S Meter Reading after Service

Mike-VA3MW

Hi All

Since this thread produces a lot of email, can we keep to the topic please.

many thanks

Erik EI4KF

Anyone receiving a report in dBm who is not a SDR user is unlikely understand it and therefore the sender is, in effect, being esoteric. Also try to enter dBm into a logbook program and then perhaps you'll see that you are not really communicating efficiently with your QSO partner. Yes, as Burt says, S units are contrived but they have a communicated meaning.

Mike-VA3MW

Joseph

We can't really compare the IC-7300 to the Flex as we have no way of known what math Icom uses in their S-Meters.  

I would not be surprised that their is some **** (either physical or virtual) that is part of the alignment procedure that says.   

" VFO to 20M, connect dummy load, adjust S Meter to a reading of Zero. " 

I have aligned a LOT of radios in my lifetime and still have the tuning sticks to prove it.    This is not an uncommon step.

As an example, this is the procedure for the FT-897.  

SSB S-Meter Adjustment

a. Tune the transceiver to 21MHz band on CW mode. Inject a RF signal from a signal generatorat 36dBu output.
b. Set the parameter “SSB-S9” at the value of lighting the 6 dots of the S-meter (S9) on the LCDby rotating the main dial.
c. Tune the transceiver to 21MHz band on CW mode. Inject a RF signal from the signalgenerator at 86dBu output.
d. Set the parameter “SSB-FS” at the value of lighting all the dots of the S-meter on the LCD byrotating the main dial.

We aren't comparing apples to apples at this point.

Howard's description is about one of the best I have seen while trying to keep it high level enough.

Mike

Mike-VA3MW

Hi Eric

You also can't enter 20 over s9. 

The S in RST has nothing to do with an S Meter at all

The S stands for "Strength". Strength is an assessment of how powerful the received signal is at the receiving location. Although an accurate signal strength meter can determine a quantitative value for signal strength, in practice this portion of the RST code is a qualitative assessment.

1. Faint—signals barely perceptible
2. Very weak signals
3. Weak signals
4. Fair signals
5. Fairly good signals
6. Good signals
7. Moderately strong signals
8. Strong signals
9. Extremely strong signals

John - K3MA

Joseph Rodick now you on the right track.  Put the S meter argument aside and compare the rx sensitivity on the Flex side by side with the 7300 on the same antenna listening to the same weak signals at the same time.  After a short period over several samples you will know if the Flex receiver is more, less or the same in sensitivity as the 7300.  If you do not find it to be more sensitive or the same sensitivity as the 7300 then something is indeed wrong.

I have compared several Flex radios this way between at least a dozen different receivers and have never found a signal I could not hear on another radio and not the Flex.  Although I have found some low priced radios that people do not give credit too for their excellent receivers that on the bands receive as well as the Flex.  Maybe they do not get as good results on the test instruments but they sure do when listening for weak DX signals.

Joseph Rodick

Right now on 80 meters with a shorted RF input my S meter is reading S6.  So if I hear a station and the S meter goes to 7, do I give a report back that they are an S1 orS7? 

Joseph Rodick

Thank you Michael, you are right on the RST.   However, I use RST only for my CW use.  On phone, I usually give an S meter report unless they are distorted or over driving.

Joseph Rodick

Later this week I am going to do a weak signal test using my 6400M and an Icom 7300.  However, I do feel confident Flex service did a good job and that my 6400M will be shown to be the better of the two.

Michael N3LI

An actual calibrated and accurate S-Meter is apparently disruptive technology!

This topic is one of the most badly beaten one on this reflector.

One of the first things that comes to my mind is why it is so important that a signal strength meter reads 0 without an antenna. Looking at the display, and with narrow bandwidth, the Signal meter reads as close to the designated value as can be determined when looking at noise floor and eyeballing it. 

Even so, unless one's hobby is reading noise floors, as Shania Twain noted, "That don't impress me much." 

So What on earth should a company making an SDR do? Provide a special inaccurate and uncalibrated setting in order to keep those who insist on a Japanese style inaccurate and uncalibrated Signal strength meter? That sounds so strange, but that is what the demands are.

Then guys like me would howl because I'd look at the right hand side of the panadapter and notice that the dbm of the signal isn't related to the S-Meter. 

This is a technical improvement plain and simple.  I'd no sooner want an old school inaccurate S-Meter than I would want an old school drifting VFO.

I think the major problem is getting people used to S-Meter accuracy. It will take a while, because so many don't know the bogosity of legacy radio S-Meters.

Mike-VA3MW

Hi Joseph

Shorting out the antenna is not the right way to make this measurement since it provides an improper load to the receiver.  Receivers also require matched loads.  

You have to do it with a dummy load.  

I have a pretty quiet HF station, so at local noon on 80M, my noise floor is about S4-5 with +32 dialed in.  And, the same with -8db (attenuation).

It was not until SDRs came into the Mainstream that S unit reporting was done correctly.  Prior to that, it was an approximation since there was no practical way to do the math in analog circuits.  Like Howard mentioned, the meter was driven from a voltage that made an approximation.  Every superhet radio we have used in the past was not worthy of using the S meter as an accurate measurement tool.

To answer your question, all you can do for an SSB station is report on the peak value and based on your comment, that is S7 since that is what you saw/heard. 

I know that is contrary to all the HF radios you have used over the decades.  Welcome to the correct way to measure it and that is the actual energy in the passband.

Mike

Michael N3LI

Also, if a 7300 compares favorably to a Signature series radio in the upcoming tests there is definitely something wrong with the Flex. 7300's are really cute and inexpensive, but after using one during Field Day and during a SET, I compare it to other entery level radios. 

Michael N3LI

dbm is not contrived. It is a very specific level as measured at some point in the receiver.  A calibrated and accurate S-Meter reads a specific level at a specific place in the radio.

We'll assume that the sensing point on the radio is appropriate.

The antenna is then a variable, but if we compare a "good" antenna to a "poor" antenna, the good one simply provides a larger signal to the place that is measuring the dbm.

The poorer antenna is simply providing less signal to be measured.

This in fact is how we determine some of the characteristics of an antenna. And to do this, you need a calibrated and accurate meter system.

I first learned of the terrible innacuracies of the legacy meters when I was  making a test of vertical versus dipole antennas on my venerable IC-761. Using a calibrated pad to knock a signal down while I was setting up, the amount of attenuation needed to put the needle on the expected S level had nothing to do with the proper padding. 

Near S-9 it was fairly close to the proper 6 db per S unit, but Below S-5, it was closer to 2 db per S-Unit,  And above S-9 was around 8 db. 

So the verdict was a really nice radio with a not so accurate S-meter. After doing some research, I concluded that this was actually pretty normal for Amateur equepment at the time. 

Rick Ciotti W3DIY

With my 6600M set exactly as pictured the S meter reads slightly above S3. With a dummy load, coax shorted or no connection to the antenna port this reading is unchanged. With the 40M monoband antenna the reading is S4. 

Burch - K4QXX

I have never understood why I would care if my S meter read 0 when I don't have an antenna connected.  I can't use the radio without an antenna so why would I care what the S meter reads? 

FISHULA X

So riddle me this.  On my 6300,  lets say on 40 meters  LSB , why when I have an antenna connected and have a noise floor of S 4 or so and then I engage the RF gain, the noise floor stays  pretty much the same but,  when somebody is talking it does not change the signal either other then peaks however, when I engage the RF gain, It makes the person talking more clearer,  less space between us, or as if I instantly got closer, a better arm chair copy. Or as if I had a beam and locked on the signal. All of this done with no significant increase in signal . Perhaps this is the same issue and the topic is related.   I feel that the S meter is just  a point of reference.  Everyone's station and situation is different . I focus on the important things like  ((((((ONE OF THE BEST RX ON THE PLANET)))))))  I have owned many radios and live in a noisy area. The 6300 is thee best RX I have had to date. So when It comes to a perfect S meter reading as I said, It's a reference point. An approximation at best.

Lawrence Gray

On my 6500, LSB, 80 meters, bandwidth 2.1, I see -114 dBm ("S2") with no antenna connected, with the antenna input shorted, and with a dummy load connected.  This is with the preamp setting at 0.  With the antenna connected, I see -102 dBm ("S4").
  
In my view, a "no antenna" noise level of S6 is an indication of local noise?

Stan VA7NF

I have stayed out of this until now - Same players, same ignoring facts.

Joseph, I will take your request and questions as not understanding what is happening, so this "Season 2 - Episode 4" of S-Meter Chronicles follows:

The Theory:  1) The front end component of a Direct Connect (directly connected to the antenna and PreAmp) SDR radio is a analogue to digital converter that captures the exact voltage as over 2,000,000  binary numbers per second (avoiding the conversion details and math involved)
2) Every A2D has both maximum (overload) and, significantly in this case, MINIMUM sensitivity levels (binary 0 numbers produced)
3) On all bands, more so as the received frequency is reduced (some at 40M and more so at lower frequencies - check this out on the BC band).

Your Case:  1) Your Maestro is set at -8db attenuation which is delivering signal to the A2D converter SIGNIFICANTLY below the MINIMUM THRESHOLD (more to follow).
2) While this is great for normal signals, you are complaining about the noise floor or very low level signals and the S-Meter reading.
3) Raise the amplifier gain and watch your "noise floor" SEEM to drop

Repeating this on my 6700:
Table of Readings at 7.000 Mhz, CW 500Hz bandpass (standard filter), No Antenna:
Preamp  dbm  S-Meter  Delta            Notes:  
 -8           -106    S3         N/A             Noise < A2D minimum threshold
0              -119    S1      (-13)              Noise level still < A2D minimum
+10         -126     S0.5   (-20 / -7)        Noise level still < A2D minimum
+20         -133    <S0     (-27/-14/-7)   Noise level NEAR or ABOVE A2D minimum
+30         -133    <S0     Same as above  Gain now OVER minimum

This demonstrates that the REAL NOISE level is around -133dbm, <S0.  The -106 level shown at 8db attenuation is STRICTLY because you have chosen to extend the A2D dynamic range in favour of sensitivity to very low strength signals.

So, if you want extreme sensitivity then raise the PreAmp until those signals are within the A2D passband.  If you want more tolerance for very near stations then reduce the gain.  If you want a quieter background then modify the AGC-T, likewise adjust the AGC-T to the sweet point (see other online postings) where the A2D actually expands the gain of the near noise signals. 

Mike-VA3MW

Stan

Thanks for posting that.  I had just got off the phone with Eric in engineering who explained the same thing to me, in a slightly different way.  And, that the PreAmps lower the noise floor resulting in a lower S meter reading.

Nice work,

Mike va3mw

Bill -VA3WTB

FISHULA, Your radio is taking the signal from the antenna port and converting that into v. And that = S meter information. In almost all conventional radios the signal is through several circuits, the main one is the AGC.

On your Flex the S meter reading peaks are **** on with the signal that is picked up at the antenna connection. It is not an  approximation.

N2WQ

The purpose of the S-meter is to measure the voltage applied to the antenna input. The purpose of the S-meter is NOT to measure the internal noise of the AD converter. All that talk about decimation and bins ignores the fact that when the input is shorted the S-meter is showing the internal AD noise, not the input voltage, that we all know is ZERO.

So, yes, the S-meter is dead wrong if it shows anything but ZERO with shorted input.

Mike-VA3MW

Sorry Rudy

That is not the way it works.  Read Howard's and Stans's comments above.

N2WQ

Yes, I read it. But all that ignores the basic fact that Ohm’s law applies to Flex as well. When the input is shorted the input voltage as seen by the AD is ZERO. The reason the AD reports values other than zero is because of the noise associated with digitization and the noise present in any semiconductor component. I assume we humans don’t care about the noise and want to measure the voltage applied to the input. A short equals zero and, personally, I don’t care about what the AD is telling me.

Stan VA7NF

N2WQ - That is not necessarily so.  ALL receivers amplify the difference between the antenna port and the chassis for unbalanced input.
I like to tell my students that the radio will also work if you ground the antenna port and have signal pickup on the ground side of the radio.
In other words, a very good ground system will reduce the actual received noise as the ground wire is also an antenna, and for that matter, so is your safety ground connection to your electrical panel.  This short comment could expand into a massive discussion of RF dedicated grounds, ferrites on the safety ground, isolation of towers from RF grounds (never from the electrical panel unless your electrical code permits ungrounded towers over 200'), and liberal application of high impedance (ferrites) on the coax lines

Bill -VA3WTB

Oh no Stan, don't start that!!!!

Mike-VA3MW

What Stan wrote!

FISHULA X

 Thank you Bill for your reply.

KY6LA_Howard

@N2WQ.   You are stuck on the Frozen Thermometer Paradox.   You want to read a temperature below the point where the mercury is frozen solid 

You are also misunderstanding the value of S0 which is BY DEFINITION-127dBm and NOT the total absence of RF. 

So unless you Were able to short the antenna with a Perfect Ground with the entire system at -273K you will still have some signal.  So it definitely follows Ohms law. 

Lionel

Checked today 40/20/10 CW  0.4  0dB preamp, into 50 ohm DL with 2 feet LMR400.
S-meter hover S1 +/- and -118 to -119 dBm.
BW at 2.7 -109/110 dBm or S3 +/-
Same as data taken 6 months ago.  I have a good ground system, a lot of copper and many ferrites. 

So, all seems to be as intended by designers.

S6 seems like noise getting into the rig, maybe on ground, outside of coax shield.  

N2WQ

The OP states that his S-meter reads S5 when the input is shorted. I seriously doubt it that a short at room temperature yields S5. If memory serves me well, S5 is about -95 dbm, I.e. a pretty strong input signal.

Rick WN2C

S-reading    HF        Signal Generator emf
μV (50Ω)    dBm    dB above 1uV
S9+10dB    160.0    -63    44
S9    50.2    -73    34
S8    25.1    -79    28
S7    12.6    -85    22
S6    6.3    -91    16
S5    3.2    -97    10
S4    1.6    -103    4
S3    0.8    -109    -2
S2    0.4    -115    -8
S1    0.2    -121    -14