HELP: 10.7Mhz NBFM IF Stage

Discussion in 'Wireless & RF Design' started by hamopp, Mar 30, 2010.

  1. hamopp

    Thread Starter Active Member

    May 13, 2009
    68
    1
    Hi all
    I'am having some probs with this MC3359 NBFM IF System.

    If you look at the mc3359 test circuit, you will see that X1, C14,
    FL1 and C12 are connected to VCC(6v), while in the circuit that i built
    (i found on the net) has X1, C14, FL1 and C12 are connected to gnd, which
    one is right ??
    In both circuits the AFC and Scan Control are NOT used!
    I am using a Marconi 2955b test set, freq set to 10.7Mhz with a 1Khz tone
    @1mV output.
    The one that i made works, but seems to have little gain, the squelch sort
    of works, but when i adjust the squelch pot i get different audio levels, and
    sometimes the level seems to drift away (can't hear the 1Khz tone)

    Any thoughts on these circuits ??

    Regards
    Howard
     
  2. skeptic

    Active Member

    Mar 7, 2010
    51
    9
    If you look at the schematic of the IC you will see that pin 1 is just a diode junction from Vcc so the DC voltage at pin 1 will be about 5.3 V. By connecting the other side of the crystal to +6 V they are minimizing the DC bias across the crystal. The DC bias may shift the crystal's frequency slightly and by connecting the crystal to ground instead, you're putting about 5 V more bias across the crystal. The filter may also be affected by DC bias.

    The second thing is that the crystal's frequency may be shifted slightly by the capacitance across it and the capacitance you are using is somewhat different than that in the example.

    The third point is the Circuit_Built schematic doesn't show what is feeding the 10.7 MHz input. If you are putting a DC bias on that pin, that could affect the gain of the circuit. In the example, DC on that line is blocked with a capacitor.

    Have you checked the frequency of the 10.245 MHz crystal?
     
  3. hamopp

    Thread Starter Active Member

    May 13, 2009
    68
    1
    Hi
    Yes i have just checked the frequency of the 10.245 Mhz crystal, and it is about
    3Hz high. I also fitted a 11.155 Mhz crystal and that was about the same.

    The 10.7Mhz signal is fed into a Toyocom HCM 10M-2A 10,7Mhz filter, which is not
    shown on the Circuit_Built schematic (sorry)

    I'am about to do some more tests, gonna put a 51Ω on the input to the filter to gnd,
    and feed the 10.7Mhz signal in via a .1μF cap.

    Will report back soon

    Howard
     
  4. hamopp

    Thread Starter Active Member

    May 13, 2009
    68
    1
    Hi

    Right i have got this sorted, there's now't wrong with the circuit that i built :cool:
    The problem was with the switching circuit that was from Sgt Wookie's circuit
    (4066 switch) it was blowing the 4066 chip and stopping the audio from getting
    through.

    When switching from FM to SSB mode it seems that the diodes D1,D2 and D3 blow,
    i have replaced the diodes and 4066 a few times, and each time the circuit works ok
    (power on in FM mode) but as soon as i switch to USB the diodes blow, and when i
    switch back to FM the audio is very faint!!

    I have also tried a version with relays (my design) (NEW RELAY.png) but this also seems
    to have the same problem, when i power on in FM mode, rly1 and rly2 switch ok
    and the FM audio and FM rssi are ok, but as soon as i switch to USB the diodes D3,D4
    and D5 blow, but if i remove D2 from rly3 and rly4 D3,D4 and D5 don't blow,is this to do
    with back emf ??

    Any thoughts on This ???

    Regards
    Howard
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    The 4066 circuit looks fine, I would suspect a solder bridge or wiring fault somewhere - or:

    Are you just switching the diode inputs to the same VDD supply as the 4066? If you use a different supply, that could blow the ICs then the diodes.

    If you use a different switch supply, add a 10K resistor between the existing diodes/R11 junction and the feed to the control pins of those lower 4066 sections.

    That will limit the current and protect the chip.
     
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