Calibration of SLX signal meter
The SLX DTTV signal meter has a set of four level-indicating LEDs. To calibrate the frequency response of the meter I measured the signal level that just caused the ‘70dBuV’ LED to light. The meter is said to work down to 40MHz so I did an initial measurement that extended down to 30 MHz.
The above graph shows the results of this measurement. Note that two plots are given. One shows the levels when the ‘receiver’ socket on the SLX meter is left unconnected. The other shows the levels when this meter output is loaded with (approx) 75Ohms to simulate the meter providing a passthough signal to a receiver.
Looking at the results we can see that although, overall, the meter under-reads the indicated power by around 10dB it can serve as a useful power meter for frequencies down to around 40MHz. The good news is that the meter is therefore useful for measuring signal levels for purposes like VHF/FM and DAB. The bad news is that this also means the meter may give misleading readings if unexpected signals are being fed down from an antenna. e.g. It may include TETRA or other inputs when the user is trying to use the meter to measure DTTV levels.
I also made a set of spot measurements at the centers of the DTTV channels, and the results are shown above as a more detailed result for the DTTV region. Note that in this case the results are plotted for when the receiver output of the meter is loaded. You can see that the meter under-reads by about 5 and 10 dB over this band. Although the above detailed measurements all used the 70dB LED I also checked the other LEDs. These under-read in a similar manner. So they do seem to be within a dB or so of being 10dB apart, but consistently under read.
