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4.10 - S-Meter

S/agc/dBm-Meter (model 1)

The S-Meter is precisely calibrated for use with a 50-ohm impedance antenna when using an RFSPACE SDR. S9 indicated by the moving triangle indicates 50 microvolts of signal is being applied to the input of the SDR. dBm readings are available at the lower left corner of the S-Meter. Readings in microvolts are available at the lower right corner of the S-Meter. A bar at the bottom of the S-Meter shows you how much gain the AGC subsystem is applying.

Note: I have made an effort to achieve as much accuracy as possible with SDRs other than those from RFSPACE, but this has proven to be quite a challenge. Please consider such attempts a work in progress.
Tip: In the dialog accessed with SDR, you will find   Post-Attenuator S-Metering . The default is unchecked, which means that the S-Meter should read the same (assuming the attenuation in the SDR is accurate) no matter what attenuation you select. This behavior is effectively telling you the signal at the antenna jack. If you check the box, then the S-Meter tells you the signal strength after the attenuator.

If you're not using an RFSPACE SDR, you can calibrate S-Units using ^ D or by right-clicking PEK (some operating systems do not allow the use of ^ D, in which case use Ref to change the key assignment.) A value of 500 equates to no correction. Less than 500 reduces calibration, more increases calibration.

Note: In scope mode ( SCP ), the oscilloscope at the bottom right displays the AGC and S-Meter values in channel two, along with a guide for squelch setting, and, if SSP, the hysteresis band for that. Very handy... well worth checking out.

With SsM SdrDx can display a set of S-Meter referenced rules on the spectrum portion of the display in place of the standard 10 dB rules. This allows quick estimation of s-levels for signals visible on the spectrum, but not tuned in. The brightness of these rules is set by GRT

Caution: When ANF is on, the spectrum s-meter indication enabled by SsM is no longer accurate; the same is true for the absolute dB reference indication when SsM is off. Relative dB indication, however, remains accurate.

S-Meter (model 2)

Unless... you left-click on the S-Meter. In which case, you get a different kind of S-Meter. Try it. Like that? Click again. The additional green needle is AGC. You can adjust AGC needle deflection using  D. The types and variations of S-Meter model switch with each click, and loop around once all modes have been shown. There are more models. Keep clicking! Also, you can right-click to go backwards.

S/agc-Meter (model 2b)

The initial S-Meter model can provide different types of information for you. First, on top, is the classic "S-Meter" display. This varies from from S1 to S9, and then 10 dB over S9 to 60 dB over S9. This is a calibrated S-Meter; the input impedance of the SDR is 50 ohms, and the A/D acquiring the data is precise, so you can be sure of the reading. Remember that your antenna is a significant factor in this: a signal that is S9 on one antenna may only be S7 or S5 on another.

Note: Internally, although responding to S-Meter levels, AGC levels vary widely depending on the SDR sample rate / bandwidth, AGC time constants, and other factors. Because of this, quite often, the AGC indication on a meter will be to the right or left of the S-Meter needle; they weren't really intended to match.

Despite this, you can correct for this over/under tendency at a particular signal level by pressing  D and adjusting the slider in the resulting dialog while observing the AGC indication.

Ideally, make the adjustment in the presence of signal (so as to match peak readings) or with the AGC response (DCY) set to 250 ms or less (so as to get the indications to follow one another quickly.)

S-Meter (model 3)

Another thing to be aware of is that when you set the SDR attenuator, the S-Meter is adjusted to compensate: So, for instance, if your signal is S9 at 0 dB attenuation, it will still read S9 at 10 dB attenuation, even though what reaches the rest of the radio software is in fact 10 dB weaker. Just remember: The S-Meter is telling you what is arriving at the antenna input. Not what comes out after the attenuator.

S/agc-Meter (model 3b)

The red and black bar in the first S-Meter model is an AGC state indicator. It shows you by how far the red bar extends towards the right, how much absolute gain is being applied to the signal. The weaker the signal, the more gain; also, the further DCY is to the right, the slower the decay.

S-Meter (model 4)

Third, at the bottom left, is the signal level in dBm. Again, this is precisely known because of the known impedance of the SDR itself, and because of the precision of the D/A. dBm is measured from a reference level of .224 volts across 50 ohms for 0 dBm.

S/agc-Meter (model 4b)

Fourth and last, at the bottom right, is the signal level in micro-volts. This tells you in the most familiar electronic terms (voltage) exactly what your antenna system is presenting to the SDR for any one signal. Precision is to a 10th of a micro-volt.

4.10.1 - VU Metering

This meter is measuring audio output from the current demodulator engine within the bounds of 16 bit amplitude, where +3 dB is absolute maximum amplitude, which is +/- 32767 levels; however, because the signal processing chain can analyze (and process) the audio before it sends it along, AGC is adjusted prior to output, and this reduces the system gain to put the max audio at exactly 0 dB.

So you're looking at the consequences of instantaneous limiting past 0 dB, and linear response up to 0 dB based on current AGC levels.

The reason SdrDx stays at, or below, 0 dB (technically, the audio processing could push right to max without ever distorting) is to leave 3 dB headroom for post-audio processing such as that provided by Dream, Multimode, fldigi, Audio Hijack Pro with SoundSoap, VST effects and so on.

S/Vu-Meter model 5

S/agc/Vu-Meter model 5b

S/Vu/DNR-Meter model 6

S/agc/Vu/DNR-Meter model 6b

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