This is a section of the User's Manual, Chapter 4 -DSP-10 Software Setup

RF Gain Calibration

Traditionally, S-Meters are crude indicators of signal strength. One of the goals of the DSP-10 was to improve on this! To get the best accuracy in signal strength readings, it is necessary to calibrate the RF Gain control and possibly the overall gain.

The steps of the RF gain conrol are shown on the screen as their nominal value of 6 dB per step. The actual corrections used for the S-meter and power-meter functions are the measured values of RF Gain. These are obtained from the UHFA.CFG configuration file . To adjust their nominal 6 dB steps to precise values, the following procedure can be followed:

1 - Go into the UHFA.CFG configuration file and change the rfgdb entries to 0.0.
The line should be: rfgb 0.0 0.0 0.0 0.0 0.0 0.0
2 - Put the mode to FM (ALT M) and tune in a strong steady signal in the -70 to -85 dBm range. It is best if it comes from a signal generator, but off-the-air signals can gve satisfactory results.
3 - Step through the 7 RF Gain settings (ALT F7 or ALT F8) and make a note of each reading. Add a last column that is the difference in the reading and the reading obtained with the "100" setting of RF Gain. My table of measurements looked like this:

Setting   Reading   Difference from "100" value
-------  ---------  ---------------------------
 100       -77.33     Reference point 0.0 
  94       -80.63           3.30 
  88       -85.13           7.80 
  82       -91.11          13.78 
  76       -96.07          18.74 
  70      -100.92          23.59 
  64      -105.26          27.93

4 - The 100 point (-77.33) is my reference level and for now is assumed to be correct. We want to make the 6 entries in rfgdb such that we will measure -77.33 at all RF Gain settings. For the "94 setting" the measurement is 80.63-77.33=3.30 dB too low. To correct this, the first number in the configuration line will be -3.30. Then continue this process for all six correction numbers. (ALL ENTRIES ARE NEGATIVE) Enter them into the UHFA.CFG file. My new line looked like:

  rfgdb -3.30 -7.80 -13.78 -18.74 -23.59 -27.93

5 - If you retest on the signal of step 2, the indicated signal strength should hold to a small fraction of a dB as you change the RF gain.

6 - If you have an accurate signal generator available, you can improve the absolute accuracy by gain_adj entry in UHFA.CFG. My readings were about 0.7 dB higher in level than the signal generator, i.e., when I measured -77.33 the generator said -78 dBm. Making the assumption that the signal generator was correct, I entered gain_adj -0.7 into the UHFA.CFG file in place of the existing entry, and now my DSP-10 agrees with the signal generator. If you don't have an accurate generator, go with the default 0.0 and you will probably be more accurate than your Icom, anyway.

The principle that makes this work is the high linearity of the Delta-Sigma A/D converters. Our procedure uses this to measure the difference in signal strength and with some care, the results are suprisingly good. There will be temperature variations, so don't expect the accuracy to be maintained if you are operating out-of-doors. Maybe someone will measure the change with temperature and report it back to us.

The bars on the S-Meter bar-graph are honest-to-goodness "6 dB per bar" levels.

Also, if you use transverters or preamps, you can correct the S-Meter readings for the added gain in the TVTR entries of the .CFG file.

Have fun, and try giving your signal reports in dBm, to keep people thinking!

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