HELP: Probs with S Meter driver circuit

Discussion in 'Wireless & RF Design' started by hamopp, Jul 19, 2011.

  1. hamopp

    Thread Starter Active Member

    May 13, 2009
    68
    1
    Hi All

    I'am building a SSB Audio AGC amp with a meter driver.
    The circuit uses a LM324 Quad op amp, i use the spare
    gate (ic1b) for the meter driver.

    The thing is the agc amp works as it should, the agc voltage
    increase with stronger signals, but the "s" meter reads backwards
    it should go from left to right on speech peaks.

    Not to sure what the prob is :confused:
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,451
    3,371
    Is this an existing circuit design or is it a new design that you are trying out?
    The output voltage at the junction of R16 and D14 increases with signal strength.
    The S-meter driver stage IC1B is configured as an inverting amplifier. You can either use a non-inverting amplifier or you can add an inverting stage to invert the signal.
     
  3. hamopp

    Thread Starter Active Member

    May 13, 2009
    68
    1
    Hi MrChips

    It is a new circuit that i am trying out.
    Could you show how to configure the S-meter driver stage IC1B
    as a non-inverting amplifier ?

    TIA
    Hamopp
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,451
    3,371
    I have not checked the component values and bias voltages you are using.
    Op-amp IC1B is configured as a unity-gain differential amplifier the way you have configured R20, 21, 22 and 23 with equal values. You could try interchanging the inputs at R20 and R21, that is, move the input to R21 and put the reference voltage on R20.
     
  5. hamopp

    Thread Starter Active Member

    May 13, 2009
    68
    1
    Hi
    The output swing of the agc amp is 5v to 8.5v, i just need
    to drive a meter without loading the agc output.

    With having a spare gate on the LM324 i thought that i would
    use it, i think that it not have to be so complicated.

    What about if i put the agc line into pin 5, a 1m from pin 6 to
    pin 7 and a 10k from pin 6 to ground, then pin 7 to r24 ??

    TIA
    Hamopp
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,451
    3,371
    Yes, there probably is a simple solution using the spare op-amp on IC1B.

    Basically, you would set an adjustable reference voltage on the inverting input (pin 6) to remove the 5V offset. This leaves you with a 0-3.5V signal.

    What is the S-meter? Is it an ammeter or voltmeter? What does it take to drive it to fullscale?

    I don't have the time now but maybe later or someone else can offer a solution.
     
  7. hamopp

    Thread Starter Active Member

    May 13, 2009
    68
    1
    Hi
    It is a voltmeter showing signal strength.
    About 120mV for full scale.

    Using a multimeter i read across the s meter about 630Ω, and it showed
    about half scale on the s meter.

    TIA
    Hamopp
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,451
    3,371
    Ok, you have tons of sensitivity on the S-meter. I will post a solution later today when I find some time.
     
  9. hamopp

    Thread Starter Active Member

    May 13, 2009
    68
    1
    Thank You MrChips :cool:

    Hamopp
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,451
    3,371
    Here are the mods to your circuit:

    [​IMG]

    All I did was interchange your inverting and non-inverting inputs and gave you an offset adjustment.

    R19 variable resistor is a pot to remove the 5V offset.
    R25 variable resistor is a pot to adjust the sensitivity and thus allow you to calibrate the meter. You can play around with the R18 and R24 values.

    Hope this works for you.
     
  11. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    I think that with "conventional" AGC control the AGC level increases as the received signal level drops - to boost the overall (RF or IF?) gain. The AGC backs off as the received RF signal level increases.

    That's why you would have an inverting stage for the 'S' meter - as AGC increases (falling received signal) the 'S' meter indication reduces.

    I guess if you are using the audio signal then that convention doesn't necessarily apply.

    Is this your design or something you obtained from elsewhere?

    How does the +12V Tx control input supposedly effect the 'S' meter indication when switched to 12V?

    Also do you want an 'S' meter indication of zero with your AGC signal reading 5V? Or something different?
     
    Last edited: Jul 21, 2011
  12. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Mr Chips - I'm not sure your circuit modification actually does what you intended.

    Suppose we consider the circuit with values as shown.

    R19 wiper would sit at about 4.1V as shown.

    Suppose the input is 6V. The op-amp +ve terminal would be at ~3V. The -ve terminal would follow to the same value.

    The op-amp output would therefore be ~2.1V

    Suppose R19 was set (at about 30%) to give 2.5V at the op-amp -ve input. This would presumably "balance" a 5V input. Even so, the op-amp output would still be ~2.5V.

    Again, I'm not sure of the intent of your modification.
     
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