Re: K3S Crystal Filtering
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Ah. Much better. Thanks!
Sent from my Verizon, Samsung Galaxy smartphone
On 2/15/2021 2:00 PM, Bob Novas via groups.io wrote:
-------- Original message --------
From: Jim Brown <k9yc@...>
Date: 2/15/21 17:35 (GMT-05:00)
Subject: Re: [Elecraft-K3] K3S Crystal Filtering
> I’ll crack open my copy of Oppenheim and Schafer, and have a read. I
> think the next step is to test the hypothesis – whether subjectively
> (e.g., listening) or objectively. The trouble with objective
> measurement is how to get the test signal to test with. Seeing what the
> filters do to an impulse would be a good way to compare the filters, but
> where do you get an RF impulse?
There are many ways to measure the complex response of a system. In the
world of pro audio, we've been doing it since the early '80s by
measuring the complex frequency response with a swept detector
synchronized with a swept generator and doing an inverse FFT of the
result. That first instrumentation was called Time Delay Spectrometry,
invented by Richard Heyser, an engineer employed at JPL at the time, and
they held the patent. In later years, we adopted the method of
dual-channel FFT for data acquisition, and computing the inverse FFT.
The DG8SAQ software does an inverse FFT on swept complex response to
compute and plot TDR with the excellent VNWA3 that he designed.
These methods are all far superior to using an impulse to excite a
system because a swept measurement provides FAR greater signal to noise
and dynamic range.
73, Jim K9YC