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Message |
    
Triplecguy
Intermediate Member
Username: Triplecguy
Post Number: 224
Registered: 12-2001
| | Posted on Tuesday, September 04, 2007 - 1:34 pm: | 
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Hi guys, just wondering if anybody can tell me what the senisitivity should be on my Midland 79-290. On most of the commercial FM radios I work on we like it about .13-.19 microvolts. My Midland is much higher at about .65 microvolts. This seems really high to me. I have the channel guard in it and I was thinking that I may have lost some sensitivity in it and I need to allow it's amplifier to make up a bit more of the loss. Any info greatly appreciated. By the way, I injected a 1000hz tone to check the sensitivity.
Dan
N0RAD/Colorado 813
International Man of Mystery
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Bruce
Senior Member
Username: Bruce
Post Number: 4502
Registered: 9-2003
| | Posted on Wednesday, September 05, 2007 - 7:03 am: |
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DAN .....
NOISE FIGURE, BANDWIDTH AND GAIN ......
To get limiting a FM radio has as much as 120DB gain. At this much gain PROVIDED you frount end and IF noise figure is ZERO you would be looking at a MAX sensitivity at 10 kHz wide of -134 dBm ... about .01 UV.
Now because you can never get a ZERO noise in the real world with real noise figure a .1-.3 UV is about right.
Now look at the IF gain of a cb set you might have 100DB so a .6 UV (-110 DBM ) signal will be still at a low level while in that FM set that detector is overloaded which is how a limiter works ....
Since AM detectors are linear over a small range adding gain will not do much except to make the radio overload....
That channelguard is not the problem everyone i have ever used runs close to 0 db loss at center frequency.
Yes .6 UV is common ..... and good enough .
On 6 since 66
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Triplecguy
Intermediate Member
Username: Triplecguy
Post Number: 225
Registered: 12-2001
| | Posted on Saturday, September 08, 2007 - 11:00 am: |
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Thanx Bruce, well stated and it makes sense.
DAN
Dan
N0RAD/Colorado 813
International Man of Mystery
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