narrowband reciever thread, and someone asked if anyione did 'direct conversion' and I at first thought 'what a silly idea' because I remember making a broadcast FM tuner once with 100khz IF, and thinking it wasn't working when I could pick up the same BBC station in 6 different places...later I realised it was working really well, as the station was on three transmitters all in range, and I got each one as itself and an image...anyway..on reflection, I thought he might be on to something. If you mixed down to a very low frequency - say a couple of hundred hertz or less, and used a bandpass auidio amplifier on that when you kicked the transitter frequency sideways, you would get a short burst of tone - probably a couple of cycles only - at maybe 2-3khz.

Image frequencies no problem, because they would be at +- a few hundred hertz only - in youir own band. The audio output itself would be highly suitable for pumping into a DSP, and all in all it might allow you a lot more sensitivity, since one of the problems with high gain sets is RF/IF instability, and that's easier to manage at audio frequiencies.

You would use a transmitter crystal in the local oscillator, and an RC active filter instead of the ceramics.

It COULD be extremely lightweight - those filters and coils add weight - and maybe actually quite accurate. You might need to slighty tune it for each transmitter though.

I sadly lack the test gear to make one. But I think it has some promise. You could probably adapt a scrap existing receiver with TX Xtal and couple the IF input to an audio amp and a scope to at least see what happens.

The best way is to use a phase locked loop and pull the crystal with that, so your local osciallator exactly tracks the transmitter, but that is a bit more complex.