1. S Meters

2. Original S Unit Definition
Readability
Unreadable
Barely readable, occasional words distinguishable
Readable with considerable difficulty
Readable with practically no difficulty
Perfectly readable
Strength
Faint signal, barely perceptible
Very weak
Weak
Fair
Fairly good
Good
Moderately strong
Strong
Very strong signals
Tone
Sixty cycle a.c or less, very rough and broad
Very rough a.c., very harsh and broad
Rough a.c. tone, rectified but not filtered
Rough note, some trace of filtering
Filtered rectified a.c. but strongly ripple-modulated
Filtered tone, definite trace of ripple modulation
Near pure tone, trace of ripple modulation
Near perfect tone, slight trace of modulation
Perfect tone, no trace of ripple or modulation of any kind
Started out as part of the R-S-T system
In wide use by 1912
Qualitative measure
S0 and below are not defined
Digital modes use R-S-Q system

3. Updated S Unit Definition
International Amateur Radio Union (IARU) Region 1 (1981):
Defined signal level at the receiver for S9 as:
HF bands (<30 MHz): dBm (50 uV into 50 ohms)
VHF bands (>144 MHz): dBm ( 5 uV into 50 ohms)
MHz: Not defined
Defined an S-unit to be a difference of 6 dB

4. Typical AGC Thresholds
The S Meter Scale
Note: Many S meters read S0 when the signal is < AGC threshold
*0 dBm = 1 milliwatt into 50 ohms
Typical AGC Thresholds

5. Sources of S Meter Errors:
On older radios “S9” can be either 50 uV or 100 uV
“S0” can be anywhere from S0 to S6 signal level
Depends on AGC design
Scale not 6 dB per S unit
Some radios designed for 3 dB per S unit
On many radios the S meter readings move:
UP with increasing Pre-Amp gain
Down with increasing Attenuation
Some modern radios do compensate for gain/attenuation settings (Elecraft K3S)
S meter not calibrated properly
Receiver input impedance not equal to 50 ohms
Some hams rarely give out reports less than 5x9 or don’t even look at the S meter

6. Icom IC-756 PROIII S Meter Pre-Amp: OFF Attenuator: 0 dB
Measured After Recalibration*
* Scale re-adjusted to be closer to 6 dB per S unit 8 9 7
+20
+40 6
+60
5.5 7 9
+20 5
+40 3 1 9
+60 8 7
S Meter Scale 6
Measured with Original Factory Setting

7. Icom IC-756 PROIII S Meter – cont’d
Pre-Amp: OFF
Attenuator: 0 dB 9 7 8
+20
+40 6
+60
5.5 7 9
+20 5
+40 3 1 9
+60 8 7 6
“Quietest receiver ever produced”
“I get Q5 copy on signals that don’t even move my S meter”

8. Receiver Sensitivity

9. What Is “Receiver Sensitivity”
Receiver Sensitivity: Signal power needed to yield a specified S/N in a specified BW
Two types of specs used for ham receivers:
10 dB S/N:
Icom: uses 2.4 KHz BW
0.16 uV (SSB/CW) => -123 dBm
2.0 uV (AM)
3 dB S/N in a 500 Hz BW
Called Receiver Noise Floor or Minimum Detectable Signal (MDS)
Rob Sherwood uses MDS
Elecraft K3S:
-135 dBm
8 dB below S0
-145 dBm with Preamp 2 (only for 12M thru 6M)
2 dB above thermal noise floor (best performance of all receivers on Sherwood’s list)
Measurements are actually (S+N)/N

10. *AGC can’t be turned OFF on many receivers
Test Setup For MDS
Receiver
BW =500 Hz
AGC Off*
Pre Amp Off
Atten = 0 dB v
Signal
Generator
Power
Meter
Atten RF IN
Audio
Out
*AGC can’t be turned OFF on many receivers

11. Types Of Noise
Thermal noise: Generated by the thermal agitation of electrons
Does not vary with frequency
Varies with temperature
System noise is never less than thermal noise
Atmospheric noise: caused by lightning discharges in thunderstorms
100 lightning strikes/sec world wide
Varies with frequency, time of day, location, time of year
Man-Made noise:
Varies with frequency & location
Extraterrestrial (Galactic) noise:
Varies with frequency

12. Thermal Noise
Thermal noise power = KTB = -174 dBm/Hz at room temperature (290o K)
Power spectral density
Independent of frequency CW
SSB
Freq

13. Receiver Performance (Rob Sherwood)
MDS

14. Receiver Performance (Rob Sherwood)
MDS
2 dB above thermal noise

15. Receiver Performance (Rob Sherwood)
MDS
29 dB above thermal noise

16. How Important Is Receiver Sensitivity
dB Above
Thermal
Noise

17. How Important Is Receiver Sensitivity – cont’d
dB Above
Thermal
Noise S9
-107 dBm S3 S0
Thermal Noise Floor
(-147 dBm/Hz in 500 Hz)
160 M
10 M

18. Sensitivity vs. Dynamic Range
Design tradeoff between sensitivity and dynamic range
Common solution is to use multiple preamps and attenuators
K3S MDS Flex 6700
-135 dBm (no preamp) dBm (no preamp)
-138 dBm (preamp 1) dBm (with preamp)
-145 dBm (preamp 2 - only used for 12M thru 6M)
Audio gain vs RF gain
With receivers that have a good AGC design, and
For some conditions:
Strong signal to noise ratio
Low QSB
No lightning crashes
Set AF gain near max and adjust volume with RF gain
Significantly reduces background noise

19. Demonstrations Receiver Noise Floor (MDS) S Meter on K3S Panadapter
Spectrum
Audio CW
SSB
S Meter on K3S Panadapter
S9 = dBm (50 uV)
S5 = dBm (24 dB below S9)
S1 = -121 dBm (48 dB below S9)
Interference mitigation
Feb ‘18 QST: FCC study => receivers (not transmitters) must deal with interference!!!