Why does my S-Meter read S5 all the time?

Homer1952

Bill - I have been using the 6400M in rig mode 90% of the time.  Next time I fire it up in PC mode, I'll look at that, thanks.

Jay - My little mind still expects 0 S units when the receiver input is shorted.  For 50 years it has been that way for me.  I think you are right that it is normal behavior for a Flex. - Bob

Bob W8RMV

PS- Homer1952 is my alter ego ;-)  I forgot my get satisfaction login & had to login thru gmail.  I just want to make one more comment:  As I try to understand the 6400M I don't want all the good about the radio to get lost.  I am just trying to maximize my understanding of the rig.  I have a lot of other questions that I will leave for another day.  73 Bob W8RMV

Jay Nation

I won't argue further, actually I'm not arguing at all. It is what it is. Maybe that's how they were calibrated in the past. Makes sense. But 50 years of accuracy work, might have achieved something. What it is now, is almost never equivalent to what it was then, or what it might someday be.  
 
#Flexradio

73, Jay / NO5J

KY6LA_Howard

dBm Metering is on the wish list... hopefully sooner than later

HOWEVER if you hover your mouse over the S-Meter you can read dBm now

Bob W8RMV

Paul- I tried it first with a 51R non-inductive dummy load & got the same result.  That is why I decided to try it with a dead short. - Bob

Ria

PowerSDR has it, so not exactly new. As Howard said - on the wish list. Upvote it if you want it. 

Ria

PowerSDR has it, so not exactly new. As Howard said - on the wish list. Upvote it if you want it. 

Bill -VA3WTB

I think as you can see it if you hover over the signal bar, Flex may feel this has been answered. 

KY6LA_Howard

@Bill. It clearly has NOT been addressed,by Flex yet. dBm needs to be visible 100% of the.time not only after a tortuous mouse movement. So upvote it please.

Mark K1LSB

Gents,

I’ve seen a number of different explanations in this thread of what the S-meter should be measuring, such as integrated (average?) power, and statements to the effect that if the RX bandwidth changes, the S-value should also change, even if there’s nothing but background noise.

I disagree.  The S-meter should only report the peak signal strength measured within the audio passband.  If the background noise is at an S-2 level then the S-meter should read “S2” regardless of how wide my receive bandwidth is.  If the S-value is expected to vary as a function of receive bandwidth, then there would arguably come a point (stay with me here) where the RB is wide enough that the S-meter should be reading S9 +10, but the strongest signal in the RB might only be an S7!  So how would anyone know the actual strength of any signal in such a scenario?

No, the S-meter should only be reporting the peak signal strength found anywhere in the audio passband.  I don’t care how wide the passband is.  It could be 500 Hz wide, or 2.7 kHz, or 250 kHz.  If there’s nothing but noise in the passband, and the noise floor anywhere in the passband is at a steady -115 dBm, then the S-meter should never read above an S2 regardless of the width of the passband.  The strongest measured signal strength (whether it be noise or a received signal) at any single spot in the passband is what the S-meter should be reporting.  Nothing more, nothing less.

Perhaps there is some other information that some users would like to also see reported, but that’s not the S-meter’s job.

Mark

[edited to correct typos]

Jay Nation

Bob

Since I don't use the RXA inputs here, I keep a 51R **** onto it. 
When I Switch the RX ANT to RXA, the preamp set for 0 dB,and set a 500 Hz filter passband, I read a minimum -123.8 dBm +/- 2 dBm delta, using Flexmeter. That's the lowest reading I can get on 40m, it also varies 1 - 2 dBm depending on the band selected. Pretty much an S-1 on the Maestro's S meter. S-0 or "S" doesn't happen. That's with Flexmeter v1.3.0.5 set for normal response, not average or peak hold.

Your @S-5 reading does suggest something's wrong. HelpDesk Ticket time.

All of these levels are actually being measured inside the radio continously, and the numerical values sent out over the Ethernet connection, Flexmeter just reads and displays the numerical values being outputted.  

If you don't already have it ...

http://www.denzone.com/uploads/FlexMeterSetupV1.3Beta.zip 

The new beta has some nice improvements. 
 
#Flexradio

73, Jay / NO5J

(edited to fix the units used)

Bill -VA3WTB

From Tim,,,
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

Bill -VA3WTB

More info from Steve at Flex


The panadapter simply measures the signal in a given bandwidth and draws what it hears. If you look at any given pixel, it represents a certain amount of bandwidth. We call this the "bin size" of the FFT that is used to produce the display. If you cut the bin size into two pieces, the amount of noise in each piece goes down by half (3dB). In PowerSDR, the bin size is generally fixed for any given setup and does not change when you zoom. This is why the resolution gets worse as you zoom in on PowerSDR -- you begin to show one bin with multiple pixels. But for SmartSDR, we knew we wanted to have a larger range of zoom and this method was no longer acceptable. So we vary the bin size across a 1000:1 range. So the noise in each bin also varies. 1000:1 is a change of 30dB so from min zoom to max zoom, the noise in a bin will lower by 30dB and you see this change in the panadapter as you zoom in and out. 

When people talk about noise floor in ham radio they are generally talking about the noise level with a 500Hz bandwidth. When the panadapter is zoomed in to the max level, the bin size today is about 5.8Hz. This is a 19dB difference in noise from where a ham would say the noise floor is to what you can see on the panadapter. This means that the panadapter can see 19dB below what most hams would call the noise floor. Your ear and brain are also able to hear below the noise floor in 500Hz because of how they work. But there are limits to how well you can hear. If you've ever worked JT65 or another long-term integrating mode, you have noticed that your computer can copy signals that you cannot hear. 

So if you ask another ham "where is your noise floor on 80 meters" and he says "S5," what has he told you? Well with most hams, you don't know because you don't know the answer to these questions: 

1. What bandwidth are you using to measure the signal? 
2. Is your S-meter calibrated? 

An S5 signal corresponds to -97dBm. And if he's getting this on sideband set to, say 2.8kHz bandwidth then the actual noise floor in 500Hz would be -97 - 10*log(2800/500) = -104dBm. There's nothing magical about 500Hz, it just happens to be the convention for measuring noise in the ham radio world. In SmartSDR, if you set the passband filter to 500Hz, the S-meter in the slice will show you the 500Hz noise floor. 

If you start at maximum zoom and begin zooming out, you can see a point where the noise reading of the panadapter equals this number. What do you think this point is? ... if you've been following along, you will realize that this is the point where the FFT bin size is 500Hz. To get a rough idea if this is right, you could measure the width of your panadapter window and divide the amount of frequency displayed by this number. It should be in the 250-1000Hz range. The answer will not be exact because we do not continuously vary the FFT bin size -- we adjust it in steps and don't tell you where the steps are or what size they are. We do what's right for what you are viewing. 

I know that was a long-winded answer, but I hope it provides some insight into noise and how it changes what you see and hear. 

Mark K1LSB

A level of S9 corresponds to 50 microvolts at 50 ohm impedance.  That value (50uv) has been the de facto standard since the 1930s (the 50 ohm reference was nailed down in 1981 by the International Amateur Radio Union), per Wikipedia.  If the background noise power level is 50uv, then your S-meter should read S9 regardless of zoom level or bandwidth (assuming there's no signal, only noise).

I've recorded two videos of me running a session of SDR Console V3 Beta 2, using an SDRPlay RSP1A with a 50 ohm dummy load connected to the antenna port.  I varied the receive bandwidth everywhere between 10 Hz and 350 kHz, and the zoom between 200 Hertz and 1.4 MHz.  The S-meter is reporting the maximum voltage measured anywhere in the vertical green receiver stripe in the display (the defined receive bandwidth).

Here are the videos (the S-meter is in the upper left corner of the band scope, and the Receive Bandwidth value is in small yellow font to the left of the S-meter):

https://drive.google.com/open?id=1jSQUm_SXCRfpWiPGnADd-Ple3mk5x6My

https://drive.google.com/open?id=1KJwBKiJXdHBCaw0MpGjGg6dNYp4Upgjv

Both videos show virtually no change in S-meter reading, regardless of any bandwidth or zoom changes I make.  Which is exactly as it should be, because the noise level doesn't change despite any changes I make to the zoom or bandwidth.

Simon Brown, author of SDR Console, understands that concept well.  His S-meter doesn't care what the bandwidth or zoom level are, it only cares what the maximum measured voltage is anywhere in the defined receive bandwidth, which is exactly what the IARU Region 1 Technical Recommendation R.1 says an S-meter should be measuring.  The maximum voltage in the RB is, and should be, independent of any zoom level or bandwidth.

Carry on.

Bill -VA3WTB

I think Steve explained it well...  maybe if Steve had time he could walk you through it a bit more. Simons software does not work the same as the 6000s and SSDR. So as Steve mentioned as you increase the size of the bin samples, it changes noise.

Mark K1LSB

One more time -- the S-meter should only report the maximum voltage as measured anywhere within the receive bandwidth.  Period.  There is no "integrating the total RF power" or "mean band limited power" or "spectral density plot area" or "expectations" or "what we wish to convey" or "intuitive manner" that should be considered -- there is only the peak instantaneous voltage anywhere in the RB, regardless of how wide or narrow the RB may be.  That is the only measurement in conformance with the IARU definition of what an S-meter should be measuring, with S9 being referenced to 50 microvolts (for HF).  If the highest voltage peak at any point anywhere in the RB is 50 microvolts, then the S-meter should be reading an S9, regardless of bandwidth or frequency or zoom level or time of day or grid square or anything else.

My apologies if I've been testing the patience of some readers, as that was never my objective.  My only objective has been to clarify what an S-meter should (and should not) be reporting, per the IARU definition.  For the S-meter to report anything else is to open up a whole new realm of latitude with long-accepted standards, with no limits of imagination to constrain it.

Disclosure:  I don't own a Flex, but only because my good friend across town hasn't received his 6600M yet -- he's selling me his 6300 as soon as he gets notification that his 6600M is on the brown truck. 

Mark K1LSB

Thanks, Paul.  I hold all posters here in the highest regard, and you are certainly among them.  If you're also one of the Pauls on Simon's groups.io page, I greatly value your posts there too!

Best regards to all,

Mark

Bill -VA3WTB

As said before, Steve answered this above and is the authority all Flex. He has explained this clearly.

Mark K1LSB

I read what you posted that Steve said.  With all respect to Steve (and I respect his technical knowledge greatly), he may be the authority at Flex but the IARU definition is the authority on what an S-meter should be reporting.

I've posted a link below to a thread in Simon Brown's groups.io site where he replies to a number of statements that people were making about what they were expecting an S-meter to report.  One person asked why the S-meter reading didn't change whenever he adjusted the bandwidth (I wondered at the time where on earth did he ever get the notion that it should), and another person couldn't even make up his mind what he wanted -- first he says that he thought the S-meter should report the highest value of all (he means "any") FFT lines in the RB (and he's correct on that, because he's actually referring to peak voltage), but in the next sentence he says he also thinks it should display the "sum" of all the FFT lines, which is a completely different thing.  Yet another person wanted the S-meter to report signal-to-noise ratio. Simon replied with the same thing I've been saying here all along -- this is his statement:  "Let me make this very clear: the Signal Meter (S-meter) shows the peak signal level, it is not designed to show Signal to Noise [or anything else]. What you expect is a different signal indication -- I am interested in what you want to see but the S-meter logic you see now will remain as it is."

Here's that thread in Simon's site:

https://sdr-radio.groups.io/g/main/topic/5714841#31952

Mark K1LSB

Paul,

To the question in your last paragraph (an excellent question, btw), the answer is a resounding Yes.  Your statement that noise is simply random frequency and amplitude is exactly correct, so there can (and will) be situations where narrowing the pass band down very tightly will cause the S-meter to fluctuate because the maximum voltage of any noise inside that tight area will drop and rise randomly, and will not always be as great as the maximum voltage measured in a hugely wider slice of noise.

That was actually captured distinctly in my videos above -- if you observe the S-meter behavior during those times when I had the receive bandwidth severely reduced, you will see the readings jumping back and forth between a low reading and a high of pretty much what the steady reading was at the much wider RBs.  That's due to exactly what you described -- at one moment there's no high-amplitude noise peak, and the next moment there is.

And as you also undoubtedly noticed, at the wider receive bandwidth settings the peak noise level is much more steady and independent of RB width, because the peak voltage measured anywhere in the RB is almost certain to be virtually equivalent whether the RB is 20 kHz or 250 kHz.

Bill -VA3WTB

Repeat,,
A statement from Steve at Flex
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!
(Edited)

Mark K1LSB

Bill, you've posted the same quotes from Steve at least four times in this thread.  Enough already.  Do us all a favor and put what Steve said into your own words and post it here, so we can all see whether you actually understand what Steve said.

I've read everything you've cut-and-pasted of what Steve said, and it substantively agrees with what Paul said in his most recent post: at extremely tight receive bandwidths, the noise level will fluctuate.  When you're talking about FFT bin size (which Steve is doing) you're talking about very small sample size.

So I await your reply, in your own words, please.

Bill -VA3WTB

I always re post comments from a trusted source, people who designed the Flex.
There is a misunderstanding how Flex works in this regard.
I will not go as far as to question your understanding, but I could.

Bob W8RMV

I am cursed.  I wish I never brought this up, as I am unable to unsubscribe no mater what I do.  I have unsubsribed on the thread, on the 6400M tab & on my get satisfaction acct. (where I driven to when click on the CLICK HERE at the bottom of the email)  I unchecked all the boxes.  And still keep getting these.  After all the words about this, I still don't understand the S meter & have moved on.  Flex could have taken those useless 5 S units, eliminate them & spent the money elsewhere. (tongue in cheek.  Pls no hate mail.  I have moved on. Or at least I am trying if I can get off this thread).
-Bob

Mark K1LSB

1)  You didn't answer my request at all.

2)  If Flex's S-meter works any differently than what the IARU definition specifies, then it works incorrectly.

3)  Correct my understanding, if you are able.  The IARU definition is clear and unambiguous.  Simon Brown's understanding of that definition is the same as mine.  Once again, here's Simon's statement: "Let me make this very clear: the Signal Meter (S-meter) shows the peak signal level, it is not designed to show [anything else]".

Edit:  In respect of Bob W8RMV's angst (below), I'm done posting in this thread.

Bill -VA3WTB

Bob your sort of like me, this is all interesting but a little hard to grasp. If you go back and read my original thread were I used Steve's explanation it may come clear. To break it down, there is a big difference between how PSDR and SSDR works in this regard. In PSDR the bin size stay much the same, so when we zoom in and out the noise stays much the same and the signal meter reflects that.
In SSDR as you zoom in and out, the bins change in size so you hear more noise as you zoom in.
Because SSDR can zoom in so tight they needed to make the bins change accordingly.

KY6LA_Howard

Rather than dealing with all this theoretical mumbo jumbo, I decided to do some experiments with a beam and a dummy load on 40M 7.074 to see exactly how the S-Meter actually worked in the real world

Obviously my dummy load (MFJ) is not a perfect Faraday Cage.

Here are the results

By VISUALLY LOOKING at the Panadapter screen I could definitely see signals peaking at the peak readings

So I guess I really do not understand what the controversy is?

Sergey R5AU

Hi Paul !
just back to you initial post:



all of us well know how to S-meter showing thereflect signals in dBm or mkv -https://en.wikipedia.org/wiki/S_meter#Examples
and S9 usually under interpretation as Strong signal - see ex. here(with links to the IARU recommendations) -
http://www.rfcec.com/RFCEC/Section-3%20-%20Fundamentals%20of%20RF%20Communication-Electronics/33%20-....

https://en.wikipedia.org/wiki/R-S-T_system 

however, sometime very usual situation on 160 or 80:   level of noise over S9(see above - strong signal), stations are calling with level looks like S9+1-3db (just 1-3 db over the level of noise), what kind of signal marl you will set? :  weak - strong - masked with noise ?
S-meter in SDR is an precise meter in case you follow with exact signal strength however in the same  time we can use the  R-S-T system https://en.wikipedia.org/wiki/R-S-T_system  were both system together are add each other isn't it ?

i like to use both system together.

Lee

OK, Guys....forget the S meter for a moment.
Let's say )(it does) I have S5 on 40m SSb BW 2.7... IF there is a signal at S3 WILL I HEAR IT?
If not...FRS has a problem as many locations actually have a very low actual noise florr in there area at certain times of the year.
Will I copy the guy at S3 if all the lower the 6400 I have will go is S5???
Thanks,
Lee

Bill -VA3WTB

Yes, the s meter is telling us what RF is being detected in the FFT bins. so it says s something. This is not the signal we hear when an antenna is connected. The Flex can hear below the noise floor, that is why it is so good for low signal work.