Saturday, April 24, 2021

MDS measurement of IC-705

Gigatronics 2420M sig-gen with 40 dB of attenuation on output

A Gigatronics 2420M just back from being calibrated baited me with its freshly-attached green 'PASSED" tag. We use these heavy pieces of gear at frequencies much higher than the IC-705's range so I was curious to put the two together at HF to 433 MHz and see what the results might be.

My main reason for wanting to do this is that the IC-705 Owner's Manual specifies receiver sensitivity in "dBuV at a 10 dB S/N ratio".

This makes no sense to me and does not conform to ARRL reviews of ham/SW receivers. Receiver sensitivity is more commonly measured in dBm and, although I could convert Icom's values to dBm, I would still end up with the theoretical (advertised) rather than the actual values for my particular radio.

I made some measurements outside the ham bands for ranges of frequencies that are of interest to me - the weather band and civilian aviation (the military aviation band is not covered by the IC-705).

Also, the IC-705 receiver covers two broad ranges: 30 kHz to 200 MHz and 400 MHz to 470 MHz. Two of the measurements are the top of the lower range and the bottom of the upper.

In all cases, bandwidth is set to 500 Hz and receive mode to CW.

Here is the page from the Icom manual. I am familiar with the parenthetical (0.15 uV) value specifying receiver sensitivity but the dBuV unit is a new one to me. Either way, I prefer MDS in dBm since it makes more sense for a variety of reasons:




  1. uV at a 10 dB S/N ratio (at 50 ohms) is historical in nature. If you look at ads in old radio magazines from the 70's and earlier, you will that being used. I recall figures of 1 uV or .5 uV being considered good sensitivity for SSB/CW HF receivers.

    dBuV? That's a bit out of the ordinary. Makes me wonder if that is a misprint.

    1. I've added the page from the manual that shows the specs in this odd value. uV can easily be converted to a dB ratio (20 log * Out/In) so I guess dBuV is a roundabout way of expressing the same thing. Just curious why they chose that unit rather than the more universal (for ham rigs) dBm.