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Re: [Amps] IMD

Subject: Re: [Amps] IMD
From: Jim Brown <>
Date: Tue, 10 Jan 2012 10:02:29 -0800
List-post: <">>
On 1/8/2012 6:35 AM, Ron Youvan wrote:
>   This "different delay for different frequencies" is called "group delay."
> It is something that messes up dial up modems.  (so you don't get 56 kB/s)

In the world of pro audio, where I made my living as a consultant for 30 
years before retiring several years ago, we had to go back and study the 
fundamentals of circuit theory that says that any variation in the 
amplitude response is accompanied by a corresponding change in the PHASE 
response.  ANY filter produces phase shift in the part of the spectrum 
where its amplitude response is changing -- that is, its skirts, AND in 
the passband NEAR its skirts. The greater the number of poles (circuit 
elements) the greater the phase shift.  Years ago I used a K2, mostly 
for CW, but occasionally on SSB. It was a decent SSB RX if you used the 
TX filter, which it did for widest RX bandwidth, but it was an AWFUL RX 
for narrower bandwidths, which were accomplished by stagger-tuning the 
crystal filters. The response looked like the Rocky Mountains, and the 
resulting phase distortion made signals quite difficult to copy.

This phase shift is also present in filters produced by digital signal 
processors DSP) --  DSP filtering is simply a simulation in the digital 
domain of analog filters, and the phase shift of an analog filter in the 
digital domain will be the same as that filter in the analog domain. One 
advantage of filtering in DSP is that if you have enough DSP, you can 
use more complex filter topologies that have less phase shift, or less 
problematic phase shift.  The best DSP rigs can produce excellent 
response, and sound very good. The K3 and Orions are in that category.

73, Jim K9YC
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