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

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