Cheap, Simple RF demodulator

Discussion in 'Wireless & RF Design' started by loyodea, Oct 13, 2013.

  1. loyodea

    Thread Starter New Member

    Oct 13, 2013
    I am looking to broadcast a simple signal "yes/no" to a large area (1Km radius).

    "Yes" could be represented by presence of signal.

    The signal needs to travel on a non regulated band (ISM), transmit up to 1 watt.

    I looked into different Radio front ends, but I am interested for something simpler, no need to serialize bytes over the air, just carrier detect the frequency.

    What kind of circuit should I study, that can be easily incorporated into a toy?

    Cheap, Simple, Small are the main circuit design requirements (excluding the antenna), battery lifetime is important but not critical.

    Please send me pointers to start studying.
  2. MrChips


    Oct 2, 2009
    RF transmission of 1W power over 1Km would require a license.
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    The ISM band permits 1W input power to the antenna with EIRP 4W permissible.
  4. davebee

    Well-Known Member

    Oct 22, 2008
    Suppose you were to make a simple on/off transmitter and a simple signal-presence detecting receiver.

    Would it be a problem if the receiver picked up some RF energy from some completely different source and turned on or off outside of your transmitters control?
  5. WBahn


    Mar 31, 2012
    Leaving legalities completely aside, you have to consider a couple of practical points.

    If you use a simple carrier and detect the presence or absence of that carrier, you have what is often called OOK -- On-Off Keying -- which is an extreme form of ASK -- Amplitude Shift Keying. So those are some buzzwords for you to look up and read about.

    Your receiver will see always see a signal at that frequency even if the transmitter is not transmitting -- it's called noise and it can come from the devices in the receiver itself or from the myriad other sources that are producing EM fields from the power supply in someone's laptop to the cell phone signals to the sun to the quasar in the third galaxy on the left. So you can't just look for the "presence" of a signal at some particular frequency. Instead, you have to look for the amount of energy in a signal at that frequency being above some threshold. Of course, as soon as you do that then you have the possibility that the signal from the legitimate transmitter won't produce enough signal in the receiver to rise above the threshold.

    From a practical standpoint, picking the threshold is going to be one of your hardest problems, unless your signal is just so strong that even at the furthest distance of interest and in the worst location in terms of signal scatter and shadowing and fading that the receiver will still get so much energy that it will be clearly detectable above the noise.

    Here's a hint -- very unlikely that you will be that lucky, not with 1W in something that you can pack into a toy for a few dollars at a distance of a kilometer.

    And, as noted in a previous response, if someone's garage door opener transmits some signal that has enough energy at the frequency you are monitoring, they've just activated your system. Remember, the system you described has no way to discern that the signal it is responding to came from the intended transmitter or something else (either accidentally or intentionally). Is that acceptable?

    If not, then you start getting into modulating your signal in a particular way and having the receiver demodulate it. This allows you to enocode an identification. The "encoding" could be very simple or very complex -- you could just transmitting a signal that is on 33% of the time and off 67% of the time or as sophisticated as direct sequence spread spectum. In any case, your receiver looks for the amount of energy that is in a signal having that particular pattern and the more unique and nonrandom (pseudorandom is fine) the pattern, the better your receiver will be able to reject unwanted signals.

    interference a signal that is smaller as the distance increases. Thus you will have to have some threshold for the amount of signal that you see
  6. gootee

    Senior Member

    Apr 24, 2007
    Yeah, that about covers it. Excellent post(s).

    Is this toy likely to be near the ground? What about the transmitting antenna? The signal power level at the receiver might be likely to rise and fall by a factor of a hundred, fairly rapidly, as the distance between the two antennas changes. You really shouldn't use the simplified "two-ray propagation model", to try to see that.

    Which frequency band(s) are you considering?
  7. Art

    AAC Fanatic!

    Sep 10, 2007
    Isn't the transmitter the modulator?
  8. WBahn


    Mar 31, 2012
    As a rule, yes. This question is without discernable context, so I can't provide a better answer than that. I'm assuming you are asking it in response to something I or someone else wrote, but I can't spot it. Please provide the context in which the question is being asked.
  9. Art

    AAC Fanatic!

    Sep 10, 2007
    I lack some understanding of how it takes place, and I know the OP doesn't
    have to modulate audio in this case,
    but is my understanding that a transmitter modulates, and the reciever is the demodulator.
    I wondered why the OP wasn't corrected.

  10. WBahn


    Mar 31, 2012
    What was to correct? Are you talking about the fact that he used the word "demodulator" in the title? That was pretty immaterial as he is looking for both a transmitter and a receiver and he isn't looking for modulation (or at least wasn't initially) but only detection.
  11. Art

    AAC Fanatic!

    Sep 10, 2007
    ok, because it only needs to transmit a carrier wave, and nothing else,
    for the radio receiver to detect the difference between on and off.

    Yes that's true, sorry, the topic is not just about broadcasting, but both Tx and Rx.