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

Sparks

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.

Steve Walker

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

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.

Tim - W4TME

The S meter isn't broken so it doesn't need fixing.  The "fix" you are asking for would actually break the S meter by assigning an arbitrary signal strength value to the meter based on user perceived signal or comparison to an inaccurate standard (traditional radios).  Let me explain.

First, there is always signal present.  Blackbody, atmospheric noise, cosmic noise and thermal noise are "signals" at RF energies and all contribute to the RF noise floor.  Even with a dummy load connected, there is still signal being digitized.  There is no such practical thing as "no signal" (unless the radio is in an RF isolation chamber at absolute zero).

The signal strength meter design in the FLEX-6000 is based on how all lab grade digital spectrum analyzers operate; by accurately integrating the total RF power in the FFT bins within the receiver's passband.  This is a very important point.

Where a traditional received estimates signal strength by calculating a value from the AGC voltage, the FLEX-6000 actually measures the signal power in each FFT bin. Since S units are a standardized unit of measure for RF power as defined by the IARU, we calibrate the S-meter based on a standard, making the FLEX-6000 very accurate in this regard.

In the second paragraph, I emphasized an important point about measuring in the FFT bins, as opposed to estimating, RF signal within the receiver's passband.  This is very easy to demonstrate.  With your dummy load connected to the antenna port, change the slice to CW mode and change the filter to 50Hz.  At this RX filter passband size, there are only a few FFT bins measuring RF power and the S meter is barely moving.  If you put your cursor on the meter bar to display dBm, it may be as low as -134 dBm.  Now the IARU defines S0 as -127 dBm.  So the S meter reading in this case is actually less than S0!
  

KY6LA_Howard

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

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

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

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

No problem, education is the name of the game!  73 and enjoy the weekend.

Dave KD5FX

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

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

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

  
   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

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

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