Best Quad Coil for Narrowband FM

Discussion in 'Wireless & RF Design' started by mossman, Aug 26, 2010.

  1. mossman

    Thread Starter Member

    Aug 26, 2010
    131
    3
    I need some help selecting the best variable coil for an FM narrowband receiver 455 kHz (quadrature detector) . The one I am using now has a Q of about 30 and BW of 15 kHz. Seems like a higher Q/narrower bandwidth would give me better sensitivity.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I suppose you can always use 455 KHz for the IF freq in an FM receiver. Are you proposing to wind your own coil? One suspects the characteristics of the entire IF strip have more to do with signal bandwidth than the detector coil, which only works with the signal applied to it.
     
  3. skeptic

    Active Member

    Mar 7, 2010
    51
    9
    Mossman, are you proposing to modify so called wideband 2-way radios (25 kHz) to narrowband (12.5 kHz) by reducing the receive bandwidth and transmit deviation? Some of the manufacturers of the radios offer kits to do that. The FCC says you are allowed to do that provided the modified radio is re-type accepted as narrowband. Once type accepted, all radios you modify are covered by the type acceptance.
     
  4. mossman

    Thread Starter Member

    Aug 26, 2010
    131
    3
    I'm simply asking that if I use an adjustable coil with a sharper Q and more narrow bandwidth, can I achieve better sensitivity? The coil I am looking at has a bandwidth of 15 kHz and a Q of 30, which doesn't sound ideal for narrowband applications with deviations of 5 kHz or so.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I would think that the front end design had more to do with sensitivity than the demodulator. The better the signal to noise ratio in the front end, the more gain you can apply in the IF, and so you have more signal to demodulate.
     
  6. mossman

    Thread Starter Member

    Aug 26, 2010
    131
    3
    Ok, but wouldn't a higher Q in the 2nd IF stage result in sharper transitions/deviations thereby improving the sensitivity of the quadrature detector/discriminator?
     
  7. skeptic

    Active Member

    Mar 7, 2010
    51
    9
  8. mossman

    Thread Starter Member

    Aug 26, 2010
    131
    3
    Think I answered my own question. Since an FM quadrature detector detects phase changes, you would want a useable amplitude across the entire band for reliable detection. In other words, if the received signal deviation was +/- 5 kHz, you would want the 3 dB bandwidth of the quadrature coil to be at least 10 kHz. Am I right?
     
  9. skeptic

    Active Member

    Mar 7, 2010
    51
    9
Loading...