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

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

As another random datapoint there's a
MetOp document regarding the LRPT satellite
transmissions which says:

Frequency range Group delay
(kHz) variation (ìs)
[0-40] +/- 2
[40-60] +/- 5

-- John

[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

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.

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

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

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

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.

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