S-UNIT DISCUSSIONS

   S-UNIT DISCUSSIONS
Compiled From The Internet

ORIGINALLY POSTED ON THE FINE WA4PGM SITE

In article 
Gordon Pritchardwrote:

I've only heard of this 6dB/S-unit by way of *rumour*, too. I've followed this discussion for

a bit, butdon't know if anyone has mentioned it already. 

The 6dB/S unit figure is covered in the 3B question pool.  It's never been clear to me which

text isused as a refrence for any of the questions, but I presume (hope, actually) they are

taken from some

sort of reliable engineering text.  I think the pool needs a Bibliography!

Jim --KE6JPO
___________________________________________________________________________

Greetings to All that read this from VE7TMA:

The ARRL handbook refers to an old standard of  S9 calibrated at 50 microvolts RF input and

6 dB perS unit down from that reference point.

The section also clarifies this by stating in reality there is seldom two rigs that follow the

sameabsolute calibration curve considering different designs and gain variations from

band-to-band.

It is stated that the S meter is useful for relative comparisons on the same rig operating

on thesame band.

Although the accurate S meter calibration subject seems somewhat of a dead issue for the

transceivers that have been manufactured in the past it is interesting to play with the

numbersconsidering the sensitivity of a typical modern rig and the calibration reference

of S9 for a 50uVRF input.

I ran a little program on my calculator to calculate the RF input voltage levels for a  S meter

scales of 1 through 7 dB per S unit  with S9 set to 50 microVolts as a reference for each
calibrationcurve.

The following table gives the results:

            ___________/\________________________
                                         dB PER S UNIT                                                                             

S mtr|    1dB       2dB      3dB      4dB      5dB       6dB      7dB
=== |    ===   ===     ===      ===     ===     ===     ====

S9   |    50.0      50.0       50.0     50.0      50.0      50.0       50.0
S8   |    44.6      39.7       35.4     31.5      28.1      25.1       22.3    
S7   |    39.7      31.5       25.1     19.9      15.8      12.6       9.98   
S6   |    35.4      25.1       17.7     12.6      8.89      6.29       4.46
S5   |    31.6      19.9       12.6     7.92      5.00      3.15       1.99
S4   |    28.1      15.8       8.89     5.00      2.81      1.58       0.89
S3   |    25.1      12.6       6.29     3.15      1.58      0.79       0.40
S2   |    22.3      9.98       4.46     1.99      0.89      0.40       0.18
S1   |    19.9      7.92       3.15     1.26      0.50      0.20       0.79
S0   |    17.7      6.29       2.23     0.80      0.28      0.099     0.035

        \____________________________________________/

                          RF  INPUT LEVEL IN MICROVOLTS RMS

It is interesting to consider the sensitivity of a typical modern HF transceiver. I checked the

sensitivity specification  for three modern SSB transceivers and found it varied from 0.20

to 0.25 uVfor a 10 dB Signal\Signal + noise ratio.

If one were to consider the sensitivity specification as roughly equivalent to an S zero
thresholdthen it would appear that a calibration factor of 5 dB per S unit is the closest

round number fit tothe10 dB S/S+N sensitivity  with a calibration reference of S9=50 uV. 

The 6dB per S unit curvewouldresult in a signal to noise ratio of about 3 dB S/S+N which

most likely would be below thereceiverAGC threshold (which is typically utilized to drive

the S meter circuit) which would tendto beimpractical to implement.

A  nice quality of a 6dB/SU curve is that the RF input voltage doubles for each S unit increase

ofsignal strength (representing increase of input power of 4 times for each additional S unit)

whichwould be apractical rule-of-thumb to remember.

I included calibrations in the table down to 1 dB per S unit as the original message posted on

thissubject thread made mention of a rig with a 1 dB/SU S meter response. It is interesting

to note thatif S9 were equal to 50 uV on that scale (which it probably is not) the S/S+N ratio

of an S0 signalwould be about 48 dB which is almost equivalent to the noise performance of

atelephone circuit!

That S meter scale would be of little practical value. I have seen similar calibration

characteristicsin a multi-mode 2 meter transceiver that I own. It is reasonably well

calibrated in the SSB modebut pretty hopeless during NBFM operation.

This is a case were the S meter calibration characteristics change drastically on the same

rig whenswitched between operating modes!

I think the current technology could support an improvement in the calibration of S-meters.

For example the National SemiconductorNE604 IF amplifier ICprovides an accurate signal

strengthlogarithmic output that closely tracks the input signal level over a wide dynamic

range that couldpossibly be used for driving an S meter circuit. Considering the consistency

of surface mount andwide band technologies coupled with the ability to program multi band

calibration factors into themicroprocessor S meter firmware it should be practical for

manufactures to now provide reasonableS meter calibration accuracy.

An S meter tracking accuracy specification should be included as part of  the overall

specifications.This specification would tend to take marketing biases out of the S meter

calibration scale.

I hope the manufactures take steps to clear up this age old guess meter confusion  so that

radioamateurs have the ability to routinely measure signal strengths in a reasonably

accurate mannerin the near future.

Possibly the amateur radio community may need to lobby the manufactures to give the

issue somepriority.

73 de VE7TMA 
____________________________________________________________________________________
S-Meter microVolts vs S-Units
Based on the accepted standard of 50uV = S9

MicroVolts      S-Units
   0.2                 S1
   0.4                 S2
   0.8                 S3
   1.6                 S4
   3.2                 S5
   6.3                 S6
  12.5                S7
  25.0                S8
  50.0                S9
 158.0              S9+10dB

---

On The Other Hand -- Here is what some others measured
Readings on my IC746 are:
S1-2 1dB
S2-3 2dB
S3-4 2dB
S4-5 3dB
S5-6 3dB
S6-7 5dB
S7-8 5dB
S8-9 5dB
s9-9+10 10dB
+10-+20 10dB
To nearest dB - checked overall by dropping from S9 to S1 - total attenuation is within 1dB of

the sum of the increments.

Pre-amp seems to have about 12dB gain - at least on 17M.

---

Here are measurements of my Icom 730 S meter on 40 meters with preamp out:

S1 - 2 1.4 dB
S2 - 3 1.3 dB
S3 - 4 1.6 dB
S4 - 5 2.3 dB
S5 - 6 1.8 dB
S6 - 7 3.2 dB
S7 - 8 3.1 dB
S8 - 9 4.0 dB
S9 - S9+10dB 5.6 dB
S9+10dB - S9+20dB 7.3 dB
S9+20dB - S9+30dB 6.6 dB
S9+30dB - S9+40dB 10.5 dB
S9+40dB - S9+50dB 11.3 dB
S9+50dB - S9+60dB 13.5 dB

---

Another reports:

There is NO standard for S units, or even for S-9. There are a bunch of suggestions, but

that's all.The suggestion the ARRL makes varies with frequency.

Most of my receivers, when designed, tried to use about 5dB per S unit. They have various

S-9calibration points. Very few receivers attempt to use 6 dB per S unit compared to the

one's using5dB or less.

My dozen or so receivers (Drake R4C's, IC-751A's, Yaesu FT1000D, Collins 75S and KWM-2,

etc)all range from about 1 or less dB per S unit at S1, to maybe 3 or 5dB per S unit near S-9. 

---

Maybe time forNational SemiconductorNE604 IF amplifierICProject!

 

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