Topband: Re: Minimal discernable signal
Tracy, Michael, KC1SX
mtracy@arrl.org
Thu, 11 Jan 2001 18:15:14 -0500
> The 20 kHz or so spacing for intermod tests is useless. We need to
> know how close can 30 over 9 signal get to received frequency before
> it starts bothering the reception.
Ah, but you missed something here, Yuri. A decent high-end rig will have a
sensitivity of about -140 dBm. It would also have an IMD dynamic range of
about 100 dB at a 20 kHz spacing. That would be a 30 dB over S9 signal
(assuming Collins standard for S-meters).
In other words, a strong signal doesn't have to be within your roofing
filter to be a problem. In fact, the "ultimate" DR of a rig is usually only
a few dB better than at 20 kHz spacing. On the FT-1000MP, the IMD DR was
100 dB at 100 kHz spacing, so multiple S9+30 dB signals could create IMD
products in your receiver from anywhere in the band.
But back to Tom's point. I went through a number of swept test data and was
surprised to find that my generalization was not true in all cases. Here is
the 5-kHz data for the rigs I checked:
(In alphabetical order...)
Rig BDR IMD DR
Elecraft K2 126 88
ICOM IC-706 MkII G 86 74
ICOM IC-756 98 67
ICOM IC-756 Pro 104 80
ICOM IC-775DSP 104 77
Kenwood TS-570D 87 72
Ten-Tec Omni 6+ 119 86
Yaesu FT-1000MP 111 83
Yaesu FT-1000MP Mark V 106 78
In the vast majority of cases, the numbers are less than at the 20 kHz
spacing, but the amount of variation between similarly priced radios was
unexpected.
Note that some of the above rigs would experience a _blocking_ response from
an S9+30 signal that was more than 5 kHz away.
You can expect that I will be examining this issue further.
73, Michael, KC1SX
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