S Meters

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

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): -73 dBm (50 uV into 50 ohms) VHF bands (>144 MHz): -93 dBm ( 5 uV into 50 ohms) 30-144 MHz: Not defined Defined an S-unit to be a difference of 6 dB

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

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

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

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”

Receiver Sensitivity

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

*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

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

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

Receiver Performance (Rob Sherwood) MDS

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

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

How Important Is Receiver Sensitivity dB Above Thermal Noise

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

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) -118 dBm (no preamp) -138 dBm (preamp 1) -135 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

Demonstrations Receiver Noise Floor (MDS) S Meter on K3S Panadapter Spectrum Audio CW SSB S Meter on K3S Panadapter S9 = -73 dBm (50 uV) S5 = -97 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!!!