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RE: [FT897] RF Gain & S Meter

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  • Mark Vaughan
    It used to bug me as well. There are lost of descriptions and theories. I think the easiest theory is that as you lower the gain, you reduce the radio
    Message 1 of 6 , Jun 3, 2008
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      It used to bug me as well.

      There are lost of descriptions and theories.

      I think the easiest theory is that as you lower the gain, you reduce the
      radio sensitivity, so the minimum deflection of the meter is raised to show
      these signals can no longer be received. The other common explanation is
      that where you put the RF gain shouldn't effect how the meter shows the
      received signal strength, if it did you wouldn't be able to give a true
      signal report, but in reality the RF gain is a gain control knob, so in
      order to keep the meter accuracy the same as you wind down the gain a DC
      offset must be applied to the meter to bring everything back in line.



      It makes sense when you get your head round it.



      73 Mark



      Dr. Mark Vaughan Ph'D., B.Eng. M0VAU

      Managing Director

      Vaughan Industries Ltd., reg in UK no 2561068

      Water Care Technology Ltd, reg in UK no 4129351

      Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH UK.

      Phone/Fax 44 (0) 1872 561288

      RSGB DRM111 (Cornwall)

      _____

      From: FT897@yahoogroups.com [mailto:FT897@yahoogroups.com] On Behalf Of
      kc5prg
      Sent: 01 June 2008 20:16
      To: FT897@yahoogroups.com
      Subject: [FT897] RF Gain & S Meter



      I've noticed that when I reduce my RF Gain my S Meter reading increases
      and vice versa. It seems like it should be the other way around. The
      actual RF Gain works correctly...i.e. when I increase the RF Gain, the
      signal increases as it should. It's just the meter that seems
      backwards. Am I missing something somewhere?

      Robert
      KC5PRG





      [Non-text portions of this message have been removed]
    • Steve Yates
      The S-meter simply shows the AGC (Automatic Gain Control) voltage as is the case with most receivers. As you rotate the RF Gain control counter clockwise the
      Message 2 of 6 , Jun 4, 2008
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        The S-meter simply shows the AGC (Automatic Gain Control) voltage as is
        the case with most receivers. As you rotate the RF Gain control
        counter clockwise the AGC voltage is increased (just like when a strong
        signal is received) and this reduces the gain of the AGC controlled
        stages.

        Another way to think about it is that signals below the S-meter reading
        are attenuated when using the RF-Gain control.

        The RF-Gain control is an often under utilized function. It is great
        for reducing the irritating noise floor since the AGC under normal
        operation is designed to make all signals, including noise, the same
        amplitude. I have read a lot of radio reviews where folks complain
        about a noisy receiver and actually judge a receiver by how "quiet" it
        is. If the AGC is really working it will be noisy by design unless you
        use the RF-Gain control. I prefer to keep the RF-Gain back so that the
        signals received are relatively linear in response, kind of like no AGC
        but without the overloaded detector ;-)

        73,
        Steve Yate - AA5TB


        http://www.aa5tb.com/
      • Steve Yates
        I ll add a little more to my last comment. The AGC creates a DC bias that is applied to various stages of the receiver to control the overall gain of the
        Message 3 of 6 , Jun 4, 2008
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          I'll add a little more to my last comment. The AGC creates a DC
          bias that is applied to various stages of the receiver to control
          the overall gain of the receiver. This is to provide a constant
          amplitude signal to the detector and your ears. In most receivers
          (including the FT-897D) the S-meter simply measures this AGC voltage
          since it increases with an increase in signal. We you turn down the
          RF Gain you are simply adding a DC bias voltage to the AGC to reduce
          the overall receiver gain and this is why the S-meter increases with
          reduced RF Gain.

          You'll notice that if you turn off the AGC the S-meter no longer
          works on signals but will rise as the RF Gain control is reduced
          (the AGC control still works but it just doesn't repond to signals).

          Some simple receivers just use a potentiometer as a variable
          attenuator at the front end of the receiver. This reduces the
          signal strength but does not mess with the AGC directly, if there is
          any AGC. This type of RF Gain control will simply lower the S-meter
          reading if there is one.

          73,
          Steve Yates - AA5TB
        • Michael Stephens
          I have the RF/SQL control set for RF gain control all of the time, even on AM, the AGC I think can sometimes be a little too fast and does make for
          Message 4 of 6 , Jun 4, 2008
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            I have the RF/SQL control set for RF gain control all of the time, even
            on AM, the AGC I think can sometimes be a little too fast and does make
            for uncomfortable listening, even when you're only casually listening to
            the S9+40Db power house italian station on 20metres or China radio
            international, I think the 897 needs a slower agc option for casual
            listening, not just fast/slow and off..

            Mike M1MBZ

            Steve Yates wrote:
            >
            >
            > The S-meter simply shows the AGC (Automatic Gain Control) voltage as is
            > the case with most receivers. As you rotate the RF Gain control
            > counter clockwise the AGC voltage is increased (just like when a strong
            > signal is received) and this reduces the gain of the AGC controlled
            > stages.
            >
            > Another way to think about it is that signals below the S-meter reading
            > are attenuated when using the RF-Gain control.
            >
            > The RF-Gain control is an often under utilized function. It is great
            > for reducing the irritating noise floor since the AGC under normal
            > operation is designed to make all signals, including noise, the same
            > amplitude. I have read a lot of radio reviews where folks complain
            > about a noisy receiver and actually judge a receiver by how "quiet" it
            > is. If the AGC is really working it will be noisy by design unless you
            > use the RF-Gain control. I prefer to keep the RF-Gain back so that the
            > signals received are relatively linear in response, kind of like no AGC
            > but without the overloaded detector ;-)
            >
            > 73,
            > Steve Yate - AA5TB
            >
            > http://www.aa5tb.com/ <http://www.aa5tb.com/>
            >
            >
          • Mark Vaughan
            You can open the radio up and add an extra cap on the AGC to slow it. It s a well documented mod, on sites like www.mods.dk . Most put
            Message 5 of 6 , Jun 7, 2008
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              You can open the radio up and add an extra cap on the AGC to slow it. It's a
              well documented mod, on sites like www.mods.dk <http://www.mods.dk/> .

              Most put 4.7uF in, I personally prefer 10uF.



              73 Mark



              Dr. Mark Vaughan Ph'D., B.Eng. M0VAU

              Managing Director

              Vaughan Industries Ltd., reg in UK no 2561068

              Water Care Technology Ltd, reg in UK no 4129351

              Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH UK.

              Phone/Fax 44 (0) 1872 561288

              RSGB DRM111 (Cornwall)

              _____

              From: FT897@yahoogroups.com [mailto:FT897@yahoogroups.com] On Behalf Of
              Michael Stephens
              Sent: 04 June 2008 20:40
              To: FT897@yahoogroups.com
              Subject: Re: [FT897] Re: RF Gain & S Meter



              I have the RF/SQL control set for RF gain control all of the time, even
              on AM, the AGC I think can sometimes be a little too fast and does make
              for uncomfortable listening, even when you're only casually listening to
              the S9+40Db power house italian station on 20metres or China radio
              international, I think the 897 needs a slower agc option for casual
              listening, not just fast/slow and off..

              Mike M1MBZ

              Steve Yates wrote:
              >
              >
              > The S-meter simply shows the AGC (Automatic Gain Control) voltage as is
              > the case with most receivers. As you rotate the RF Gain control
              > counter clockwise the AGC voltage is increased (just like when a strong
              > signal is received) and this reduces the gain of the AGC controlled
              > stages.
              >
              > Another way to think about it is that signals below the S-meter reading
              > are attenuated when using the RF-Gain control.
              >
              > The RF-Gain control is an often under utilized function. It is great
              > for reducing the irritating noise floor since the AGC under normal
              > operation is designed to make all signals, including noise, the same
              > amplitude. I have read a lot of radio reviews where folks complain
              > about a noisy receiver and actually judge a receiver by how "quiet" it
              > is. If the AGC is really working it will be noisy by design unless you
              > use the RF-Gain control. I prefer to keep the RF-Gain back so that the
              > signals received are relatively linear in response, kind of like no AGC
              > but without the overloaded detector ;-)
              >
              > 73,
              > Steve Yate - AA5TB
              >
              > http://www.aa5tb <http://www.aa5tb.com/> com/ <http://www.aa5tb
              <http://www.aa5tb.com/> com/>
              >
              >





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