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Group Delay Variation - How much is too much?
On Jun 26, 11:34*am, wrote:
On Jun 26, 3:32*am, Paul Keinanen wrote: why would anyone use the receiver CW filters Probably a bit narrower than what I had in mind … *I'm currently looking at 500 KHz wide SAW filters. As a rough guesstimate, the group delay in a 500kHz wide filter will be 1/500,000 secs, or 2.0 microseconds. Now, depending on shoulder steepness the change in group delay might get to 2, 3, maybe even 5 times 2.0 microseconds. But even at 20 microseconds I don't think any of the HF digital modes you mentioned would be impacted. Most of my comments regarding group delay and ringing in filters were oriented towards narrowish (few kHz or less) filters. Wow, a HF receiver with a 500kHz SAW filter after the mixer. I don't have a clue what you're doing! I thought we were talking about HF receivers for common bandwidths! Tim N3QE |
Group Delay Variation - How much is too much?
On Jun 26, 2:40*pm, Tim Shoppa wrote:
But even at 20 microseconds I don't think any of the HF digital modes you mentioned would be impacted. Thanks ... that's the type of information I was curious about. Wow, a HF receiver with a 500kHz SAW filter after the mixer. I don't have a clue what you're doing! I'm "playing" with something resembling 0 - 175 Mhz up converted to 208 Mhz filtered using a GSM SAW filter sampled at the first IF using a 25 Msps 16 bit ADC. The silly width is because I'm interested in handling broadcast FM including RDS (among other things). I'm also interested in receiving satellite images which in some cases has a bandwidth of 150 Khz. -- John |
Group Delay Variation - How much is too much?
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Group Delay Variation - How much is too much?
[Let's try this again ... that should be microseconds (us) not (is)]
As another random datapoint there's a MetOp document regarding the LRPT satellite transmissions which says: Frequency range * * * Group delay (kHz) * * * * * * * * * * * *variation (us) [0-40] * * * * * * * * * * * *+/- 2 [40-60] * * * * * * * * * * *+/- 5 -- John |
Group Delay Variation - How much is too much?
wrote in message
... "Does anyone else have data to contribute?" I don't, and I suspect that no one has done a comphrehensive survey of various popular (to hams) modulation formats and their sensitivity to group delay variations. Doing so would definitely be valuable -- it'd be a shoe-in for a QST or QEX article. |
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 |
Group Delay Variation - How much is too much?
K7ITM wrote on Thurs, Jun 26 2008 2:45 pm
On Jun 25, 6:45 am, Tim Shoppa wrote: On Jun 24, 11:46 pm, wrote: 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. 'Ringing' and group-delay over a passband are separate things but, in practice they are are related. In passive-component filters they are quite related. 'Ringing' phenomena can be investigated analytically in any time- domain circuit analysis program. I use LTSpice from National Semi- conductor...totally free for download and works on any PC. SPICE compatible, the source stimulus can be set as a pulse of several cycles with the rise-time, fall-time adjustable. If there is electronic-cause ringing, it will show up at the output. Every single passive-component filter has time delay. If the time delay is unequal across the passband, then one will hear the ringing. Such ringing isn't always a physical-electronic thing IN the filter but more in the way the human brain perceives sound. True high- fidelity electronic music systems will have near-equal time delay over its entire passband. Unfortunately, few, if any, of the first Hi-Fi systems makers ever published specifications on group-delay or delay of any kind. [excluding speakers, of course, since those and their interrelationship with a room are so acoustically variable that no common standard could be reasonably adopted] It is very hard to describe sound that is FELT rather than measured by instruments. As Tim said about CW use, ringing causes an actual discomfort. With wideband home music systems there is very little perceived 'ringing' but there exists 'quality' which can only be graded by comparison with another system as 'A-B' testing. The one that FEELS like it sounds best would be the winner. For non-audio use, such as in AM-PM ('QAM') modulation combinations in modems, inter-symbol distortion with/without an 'eye' display can grade things...and group-delay effects aren't the only things to blame there, lots of others in the total path. Simpler FSK systems such as single-channel teleprinter need concentrate on group-delay only over a passband about twice the frequency of its frequency shift between Mark and Space. Group- delay there shows up more on demodulated pulse edge ditortion. Ringing there can be seen easily at the edge transitions. A compromise there is to have group-delay greater beyond the needed passband limits resulting in rounded transitions; demodulator output can be shaped afterwards as desired. NTSC analog video examples do not really apply since the common ACTUAL bandwidth of most smaller TV sets was so limited (down to 1 MHz in some) that group-delay effects are hidden in the resulting video passband distortion of details on objects. Broadcast FM is hard to define in felt-quality since FM's 'quiet' spectrum use is relatively narrow. 'Loud' passages of music uses more spectrum space, thus group-delay effects over a passband are more pronounced on loud passages. ... 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. Machine-in, machine-out systems such as teleprinters and modems can be compared relatively easily with computer analysis programs. Regardless of the computer program, none can substitute for what is FELT in the ear-brain sensory system with acoustic input. I see a valid test as only A-B or A-B-C (or more) comparisons using the same audio or audio-modulation input. That is a LOT more work. Most of what I've done in that delay subject are locked into lab notebooks in corporate ownership and it involves many man-weeks of investigation. The best one can hope for, in my estimation, are general guides on limits from the many and varied radio services. 73, Len AF6AY |
Group Delay Variation - How much is too much?
It just occurred to me that many don't know what "group-delay"
is. It is physically-electronically a time delay from input to output that may vary depending on the input frequency versus the magnitude-phase response of the filter within its passband. Group-delay is defined as the difference in phase delay versus frequency difference over very small differences in frequency. Most analysis programs show that as a matter-of-course using frequency-domain analyses. Using around 50 or more different frequencies of a linear source sweep input will show the actual filter time-delay, input to output, that is very close to the real thing. 73, Len AF6AY |
Group Delay Variation - How much is too much?
On Jun 27, 2:16 pm, AF6AY wrote:
.... I use LTSpice from National Semi- conductor... My friends at Linear Technology will be most interested to hear that, Len. |
Group Delay Variation - How much is too much?
On Jun 27, 10:25�pm, K7ITM wrote:
On Jun 27, 2:16 pm, AF6AY wrote: ... I use LTSpice from National Semi- conductor... My friends at Linear Technology will be most interested to hear that, Len. OOOPS! My bad! :-) Yes, LINEAR TECHNOLOGY CORPORATION did LTSpice...an improvement over their previous SPICE derivative done to promote their switcher ICs. Apologies all around to Linear Technology friends. They make fine ICs. I also have, but have not tried TINA from Texas Instruments, another SPICE derivative. Not enough time here to try everything out that is available. TINA is also free for download but it is harder to get through their web pages to do so. I don't have any friends at TI or Linear or at National Semi, I just use their good products...just like I am currently using a couple of LM337s using National's appnote information in a slightly different way. 73, Len AF6AY |
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