[TenTec] narrow filters

[Jan & Del Seay]

Hi Bill. There are definite advantages, but no so much in using
really narrow filters. Of course, the more tandom filters you have,
the steeper the walls, assuming they are well matched to each
other.
But - remember that each amplifier stage in a radio also introduces
noise, and it is usually broadbanded noise. It is ideal to put a good,
steep filter on the output of the very last if amplifier to minimize the
noise getting through to the detector stage. Remember that in a
noise figure calculation, stage bandwidth is one of the equations.
The factors are all of the noise either enterring from the antenna jack,
or generated in the succeeding stages, minimized by the bandwidth
of the signal at the detector.
With this in mind, you can surely see how a receiver with cascaded
filters could hear signals that equal receivers without could not.
Now - we have another factor here that city dwellers have to live with, 
and it is one reason why the JA rigs have found so much favor among
U.S. hams. Why have a receiver that can hear neg 140 dBm signals when
there is an ambient man made noise of 130 dBm?
The JA engineers have taken into account that 95% of users of their
radios will be urban/suburban dwellers. Therefore, they spend more time
on features than dragging the last ounce out of the receivers.
When you push a receiver to it's limits, you increase the chance of
imd product problems, and have increased the cost considerably.
Certain higher end rigs have a switchable rf amp so that they can get
a slightly better noise figure, while allowing the guys who have high
ambient noise to shut it off.

Hop I haven't bored everyone!
73  de KL7HF

   

----------
From: 	Bill_Ames@hysoft.com[SMTP:Bill_Ames@hysoft.com]
Sent: 	Wednesday, February 25, 1998 9:08 PM
To: 	tentec@contesting.com
Subject: 	[TenTec] narrow filters

Interesting conversations with respect to using many narrow filters in the
RX chain.. however, I wonder what the real results might have been for the
users of these combinations? Have they needed this capability to snag real
"rare" DX? Received critical information from the other person in the QSO?
My hunch is that for the most it is a technical exercise, to see just what
one's rig can do, rather then using the rig extensively to "do it". IMHO.
;-)


Bill
KB1LG



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