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Author Topic: Comparing Numbers on Two Receivers  (Read 1676 times)
KC2NLT
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Posts: 94




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« on: August 15, 2017, 08:33:56 AM »

I realize that in a perfect world numbers would be an indication of a receiver's performance, but they sometimes are not.

For the sake of argument, if everything else were equal, which of the following two receivers would you pick?

Receiver 1. Has a sensitivity of 1.5 μV (TYP) for 10 dB SN in the AM band 108-137.
Receiver 2. Has a sensitivity of 0.5 µV (TYP) for 6 dB SN in the AM band 108 - 137.

Can one of the specs be converted so that we can compare them apples to apples?
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WB6BYU
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Posts: 17182




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« Reply #1 on: August 15, 2017, 12:18:59 PM »

If you increase the input signal level for the receiver 2 spec (0.5uV) to the level used in the Receiver 1 spec (1.5uV), that
would be an increase of 20 log ( 1.5 / 0.5 ) = ~10 dB.  One might then hope that the S/N would also increase by 10 dB,
making it 16 dB for Receiver 2 vs. 10 dB for Receiver 1.

While it isn't a perfect comparison, and I'm often suspicious of how closely receivers actually meet their specifications,
that would imply that Receiver 2 is somewhat more sensitive than Receiver 1.


In practice I find that the usefulness of a receiver often depends on other factors.  For example, the sensitivity of some
2m ham rigs when listening on Aircraft AM band is rather poor.  In practice when we have tried to use them to listen for
an ELT, most were swamped by a local FM station whose image fell on 121.5 MHz.
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KC2NLT
Member

Posts: 94




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« Reply #2 on: August 15, 2017, 07:12:01 PM »

Thanks for that explanation.

The numbers came from two handheld units.

1. Yaesu VX-7R amateur radio transceiver .

2. Icom IC-A14 aviation transceiver.

To my surprise, the VX-7R has better reception in the 108 - 137 range.

My only guess is that it has to do with either the circuitry design, or the magnesium construction of the VX-7R housing which helps prevent interference.

Any thoughts?
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WB6BYU
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Posts: 17182




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« Reply #3 on: August 15, 2017, 10:38:09 PM »

You really have to compare them against the same signal to see
for yourself.

Because aircraft signals can carry for long distances at altitude,
many aircraft transceivers are actually less sensitive than they
might be, on the assumption that they are most interested in
relative strong local signals from a nearby airport rather than
ducted interference from a more distant one:  this would particularly
be the case for a low power transmitter like an HT.  We saw this when
we converted some old 100kHz spacing aircraft radios for use as
ELT monitors: a preamp brought them up to "normal" receiver
sensitivity.

So there isn't necessarily anything wrong with the aircraft band
transceiver:  the receiver sensitivity is probably appropriate for
the expected range and output power of the station to whom
they are talking.
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W1VT
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Posts: 2528




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« Reply #4 on: August 16, 2017, 08:13:37 AM »

It is likely that the aviation receiver has bandpass filters to protect the RF preamplifier from out of band signals.  This adds the loss of the bandpass filter to the noise figure of the preamplifier so the radio is less sensitive.  But, in practice, since there are strong transmitters everywhere--like the cell phone transmitter at your waist--this tradeoff is quite worthwhile if you want a reliable radio that isn't disturbed by nearby out of band transmitters.

Zack W1VT
« Last Edit: August 16, 2017, 08:16:19 AM by W1VT » Logged
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