[ARC5] Some info on the S-38

[Richard Knoppow]

    I didn't differentiate the purposes of the filters.  Yes, the 
filters used in carrier systems were to isolate the channels and allow 
closer spacing (more channels).  Up to at least 20 Mhz atmospheric noise 
dominates any other source in most receivers, even rather noisy ones. 
There is a tremendous amount of technical literature about this. Some of 
the older noisy RF tubes, like the 6K7 and 6SK7 have better dynamic 
properties than lower noise tubes so are advantageous where the very 
minimum receiver noise is not necessary.  I think this is one reason the 
familiar 6BA6 continued in wide use after much quieter tubes became 
available; it has better cross modulation/ intermodulation 
characteristics. I discovered this years ago when experimenting with low 
noise input stages for communications receivers.  Sometimes the plain 
vanilla kind works best.

On 11/20/2015 10:43 AM, Fuqua, Bill L wrote:
>    If you have an antenna sufficient to hear atmospheric noise on HF that is the best you can  do other than improving the effective noise bandwidth of the receiver.
> That is why the application of low noise figure devices are not common or necessary in HF receivers. The atmospheric noise becomes the limiting factor.
> Microwave systems using frequency division multiplex systems used many pole filters not due to noise issues but for isolation between adjacent voice channel rejection.
> You can still find very good mechanical filters used in these systems from time to time. It is just that they are at odd frequencies for amateur use and are relatively wide
> in bandwidth.
> 73
> Bill wa4lav
>   
>    
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL