PPM signal measurement

Discussion in 'Wireless & RF Design' started by Terp, Apr 5, 2009.

  1. Terp

    Thread Starter Active Member

    Jun 6, 2008
    32
    1
    Hello!

    I wanted to learn how to measure the PPM signal coming out of an RC transmitter, on the oscilloscope. I could not really find anything on that through google searches. Any suggestion on is greatly appreciated.

    Thanks in advance
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Hook the oscilloscope probe up to the transmitter output.
     
  3. Terp

    Thread Starter Active Member

    Jun 6, 2008
    32
    1
    Hello everyone:

    I am currently working on an RC robotics project. I want to measure on the oscilloscope the PPM signal that comes out of my Airtronics RC transmitter when it is turned on. I haven't tried it yet (I'll get access to my lab after about 2 days). I would like to learn of some insightful suggestions on measurement techniques.

    Any help is greatly appreciated.

    Thanks in advance!
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    I am not sure what you mean by "measure." If you basically just want to see what's going on, try to find an old receiver--one with parts big enough to attach a probe to. I had an old FMA Fortress receiver. There are two chips of interest. The HEF4015B that is near the pin outs. The IF mixer is the MC3371D. It is near the IF crystal. Pin 9 of the MC3371 will show you the complete cluster of channels. Pin 1 of the HEF4015 will show you the cleaned up packet, i.e., all the channels at once. You can watch the peaks move and change as you wiggle the sticks on a transmitter. You can look at the individual channels at the 4015's pins too.

    Presumably, you can find similar test points in the transmitter, but I have never looked for them. If I were to do that, I would look at something going to the RF module. If you can find a Multiplex EVO transmitter, it can show you basically the same thing on its LCD screen.

    Not all receivers will have those exact chips, but the older receivers all probably have something similar. It's great fun, but not a lot of practical use.

    John

    Edit: This reply was written to an earlier, similar post that wanted to "measure" the PPM.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I have combined the duplicates into a single thread. Please refrain from posting your question more than once.
     
  6. DickCappels

    Moderator

    Aug 21, 2008
    2,664
    634
    Didn't this question come up a few days ago?

    Here is a demodulator probe you can build for your scope:

    [​IMG]

    Use this with a 10X scope probe.

    If the signal is low enough, you can connect the antenna and ground termials to your transmitter's output through a small capacitor. Or just put the antenna from the probe near your transmitter.

    If you don't have a choke, use a 10k resistor. If you pick up too much noise, change the choke into an LC circuit resonant at the transmitter's carrier frequency.
     
  7. Terp

    Thread Starter Active Member

    Jun 6, 2008
    32
    1
    Hello!

    Thanks everyone for your valuable inputs.

    Jpanhalt (and all):
    I have an Airtronics receiver that pairs with the transmitter I am working with. If I turn both the transmitter and the receiver on, and move the sticks on the transmitter, then would connecting the o'scope probes to various channels of the receiver show me the signal transmitted at the respective channel?

    Sorry about posting the same question in two different forums. I just intended to make sure that the question was seen by individuals in different forums.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    Yes. Just attach the probe to the signal pin (white wire) and the ground to ground pin. Sync on the probe channel you are using. You will see the servo pulse get wider or narrower as you move its stick.

    If you want to see all the channels at once, you will need to open the case. There is likely a chip similar to the one I mentioned above that you may be able to test. I have several Airtronics receivers. Which one do you have? If I have it, I can take a look for you.

    John
     
  9. Terp

    Thread Starter Active Member

    Jun 6, 2008
    32
    1
    Thanks, John. Yeah, I'll give it a go tomorrow at school. I am trying to find out if the signal coming out of the transmitter is a PPM, and make measurements of the amplitude and such. I realize that "measure" was used sort of loosely in my first post. Sorry about that. The receiver that I am using is this 2.4 GHz 8-channel 92824
    (http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXTZU8&P=V). I do appreciate you offering to take a look, given that you have this receiver.
     
    Last edited: Apr 6, 2009
  10. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    The signal at the servo pin will not reflect anything about the transmitter modulation. Sorry, I don't know anything about the 2.4 GHz stuff. I suspect the insides of the receivers are a bit more complicated and smaller than for the older 72 MHz receivers.

    John
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    2.4Ghz is another ballgame. There are a large variety of transmitters and receivers in this range.

    The best are Frequency Hopping Spread Spectrum, where the radio actively seeks the available channels with the lowest interference and uses them. The other major type is direct sequence spread spectrum, where it simply hops across predetermined "jumps", so the receiver can follow along easily. There are hybrids of these as well, due to complexity of a pure FHSS solution.

    All 2.4Ghz radios are PCM with 9 to 11 bit per channel resolution, plus overhead bits. Futaba alone has 6 different protocols for their 2.4Ghz receivers, some with PCM output, some with PPM output.

    Most companies made the transfer to 2.4Ghz SS After the cordless phone market did much R&D and got the price down for encrypted digital frequency hopping.
     
  12. cambridgekiwi

    New Member

    Mar 25, 2010
    1
    0
    Not sure how you are getting on with this, but I stumbled on it while doing a google search. If you are looking for PPM signals inside a transmitter then in most cases you will be in luck - as the signals from the transmitter internals only get converted at the output stage of the circuit. Connect one end of your probe to the earth (Negative) terminal of the circuit and start testing backwards around the circuit from around where the crystal is - in some cases on an AM transmitter the PPM signal can actually be picked up on the input side of the crystal - though I don't recommend this for an FM transmitter. Do a google search on trainer port diagrams as well as you may well find there is an easier way to do a 2.4Ghz DIY module installation if you can pick up a wire from the trainer port! I'm still fairly new to this myself too and picked up a cheap scope from the local electronics store to do conversions for others. It isn't hard to do, just don't get it all finished when you are tired and then connect the transmitter battery up backwards :eek: because once the magic smoke gets out it becomes very expensive to put it back in - IF it is possible!
     
Loading...