How does my FTdx5000 S-Meter track
The following describes the process of getting to know our S-Meter.
To test the S-Meter we need some test equipment. My test equipment consists of an Elecraft XG3 RF Signal Source, a 50Ω Step attenuator and commercially made 3, 6 & 12dB attenuators.
The step attenuator is home build into a commercial enclosure from Texan. Using the XG3 , the step attenuator and a loan LP-100A power meter, I measured the following:
Good enough for amateur tests.
For the S9 to S1 tests I used the -73dBm generator output (Test1) and for the S9+40 to S9 tests the -33dBm output (Test2).
Test1
Test2
To change the S-Meter reading by one S-Unit (6dB) we add 6dB to the attenuator (ATT), which drops the input signal from -73dBm to -79dBm (-73 - -6 = -79) which equates to an S-unit of 8 (S8) according to IARU recommendations.
Test1 - 6dB
For each measurement we increase the attenuation until the meter needle rests on the next lower S-Point.
The main receiver has been set up as follows:
no VRF, no ATT, AMP1 on, SSB, Width 3kHz, AGC fast.
The following values are from the main receiver of the FTDX5000, the first checks have been done at 10m and then another rounds of checks at 40m.
The ATT column indicates how much attenuation I added to read the S-Value for that row. i.e. 10m, -73dBm signal injected, added 3dB ATT to read S9 which then equates to a signal of -76dBm or 31.0dBµV or 35.5µV.
The graphs below show the gathered values for of the main receivers (RX1) S-Meter at 10m and 40m. These graphs only show the recorded values using the -73dBm signal source. It can be seen that the S-Point difference is only 3dB instead of the recommended 6dB, however the meter is exceptionally linear across its S1 to S9 range.
On a side note I've checked the built in attenuator and selecting 6dB attenuation drops the S-Meter by two (2) S-Points, selecting 12dB drops it by another two and at 18dB it has dropped the S-Meter by six (6) S-Points.
I'm unsure about this 3dB per S-Point business and would like to know the reasoning behind this? Maybe a YAESU engineer could shed some light on to this. But why is the reading that high, at S1 around -100dBm. Even adding AMP2 into the mix, which adds 12dB more gain and will only drop the S1 point to -112dBm.
Looking at the charts, the blue line is the IARU recommendation for S9 = -73dBm/50µV with 6dB increments per S-unit.
So, now that we know how the S-meter tracks we should be able to give more meaningful S-unit reports.
Test set up