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Group Delay Variation - How much is too much?
On Jun 25, 6:45 am, Tim Shoppa wrote:
On Jun 24, 11:46 pm, wrote: For various transmission types, how much group delay variation can be tolerated in the receiver before it causes problems recovering the original signal? I realize that it all depends ... I'm just interested in getting a rough idea. For example, at what point is: 1) SSB voice reception noticeably affected? 2) the ability to receive images from weather satellites affected? 3) various amateur radio modes such as PSK31, RTTY, and MFSK16 affected? 4) HD Radio specs 600 ns as the max for the transmitter, what's the max for the receiver? I'm curious since the group delay variation of SAW filters available for IF filtering seems to vary widely. I have been playing with homebrew crystal filters (following W7ZOI and Bill Carver/K6OLG) for CW, as well as audio filters, and can tell you that on CW the difference between a super-sharp-in-frequency Chesbyshev filter (typical in ham equipment for a long time now) and a more constant-delay (e.g. Gaussian to 6dB or 12dB, or equiripple linear phase) filter is like night and day. My impressions are done "to my ears", not to a spectrum analyzer. The super sharp in frequency Chesbyshev filters have horrible horrible ringing especially on say 40M or 80M in the summer with the QRN. I can hardly listen for a few minutes without getting disgusted. BUT... they do have a real advantage during say a contest when there's competing signals every few hundred Hz. At the other extreme the constant-delay filters sound remarkably clear and transparent. They do not have such a sharp stopband, but my ear makes up for that most of the time. Ringing from summertime band noise is not nearly so tiring. Most of the filter simulation programs (AADE, SPICE, NatSemi's cool new WebBench filter tools, etc.) let you look at not just frequency response but also phase response and (maybe most importantly for summertime QRN) impulse response. Having run the simulations and listened with my ears to my experiments this summer, I cannot emphasize how much more enjoyable it is to use a Gaussian-to-6dB or equiripple linear phase filter in CW. Often when the bands are not crowded but there is QRN, I far far prefer a simple two-pole crystal filter designed for CW in the first place (e.g. my Heath HW-16) to any fancy-pants 8-pole or 12-pole modern filters in my new rigs. I notice you ask about a lot of digital modes but not CW. My ears have been listening to CW for 30-some years now and I can do a lot of processing in my brain. But what my brain cannot remove is horrible filter ringing. I don't know how those other digital modes stack up... maybe computers are better at removing horrible ringing than my brain. I don't think I have a "golden ear" or any other audiophile quality. In fact I'm pretty sure my ears are less good than they were when I was a kid doing CW. Tim N3QE Obviously, both the time and the frequency response are determined by the positions of the poles and zeros of a linear system (filter), but be a bit careful about equating group delay and ringing. It's easy to make an FIR digital filter that has constant group delay, but rings quite nicely. I suspect the people who design RF communications systems using modern modulation schemes know the answers to John's questions. There's probably another newsgroup where you'd get more answers. Or--run some simulations. I can imagine creating ideal signals in Scilab (or Matlab) and feeding them through various filters, and then demodulating them. It shouldn't be terribly difficult to do that, but I'm not volunteering. Cheers, Tom |
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