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Low Cost RF Circuits — Parallax Forums

Low Cost RF Circuits

william chanwilliam chan Posts: 1,326
edited 2009-05-23 09:01 in General Discussion
I have always marveled at how the chinese managed to implement very low cost RF circuits for their remote control toys, doorbells and such.

I recently stumbled on a simple RF receiver circuit.
It claims to receive signals in the 27Mhz band but is without any crystal.

Can anybody explain whether such a simple circuit would work and whether it will be stable?

See attachment.

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www.fd.com.my
www.mercedes.com.my
simple rf.JPG
995 x 515 - 50K

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-05-21 15:23
    I'm not an expert by any means, but this looks like a super-regenerative receiver. These do tend to "lock on" to the strongest carrier over a relatively broad frequency range and they're quite sensitive for the simplicity of the circuit. Have a look at the Wikipedia's article on "superregenerative".
  • LeonLeon Posts: 7,620
    edited 2009-05-21 19:06
    It is a super-regen. They have the disadvantage that they radiate interfering signals via the antenna, and don't have any selectivity to speak of.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • william chanwilliam chan Posts: 1,326
    edited 2009-05-21 22:01
    Why is the output capacitor (104 p) much larger than the antenna capacitor (4p) ?
    Can we use a PCB track as the receiving antenna?
    How long should the track be? and should we connect both ends of the track?

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    www.fd.com.my
    www.mercedes.com.my

    Post Edited (william chan) : 5/21/2009 10:06:26 PM GMT
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-05-21 22:12
    The output capacitor has to pass lower (i.e. audio) frequencies; therefore it has to be bigger.

    A PC track can work at UHF wavelengths but would not be adequate at 27MHz.

    -Phil
  • william chanwilliam chan Posts: 1,326
    edited 2009-05-22 09:39
    Is it better to wind yourself a 7 turns coil with 5mm core as suggested in the circuit
    or try to calculate the inductance and buy a complete inductor from Farnell?

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    www.fd.com.my
    www.mercedes.com.my
  • LeonLeon Posts: 7,620
    edited 2009-05-22 13:36
    Wind it yourself, I doubt if the inductors stocked by Farnell have a high enough Q. It's only seven turns anyway, and will only take a few seconds to make.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-05-23 04:39
    From the schematic, it indicates a "slug", so you'll need to tune the circuit anyway. The L2 and C3 form an 'LC tank' that needs to match the desired frequency that you want to operate on.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • william chanwilliam chan Posts: 1,326
    edited 2009-05-23 08:41
    Sorry for being a newbie in rf circuits.

    How can I add an led indicator to this circuit to light up whenever it locks on to a strong signal?

    For a typical car alarm key fob that doesn't seem to use any external antennas, what frequencies do they use?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.fd.com.my
    www.mercedes.com.my

    Post Edited (william chan) : 5/23/2009 8:58:17 AM GMT
  • LeonLeon Posts: 7,620
    edited 2009-05-23 09:01
    You need to rectify the output and use it to turn a transistor on.

    Remote controls like that used to use 433 MHz here in the UK.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
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