On Jun 25, 9:45*am, Tim Shoppa wrote:
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.

You wouldn't happen to know the group delay variance
of the filters you mentioned? Rough values are okay.

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 believe that in theory if the exact group delay profile is known,
then
a digital receiver can perform a certain amount of equalization. What
I'm curious about is how much varience can be introduced by the
filters in a receiver for various transmission types without needing
to equalize.

-- John

On Jun 25, 2:14*pm, wrote:
On Jun 25, 9:45*am, Tim Shoppa wrote:

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.

You wouldn't happen to know the group delay variance
of the filters you mentioned? *Rough values are okay.

I haven't actually measured group delay but I'm sure that what I hear
is group delay.

If you look at published group delay graphs for commercial Chesbyshev
filters, a 500 Hz 8th-order crystal filter has a delay around 2ms in
the middle of the passband, but within 100 Hz of the edge of the
passband the delay peaks up enormously to 4ms and then back down again
over the very steep skirt.

Maybe you can turn the 2 ms variation into some inter-symbol/intra-
symbol limit for some digital modes. It rings like the dickens when
hit with QRN, that's for sure!

Tim N3QE.

test

On Wed, 25 Jun 2008 11:14:03 -0700 (PDT), wrote:

On Jun 25, 9:45*am, Tim Shoppa wrote:
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.

You wouldn't happen to know the group delay variance
of the filters you mentioned? Rough values are okay.

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 believe that in theory if the exact group delay profile is known,
then
a digital receiver can perform a certain amount of equalization. What
I'm curious about is how much varience can be introduced by the
filters in a receiver for various transmission types without needing
to equalize.

Excuse my ignorance, but why on earth do you do some crude analog
filtering and then continue with digital filtering, in which you have
much more alternatives ?

The only reason that I can think about using sharp IF crystal filters
is that the dynamic range of the following stages (product detector
and ADC) is not sufficient.

In a typical general coverage up converting receiver, the roofing
filter will define the bandwidth the ADC must handle. Also some gain
control (not necessary automatic) is needed to set the band noise well
below one LSB (LF/MF vs VHF/UHF and antenna efficiency on LF).

Even when designing an add-on unit for audio processing, why would
anyone use the receiver CW filters apart from dynamic range issues ?

Paul OH3LWR

On Jun 26, 3:32*am, Paul Keinanen wrote:
On Wed, 25 Jun 2008 11:14:03 -0700 (PDT), wrote:
On Jun 25, 9:45*am, Tim Shoppa wrote:
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.

You wouldn't happen to know the group delay variance
of the filters you mentioned? *Rough values are okay.

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 believe that in theory if the exact group delay profile is known,
then
a digital receiver can perform a certain amount of equalization. *What
I'm curious about is how much varience can be introduced by the
filters in a receiver for various transmission types without needing
to equalize.

Excuse my ignorance, but why on earth do you do some crude analog
filtering and then continue with digital filtering, in which you have
much more alternatives ?

The only reason that I can think about using sharp IF crystal filters
is that the dynamic range of the following stages (product detector
and ADC) is not sufficient.

In a typical general coverage up converting receiver, the roofing
filter will define the bandwidth the ADC must handle. Also some gain
control (not necessary automatic) is needed to set the band noise well
below one LSB (LF/MF vs VHF/UHF and antenna efficiency on LF).

Even when designing an add-on unit for audio processing, why would
anyone use the receiver CW filters apart from dynamic range issues ?

I kind-of have the same questions too. For a homebrew project, having
all the software-designed-radio complexity in addition to the tight-
analog-filtering-from-DC-to-daylight complexity seems to just... make
everything too complicated and not fun anymore.

But the best ham receivers couple impressive front ends with
effectively tight roofing filters with SDR aspects effectively (and
quite usably) and I can see why someone would want to try their hands
at their own competitive design. But, wow, it's a lot of effort. And a
lot of ham receivers - especially the first and second generation
designs - combined all these technologies into radios that are
actually painfully complicated to use. (When the QST review starts
contrasting menu option 73 submode 4 with menu option 105 submode 13,
that's a real turn-off to me. At the same time, other younger
operators just love that sort of complexity!)

On the other hand, a truly simple analog front end (e.g. Softrock)
combined with a computer is a hell of a lot of fun. You spend a lot
more time looking at a computer screen and less listening but that's
what some like.

Tim N3QE.

On Jun 26, 3:32*am, Paul Keinanen wrote:
Excuse my ignorance, but why on earth do you do some crude analog
filtering and then continue with digital filtering, in which you have
much more alternatives ?

To undersample the signal it must be bandwidth limited which means
some
type of analog filtering. As long as filtering is necessary, it might
as well be a narrow as the widest signal of interest and as sharp as
possible so long as it's convenient and doesn't distort the signal too
much.

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.

wrote in message
...
"As long as filtering is necessary, it might
as well be a narrow as the widest signal of interest and as sharp as
possible so long as it's convenient and doesn't distort the signal too
much."

"As sharp as possible" and "doesn't distort the signal too much" are somewhat
conflicting goals: In general, the steeper the skirts of a filter, the more
group delay variation you get there at the edges (hence, Butterworth has less
group delay variation than Chebyshev which has less than Elliptic). Now, you
can certainly account for this by widening the passband a bit and then perhaps
using even steeper skirts, or you can compensate for it digitally if you can
characterize it, but the main point here is that it does get rather complex --
hence the trend to have somewhat "looser" analog filters (and thus low group
delay variation) and then do whatever you want digitally.

On Jun 26, 12:41*pm, "Joel Koltner"
wrote:
"As sharp as possible" and "doesn't distort the signal too much" are somewhat
conflicting goals:

Understood, part of the point of this thread which was to get an idea
of how much group delay variance is acceptable for various types of
transmissions without greatly impacting the quality of the received
signal.

hence the trend to have somewhat "looser" analog filters (and thus low
group delay variation) and then do whatever you want digitally.

Also understood, it's all about balance. Going narrow impacts group
delay variance which distorts signal, going wide impacts dynamic
range.

Which still leaves me with the notion that you want to go as tight as
reasonably possible and no tighter. With that in mind it sounds like
what we've determined so far with regards to IF filtering is:

transmission type receiver group delay variance
-------------------------------------------------------------
CW should be less than 2 ms

this is based on Tim Shoppa's posts which were to the point.
Does anyone else have data to contribute?

-- 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 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