[INRAD] Filter Board

[William E. Sabin]

----- Original Message ----- 
From: "Charlie Mazoch Jr." <w5vin at earthlink.net>
To: <list at inrad.net>
Sent: Wednesday, October 26, 2005 7:53 PM
Subject: Re: [INRAD] Filter Board


> Hi Bob:
> I enjoyed your comments about lowering the receivers noise through IF
> passband narrowing with cascaded filtering to better hear weak signals.
> I've had a running battle going with some amateurs who claim that if
> there is an increase in noise when the antenna is connected versus none
> the receiver is just fine. Baloney!

It is not baloney.

If I switch my rcvr antenna coax from a 50 ohm dummy load to an efficient 
antenna at 1.8 to 29.7 MHz, I should (and do) hear an increase in noise 
level of at least 4 or 5 dB.  If I do not hear that increase in noise level 
it surely means that rcvr internal noise is the limiting factor and that 
very weak signals will not be received properly.  Incoming signals must be 
greater than the total noise (antenna noise plus internal noise).

At HF, 1.8 to 29.7 MHz, the antenna noise temperature is well above 290 K, 
even under very quiet conditions. A low noise RF preamp (this is important) 
can be quite helpful in this situation, and in fact many rcvrs such as my
MP Mark V have such a preamp available especially for the 12 and 10 meter 
bands (menu item).  My homebrew  rcvr (see QRZ.COM, W0IYH) also has just 
such an RF preamp that I can switch in from the front panel.

Does that mean that "everything is just fine"? Not necessarily. A common 
problem in a rcvr is that wideband, noisy, high gain IF amplifiers that 
follow a single up-front "knothole" filter can make the overall noise figure 
higher.  This makes the ambient antenna noise override less certain. 
Cascaded filters can do good things for this problem. A low noise IF preamp 
right after the first filter and a second IF filter just ahead of the 
product detector can reduce IF noise substantially, therefore improve 
overall rcvr weak-signal performance.  The second filter also eliminates 
product detector image noise (on the opposite sideband). My homebrew rcvr 
does this and so does my Mark V rcvr.  Audio filtering after the product 
detector can also be beneficial.

The introduction of auxiliary Xtal and mechanical filters into the DSP rcvrs 
has been a major improvement in DSP rcvrs.  Early all-DSP rcvrs in the early 
1980s had some bad problems with AGC and with adjacent channel interference 
and desensitization.

Bill W0IYH