Goto page 1, 2, 3, 4, 5, 6, 7 Next
John Larkin
Guest
Wed Sep 01, 2010 5:29 am
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
John
miso@sushi.com
Guest
Wed Sep 01, 2010 7:44 am
On Aug 31, 9:29 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
John
Why wouldn't you just stretch the pulses with logic circuits? Your
delay element would be a string of inverters.
miso@sushi.com
Guest
Wed Sep 01, 2010 7:53 am
On Aug 31, 9:29 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
John
I forgot to answer your other question. The inverse sinc is a filter
where you feed it an impulse and you get a pulse out. It is the
theoretical PCM reconstruction filter. Remember, sampled data theory
presumes impulses.
In real life of course, your DAC creates steps rather than impulses,
so the inverse sinc is more theory than reality. You do need to boost
the high frequencies of a staircase signal in your antialiasing/
reconstruction filter. They call it sinc compensation.
Bill Sloman
Guest
Wed Sep 01, 2010 8:06 am
On Sep 1, 2:29 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
I've done that a couple of times now and it works fine. For one we
tapered the coefficients slightly to compensate for the loss in the
delay line, and for another I used 0.5, 1, ... 1, 0.5 to make the
filter a little closer to Gaussian. Both worked adequately, but we
didn't analyse them exhaustively.
Quote:
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas?
The first time I had to tackle this particular problem, I used a
monostable.
Ghiggino, K.P., Phillips, D., and Sloman, A.W. "Nanosecond pulse
stretcher",Journal of Physics E: Scientific Instruments, 12, 686-687
(1979).
As a solution, it was pretty horrible - the threshold depended on the
repetition rate, due to self-heating in the transistors, as is
mentioned in the paper. An emitter-coupled monstable - with similar
5GHz transistors - should have done better, but the circuit was
adequate in the application, where the repetition rate was a steady
20MHz (IIRR) set by the laser generating the pulses, and nobody wanted
to fix something that wasn't actually broken.
Quote:
What sort of filter has a rectangular-pulse impulse
response?
An infinitely long FIR filter, for one.
--
Bill Sloman, Nijmegen
Jan Panteltje
Guest
Wed Sep 01, 2010 11:29 am
On a sunny day (Tue, 31 Aug 2010 21:29:17 -0700) it happened John Larkin
<jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote in
<rakr76tpoiph198ldaiq5uvjpneebuvlbt_at_4ax.com>:
Quote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
One shot?
There must be some logic that fast.
John Larkin
Guest
Wed Sep 01, 2010 12:15 pm
On Tue, 31 Aug 2010 21:44:07 -0700 (PDT), "miso_at_sushi.com"
<miso_at_sushi.com> wrote:
Quote:
On Aug 31, 9:29 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
John
Why wouldn't you just stretch the pulses with logic circuits? Your
delay element would be a string of inverters.
It has to be analog and linear. Downstream will be amplifiers and
comparators, as noted.
Another possibility is two cascaded Bessel filters. The first shapes
the 2 ns pulse into, say, a 6 ns gaussian pulse, and the second pretty
much just delays that. The sum of the filter outputs will be pretty
much flat and will settle out fast.
John
John
Vladimir Vassilevsky
Guest
Wed Sep 01, 2010 4:07 pm
John Larkin wrote:
Quote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
A circuit made of lumped components can't have more zeroes then poles.
Hence what you trying to build is a twisted elliptic lowpass filter
where zeroes are at lower frequencies then the poles. I would approach
this as an optimization problem.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
John Larkin
Guest
Wed Sep 01, 2010 4:40 pm
On Tue, 31 Aug 2010 22:06:20 -0700 (PDT), Bill Sloman
<bill.sloman_at_ieee.org> wrote:
Quote:
On Sep 1, 2:29 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
I've done that a couple of times now and it works fine. For one we
tapered the coefficients slightly to compensate for the loss in the
delay line, and for another I used 0.5, 1, ... 1, 0.5 to make the
filter a little closer to Gaussian. Both worked adequately, but we
didn't analyse them exhaustively.
We might try that. I can buy a 5-tap, 1 ns per tap, SIP delay line for
about $5. There is some concern as to whether such parts will be
available long-term, since the demand for analog delay lines seems to
be fading.
Other possibilities: two cascaded 3 to 5-pole Bessel filters, maybe
with a buffer between, and resistive summing of their outputs. The
second one is pretty much acting as a delay.
A Bessel followed by all-pass sections, ditto.
John
George Herold
Guest
Wed Sep 01, 2010 5:31 pm
On Sep 1, 12:29 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
John
Can you slow it up at the source? Make the photodiode (PD) a bit
slower? Are the pulses straight from a PD or is there some active
element? Perhaps a bit more resistance somewhere.
George H.
Phil Hobbs
Guest
Wed Sep 01, 2010 5:50 pm
John Larkin wrote:
Quote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
John
IIRC if you use a constant-K filter, all sections except the ends have
the same L and C values, so it looks like a transmission line. Tapping
off from the intermediate nodes would work fine then.
I gather you care about the pulse height, and so can't just use (say) a
TTL gate with a cap on its output. TTL's output drive is asymmetric
enough that you should be able to stretch the pulse some that way.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
John Larkin
Guest
Wed Sep 01, 2010 9:10 pm
On Wed, 01 Sep 2010 12:50:10 -0400, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:
Quote:
John Larkin wrote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
John
IIRC if you use a constant-K filter, all sections except the ends have
the same L and C values, so it looks like a transmission line. Tapping
off from the intermediate nodes would work fine then.
Yup, that would be a homemade lumped LC delay line. They are OK for
modest lengths and modest delay/risetime ratios. They explode after
that. The number of sections goes as Td/Tr squared, even if you can
buy ideal inductors.
Quote:
I gather you care about the pulse height, and so can't just use (say) a
TTL gate with a cap on its output. TTL's output drive is asymmetric
enough that you should be able to stretch the pulse some that way.
The PD pulse will drive, probably, four comparators, each programmed
by a DAC. One will hunt for the nominal height, one will derive a
trigger at maybe H/3, and two will look for over/under amplitude
pulses. The amps, comparators, and FPGA would be happier with 5 ns or
so pulses, but we need to recover pretty fast. Hence the rectangular
output filter.
I can buy a 5-tap, 1 ns per tap SIP LC delay line for about $5, so
maybe a bessel lowpass followed by a transversal filter would work.
Rob is trying to design a classic form LC filter that has a
rectangular output, using a genetic-evolution optimizer thing. So far,
the waveforms are, well, interesting. He's calling LT Spice to get the
response, with that wrapped inside some error function + optimization
thing, running for maybe 35K shots per trial. I'm thinking in terms of
doing the design with a breadboard LC delay line and 4 or 5 trimpots.
The cool thing about transversal filters is that they can be tuned by
hand without difficulty; you practically draw the waveform you want.
John
miso@sushi.com
Guest
Thu Sep 02, 2010 2:31 am
On Sep 1, 4:15 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
On Tue, 31 Aug 2010 21:44:07 -0700 (PDT), "m...@sushi.com"
m...@sushi.com> wrote:
On Aug 31, 9:29 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
John
Why wouldn't you just stretch the pulses with logic circuits? Your
delay element would be a string of inverters.
It has to be analog and linear. Downstream will be amplifiers and
comparators, as noted.
Another possibility is two cascaded Bessel filters. The first shapes
the 2 ns pulse into, say, a 6 ns gaussian pulse, and the second pretty
much just delays that. The sum of the filter outputs will be pretty
much flat and will settle out fast.
John
John
I claim pulse stretching is not a linear operation. I don't recall the
strict definition of linearity, but it had something to do with energy
conservation (Parseval). We're talking geek here, not stuff like low
THD, etc. that is normally associated with linearity.
My guess is that you are trying to maintain the amplitude of the
pulse. I can see that being designed with a peak detector and one
shot.
John Larkin
Guest
Thu Sep 02, 2010 3:24 am
On Wed, 1 Sep 2010 18:34:22 -0700 (PDT), Bill Sloman
<bill.sloman_at_ieee.org> wrote:
Quote:
On Sep 2, 1:40 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 31 Aug 2010 22:06:20 -0700 (PDT),BillSloman
bill.slo...@ieee.org> wrote:
On Sep 1, 2:29 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
I've done that a couple of times now and it works fine. For one we
tapered the coefficients slightly to compensate for the loss in the
delay line, and for another I used 0.5, 1, ... 1, 0.5 to make the
filter a little closer to Gaussian. Both worked adequately, but we
didn't analyse them exhaustively.
In the first example, I loaded each tap of a nominally 100R lumped
constant delay line with a nominally 1k5 resistor running into a
summing junction; by starting off with 1k7 and tapering down to 1k3 we
more or less compensated for the pulse attentuation in the loaded
delay line.
The summing amplfier was something tolerably fast from Comlinear,
though we might have switched to an Analog Devices part on the second
spin of the board.
The second example was done some five years later, when you could get
quad packaged op amps of similar speed, and I buffered each tap of the
delay line with a follower amp and used the outputs of the followers
to drive the resistors into the summing junction.
We might try that. I can buy a 5-tap, 1 ns per tap, SIP delay line for
about $5. There is some concern as to whether such parts will be
available long-term, since the demand for analog delay lines seems to
be fading.
Other possibilities: two cascaded 3 to 5-pole Bessel filters, maybe
with a buffer between, and resistive summing of their outputs. The
second one is pretty much acting as a delay.
A Bessel followed by all-pass sections, ditto.
Apparently commercial delay lines use low-pass sections. The one time
I got somebody to design a home-built delay line, comment was that
that got you twice as much delay per unit R/C or L/C product. The guy
who was doing the work was very good, but the project got canned
before we got to the design review on that board (which really peeved
me - we were designing a board to replace one that had been evolving
for about ten years, and contained lots of evidence that evolution
drives you towards sub-optimal solutions).
The bad news is that few people still make analog delay lines, and
they want 4 weeks to come up with a sample. I don't have 4 weeks. So
either I make my own tapped LC delay line from parts, or try cascading
and summing Bessel filters.
John
John Larkin
Guest
Thu Sep 02, 2010 3:44 am
On Wed, 1 Sep 2010 18:04:58 -0700 (PDT), "miso_at_sushi.com"
<miso_at_sushi.com> wrote:
Quote:
On Sep 1, 4:15 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 31 Aug 2010 21:44:07 -0700 (PDT), "m...@sushi.com"
m...@sushi.com> wrote:
On Aug 31, 9:29 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
We were playing around with using a 3 or maybe 5 pole LC lowpass
filter, but summing the signals from intermediate nodes, instead of
just taking the last one. This looks promising but mathematically
messy to do really well, a "lost in space" situation maybe. A filter
that makes a beautiful output pulse can have some ghastly intermediate
waveforms.
Any ideas? What sort of filter has a rectangular-pulse impulse
response?
John
Why wouldn't you just stretch the pulses with logic circuits? Your
delay element would be a string of inverters.
It has to be analog and linear. Downstream will be amplifiers and
comparators, as noted.
Another possibility is two cascaded Bessel filters. The first shapes
the 2 ns pulse into, say, a 6 ns gaussian pulse, and the second pretty
much just delays that. The sum of the filter outputs will be pretty
much flat and will settle out fast.
John
John
I claim pulse stretching is not a linear operation.
What I want to do certainly is. A passive LC filter is linear. The
output is strictly proportional to the input.
John
Bill Sloman
Guest
Thu Sep 02, 2010 4:34 am
On Sep 2, 1:40 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
On Tue, 31 Aug 2010 22:06:20 -0700 (PDT),BillSloman
bill.slo...@ieee.org> wrote:
On Sep 1, 2:29 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I'm expecting to get some photodiode pulses that are just a bit too
fast to handle with cheapish amps and comparators and such. It would
be nice to have an analog filter that would accept a roughly gaussian
pulse, maybe 2 ns wide, and stretch it to, say, 5 or 6 ns wide,
substantially flat on top if possible. Rep-rate might go up to 40 MHz
maybe.
An LC phase-linear lowpass filter with a reasonable number of poles
would make a slower sorta gaussian blip, not very flat, with a
substantial tail, which would limit my rep-rate to some extent.
If I run the pulse through a tapped analog delay line, maybe five 1 ns
taps, and sum the signals that appears at each tap, I can get a pretty
flat pulse. That amounts to a FIR/transversal filter with all
coefficients = 1, tweakable a little maybe. That's OK if I can get and
afford such a delay line and can sum the tap signals without great
hassles.
I've done that a couple of times now and it works fine. For one we
tapered the coefficients slightly to compensate for the loss in the
delay line, and for another I used 0.5, 1, ... 1, 0.5 to make the
filter a little closer to Gaussian. Both worked adequately, but we
didn't analyse them exhaustively.
In the first example, I loaded each tap of a nominally 100R lumped
constant delay line with a nominally 1k5 resistor running into a
summing junction; by starting off with 1k7 and tapering down to 1k3 we
more or less compensated for the pulse attentuation in the loaded
delay line.
The summing amplfier was something tolerably fast from Comlinear,
though we might have switched to an Analog Devices part on the second
spin of the board.
The second example was done some five years later, when you could get
quad packaged op amps of similar speed, and I buffered each tap of the
delay line with a follower amp and used the outputs of the followers
to drive the resistors into the summing junction.
Quote:
We might try that. I can buy a 5-tap, 1 ns per tap, SIP delay line for
about $5. There is some concern as to whether such parts will be
available long-term, since the demand for analog delay lines seems to
be fading.
Other possibilities: two cascaded 3 to 5-pole Bessel filters, maybe
with a buffer between, and resistive summing of their outputs. The
second one is pretty much acting as a delay.
A Bessel followed by all-pass sections, ditto.
Apparently commercial delay lines use low-pass sections. The one time
I got somebody to design a home-built delay line, comment was that
that got you twice as much delay per unit R/C or L/C product. The guy
who was doing the work was very good, but the project got canned
before we got to the design review on that board (which really peeved
me - we were designing a board to replace one that had been evolving
for about ten years, and contained lots of evidence that evolution
drives you towards sub-optimal solutions).
--
Bill Sloman, Nijmegen
Goto page 1, 2, 3, 4, 5, 6, 7 Next
