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6300 shows S-meter 1-3 on various bands with dummy load?

Sparks
Sparks Member
So, it's normal for a 6300 to read S 1-3 on various bands with a dummy load connected?
It it reading internal noise? One would think there was a way to zero out the S meter when there is absolutely NO signal present.

Answers

  • Steve Walker
    Steve Walker Member
    edited September 2016
    Hi Bob, This thread might help answer that https://community.flexradio.com/flexradio/topics/why_does_my_s_meter_read_s5_all_the_time

    I see the same if I switch my 6300 to a DL on the Ant2 port then I see on SSB about S3 at 2.7Khz. Which as I read before is normal for the platform.

    73 Steve

  • Sparks
    Sparks Member
    edited March 2017
    Steve- I just can't accept that when NO signal is present, there is an S meter reading. Probably, this is a design consequence of using the Wideband A to D converter. Basically, Everything is two S units higher on my 6300 than IC-7600. It just goes against my better judgement- because a moving S meter with NO signal there - used to mean too much gain in the RX... that the RX chain was past the noise floor at that freq. There may be no fix for this. One would think that you could simply re-cal the S meter via software for a zero reading with no signal.
  • KY6LA_Howard
    KY6LA_Howard Member ✭✭✭
    edited January 2017
    BOB

    You totally misunderstand what the S-Meter Function is on your FLEX
    The Flex is an extremely accurate spectrum analyzer

    It gives accurate readings in dBm of the signal strength at the receiver  (Frankly I wish they never wasted space on that less than useful analog red on black line)

    https://community.flexradio.com/flexradio/topics/why_does_my_s_meter_read_s5_all_the_time

    Even with a dummy load there is noise - or at least a noise floor...

    On say 20M the noise floor of your Flex is about -120dBm without a pre-amp so it should read at least S1-S2... S0 being -127dBm

    By contrast your legacy radio S Meter is at best totally inaccurate and totally misleading - S0 varies between bands and even modes and radios... at best it is a relative indication of something... but not much that you could use to provide consistent reproducible results...


  • Sparks
    Sparks Member
    edited March 2017
    OK guys-

    I hear what you are saying. What this requires is a mental re-calibration of long-held analog s-meter understanding. There is always noise - from whatever number of FFT Bins we are being read. In order to reflect IARU specs on S units Vs. incoming uv, the meter will only read zero at 50 hz or less on a quiet band. Just tried it and - correct- -134dbm = S 0. Under normal operating situations, this results in meter readings which are several S units above what old analog Guess-meters read.
     
    Say, one other question: Why is the visual DB reading on the scope - different from the one you get when you hold your cursor over the S meter? I'm at -125 dbm on the scope (baseline noise on 20 mtrs) and the meter says around -103 dbm. Which is correct?

    Thanks!

    Bob-AB5N
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    Bob,

    Yes, sometimes we do have to unlearn old, bad habits ;-)

    Both are correct, as they are representing the digitized spectrum data in different ways.  The panadapter display is averaging the FFT bins so your eyes can better interpret the information visually.  If we didn't, due to the erratic nature of the spectrum at the noise floor, weak signals would be very difficult to detect visually.  

    The S-meter is integrating the power in the FFT bins to determine total RF power within the RX passband. 

    To get an accurate reading of the noise floor, based on conventional testing standards, you want to use a 500 Hz wide filter.

    On 20m at my QTH right now, the panadapter is at about -118 dBm but my 500 Hz CW slice is showing -113 dBm, a little over S2.  Which isn't too bad for a partially rural location in a 1000 home subdivision.
  • Sparks
    Sparks Member
    edited November 2014
    Alrightie.   I'm at -126dbm on 20 mtrs.. but it's cold out there and things are quieter than normal. 500hz for normal noise floor measurements...QSL..
    Thanks for your help!

  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    No problem, education is the name of the game!  73 and enjoy the weekend.
  • Dave KD5FX
    Dave KD5FX Member ✭✭
    edited March 2015
    Why is the noise floor lower with the pre-amp on? I notice this on 20 mtrs and above, didn't look at lower bands.

  • Sparks
    Sparks Member
    edited November 2014
    Well, you are able to see / hear weaker signals.... I interpret it as the pre-amp actually lowering the level at which you can discern a signal - which is what it ideally would be doing. We are used to Pre-amps adding noise. I'm sure one of the tech gurus will give a precise answer!
    
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    When you engage the preamp, you have not changed the signal level at the antenna terminal. You have, however; improved the signal to noise ratio with the preamp and therefore the noise will go down in reference to a fixed signal level. The previous statement holds true in all cases except one. That is if the radio is antenna noise limited.

    If you engage the preamp and the noise floor does not decrease, then you turn it off or back down the gain.

  • Ned K1NJ
    Ned K1NJ Member ✭✭
    edited December 2016
      
       In the not-so long-ago, manufacturers specified a calibration point
    of S9=50 microvolts.  Readings from manufacturer to manufacturer
    and unit to unit were reasonably comparable around S-9, and less
    comparable elsewhere.  Flex produces an instrument-quality device.
    It is designed to tell it like it is.

    Ned,  K1NJ
  • Richard Fusinhski
    edited November 2015
    I am not sure that I am buying these answers. I have a flex 6300 and it NEVER reads below S-3 whether a dummy load or antenna. In contrast my FTDX 5000 reads exactly what I put into it with a signal source, If I put in -127, it reads S-0; if I put in -116 it reads S-1; if I put in -111 it reads s-2. If I put in -105 it reads S-3. So what happened to the s-1 to s-3 signals on my flex?
    So what you are trying to tell me is that the flex meter is no good below S-3?????? I understand noise temperature and the such but I would like my meter to read what is really there in signal strength.  I checked the cal from S-3 to +40 and it is right on the money with my flex and my FTDX5000. S-9 is exactly 50uv on both.
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited December 2016
    It is the absolute truth.  Traditional analog radios provide a relative measure of signal strength based on the receiver's AGC voltage. I suspect Yaesu has done a stepped (quantized)  approximation based on a peak input signal strength from a signal generator source.  This is an assumption based on other radios as I have not looked at the circuit schematics for that particular radio.

    With our radios we measure the actual signal strength by integrating the digitized RF power contained within the FFT bins inside the RX filter.  This is how a digital signal generator operates.

    What we are saying is there is always RF power inside the filter and it is additive.  The filter bandwidth is a dependency in the power measurement process.  If you make the RX filter wider, the S-meter reading increases.  If you make it smaller, it decreases.  The standard is to measure integrated power in a 500 Hz filter.  And it is frequency dependent too.

    And you dBm to S unit numbers are a little off too for frequencies below 30 MHz

    S1 = -121 dBm
    S2 = -115 dBm
    S3 = -109 dBm

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