> I'm writing some material about sampling for the beginner. I want to
> include a bit about non-ideal sampling. Assuming that sampling is
> defined as a process that takes on the value of a continuous-time signal
> at the instant that t = (sample time) * n, the only difficulties that I
> can attribute to the sampling process itself are:
>
> 1. Finite Aperture, i.e. the signal will be filtered a little bit
> before sampling.
>
> 2. Jitter, i.e. the actual sample time will vary from the assumed
> sample time by some amount.
>
> What have I missed?
>
> -------------------------------------------
> Tim Wescott
> Wescott Design Services
> http://www.wescottdesign.com
>
Tim,
For some content you may want to look at the book "The Art of
Electronics". While the book may not go into the "WHY", you can
probably extract that out. They most likely address some of the issues
that are experienced in the real-world.
Cheers,
David
Reply by Tim Wescott●July 2, 20052005-07-02
Jerry Avins wrote:
> Tim Wescott wrote:
>
>> Jerry Avins wrote:
>
>
> ...
>
>>> Leave out the detail. It's not too deep, but it sidetracks the
>>> thought process. Describe the stepwise nature of the output, mention
>>> in passing (if you must) the high-end loss, but pass on. High-end
>>> attenuation is zilch anyway in servos with their necessary
>>> oversampling. MHO.
>>>
>> One of my goals is to explain why, if you're building a servo system
>> and the guy next to you is working on a telephony application, he
>> needs fancy anti-alias and reconstruction filters and you need "good
>> enough" anti-alias and vestigal reconstruction filters.
>
>
> Chris Bore used to be a regular contributor to comp.dsp. Explore his
> site, especially if you're putting together a course. It's copyrighted,
> but ideas are free. (Look at the on-line courses http://bores.com/.) He
> made a remark one time that I occasionally use as a sig. See below.
Thanks for the link. The missing 'www' is interesting.
>
> There's more than not _needing_ fancy filters, they hurt.
Well, that's why I said you need "good enough" filters -- I wanted to be
stronger than to say you don't need fancy ones. This is one of the
points that I intend to make -- while you do sometimes need
anti-aliasing filters, particularly if you're in an electrically noisy
environment, you also need to pay careful attention to your filter's
phase delay, and with almost all plants you _don't_ need to worry about
a bit of aliasing 'cause it all falls outside of the plant's passband
anyway.
snip
> 5 ms will kill a servo, but if a motor doesn't
> like the buzz and whistle from the aliasing, it will never tell you.
I've had to spend significant time convincing folks of this, in the
past. On one occasion I ended up just specifying the phase delay of the
filter and letting the circuit designer go ahead and implement the extra
3 poles of anti-aliasing.
>
> Servo filters should be "prompt" (close to minimum phase) and as "light"
> (short) as possible. Servos are practically a different technology:
> higher sample rates for the bandwidth, different filtering goals, flash
> or parallel successive-approximation ADCs, and so on.
>
> Jerry
Yea verily.
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply by Jerry Avins●July 2, 20052005-07-02
Tim Wescott wrote:
> Jerry Avins wrote:
...
>> Leave out the detail. It's not too deep, but it sidetracks the thought
>> process. Describe the stepwise nature of the output, mention in
>> passing (if you must) the high-end loss, but pass on. High-end
>> attenuation is zilch anyway in servos with their necessary
>> oversampling. MHO.
>>
> One of my goals is to explain why, if you're building a servo system and
> the guy next to you is working on a telephony application, he needs
> fancy anti-alias and reconstruction filters and you need "good enough"
> anti-alias and vestigal reconstruction filters.
Chris Bore used to be a regular contributor to comp.dsp. Explore his
site, especially if you're putting together a course. It's copyrighted,
but ideas are free. (Look at the on-line courses http://bores.com/.) He
made a remark one time that I occasionally use as a sig. See below.
There's more than not _needing_ fancy filters, they hurt. A substantial
delay when reproducing recorded music goes unnoticed, but even a faint
artifact in the sound is intrusive. Real-time voice reproduction is more
tolerant of artifacts, but less tolerant of delay -- more than 5 ms
becomes uncomfortable. 5 ms will kill a servo, but if a motor doesn't
like the buzz and whistle from the aliasing, it will never tell you.
Servo filters should be "prompt" (close to minimum phase) and as "light"
(short) as possible. Servos are practically a different technology:
higher sample rates for the bandwidth, different filtering goals, flash
or parallel successive-approximation ADCs, and so on.
Jerry
--
... the worst possible design that just meets the specification - almost
a definition of practical engineering. .. Chris Bore
������������������������������������������������������������������������
Reply by Tim Wescott●July 2, 20052005-07-02
Jerry Avins wrote:
> AntiSPAM_g9u5dd43@yahoo.com wrote:
>
>> On Fri, 01 Jul 2005 12:46:27 -0700, Tim Wescott <tim@seemywebsite.com>
>> wrote:
>>
>>> I'm writing some material about sampling for the beginner. I want to
>>> include a bit about non-ideal sampling. Assuming that sampling is
>>> defined as a process that takes on the value of a continuous-time
>>> signal at the instant that t = (sample time) * n, the only
>>> difficulties that I can attribute to the sampling process itself are:
>>> 1. Finite Aperture, i.e. the signal will be filtered a little bit
>>> before sampling.
>>> 2. Jitter, i.e. the actual sample time will vary from the assumed
>>> sample time by some amount.
>>> What have I missed
>>> Tim Wescott >Wescott Design Services >http://www.wescottdesign.com
>>
>>
>>
>> How about quantization by the resolution of the converter and the
>> process of reconstruction of the original signal. I suppose the
>> differences in zero and first order hold is inappropriate for the
>> beginner.
>
>
> As I understand it, no hold at all is a series of spikes, a zero-order
> hold creates a stair case, and a first-order hold is either a
> continuation of the previous section's slope, of linear interpolation.
> The only first-order hold I've ever seen was one I built myself to
> smooth the digitally generated carrier of a home-brew 212 modem.
>
> <quibble> A zero-order hold doesn't attenuate the high frequencies. It
> greatly amplifies the whole usable spectrum, just a little less at the
> high end. </quibble>
Well, that depends on how you do your math in the first place. One way
or another, however, the high-frequency energy ends up being much less
than the low.
>
> Leave out the detail. It's not too deep, but it sidetracks the thought
> process. Describe the stepwise nature of the output, mention in passing
> (if you must) the high-end loss, but pass on. High-end attenuation is
> zilch anyway in servos with their necessary oversampling. MHO.
>
One of my goals is to explain why, if you're building a servo system and
the guy next to you is working on a telephony application, he needs
fancy anti-alias and reconstruction filters and you need "good enough"
anti-alias and vestigal reconstruction filters.
--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply by Tim Wescott●July 2, 20052005-07-02
AntiSPAM_g9u5dd43@yahoo.com wrote:
> On Fri, 01 Jul 2005 12:46:27 -0700, Tim Wescott <tim@seemywebsite.com>
> wrote:
>
>>I'm writing some material about sampling for the beginner. I want to
>>include a bit about non-ideal sampling. Assuming that sampling is
>>defined as a process that takes on the value of a continuous-time signal
>>at the instant that t = (sample time) * n, the only difficulties that I
>>can attribute to the sampling process itself are:
>>1. Finite Aperture, i.e. the signal will be filtered a little bit
>>before sampling.
>>2. Jitter, i.e. the actual sample time will vary from the assumed
>>sample time by some amount.
>>What have I missed
>>Tim Wescott >Wescott Design Services >http://www.wescottdesign.com
>
>
> How about quantization by the resolution of the converter and the
> process of reconstruction of the original signal.
I have that covered.
> I suppose the
> differences in zero and first order hold is inappropriate for the
> beginner.
>
I hope so -- I'm only covering the zero-order hold.
--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply by Jerry Avins●July 2, 20052005-07-02
AntiSPAM_g9u5dd43@yahoo.com wrote:
> On Fri, 01 Jul 2005 12:46:27 -0700, Tim Wescott <tim@seemywebsite.com>
> wrote:
>
>>I'm writing some material about sampling for the beginner. I want to
>>include a bit about non-ideal sampling. Assuming that sampling is
>>defined as a process that takes on the value of a continuous-time signal
>>at the instant that t = (sample time) * n, the only difficulties that I
>>can attribute to the sampling process itself are:
>>1. Finite Aperture, i.e. the signal will be filtered a little bit
>>before sampling.
>>2. Jitter, i.e. the actual sample time will vary from the assumed
>>sample time by some amount.
>>What have I missed
>>Tim Wescott >Wescott Design Services >http://www.wescottdesign.com
>
>
> How about quantization by the resolution of the converter and the
> process of reconstruction of the original signal. I suppose the
> differences in zero and first order hold is inappropriate for the
> beginner.
As I understand it, no hold at all is a series of spikes, a zero-order
hold creates a stair case, and a first-order hold is either a
continuation of the previous section's slope, of linear interpolation.
The only first-order hold I've ever seen was one I built myself to
smooth the digitally generated carrier of a home-brew 212 modem.
<quibble> A zero-order hold doesn't attenuate the high frequencies. It
greatly amplifies the whole usable spectrum, just a little less at the
high end. </quibble>
Leave out the detail. It's not too deep, but it sidetracks the thought
process. Describe the stepwise nature of the output, mention in passing
(if you must) the high-end loss, but pass on. High-end attenuation is
zilch anyway in servos with their necessary oversampling. MHO.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by ●July 2, 20052005-07-02
On Fri, 01 Jul 2005 12:46:27 -0700, Tim Wescott <tim@seemywebsite.com>
wrote:
>I'm writing some material about sampling for the beginner. I want to
>include a bit about non-ideal sampling. Assuming that sampling is
>defined as a process that takes on the value of a continuous-time signal
>at the instant that t = (sample time) * n, the only difficulties that I
>can attribute to the sampling process itself are:
>1. Finite Aperture, i.e. the signal will be filtered a little bit
>before sampling.
>2. Jitter, i.e. the actual sample time will vary from the assumed
>sample time by some amount.
>What have I missed
>Tim Wescott >Wescott Design Services >http://www.wescottdesign.com
How about quantization by the resolution of the converter and the
process of reconstruction of the original signal. I suppose the
differences in zero and first order hold is inappropriate for the
beginner.
Reply by Jerry Avins●July 2, 20052005-07-02
RRogers wrote:
> I did a detector noise limited design for IR detection back in the
> 80's. You could practically write a whole book on the sample and hold
> section. Nothing is "perfect". You have to choose an imperfect
> material for the capcitor and imperfect switches, and then work around
> the imperfections, deal with the response time of the circuit, the
> spectral content of the noise and signal. The problems and solutions
> just go on and on;
...
> It sounds hard but it is all
> standard Electronic theory applied in detail.
> The final sampling circuit only had about 4-5 parts; it's all in timing
> and process.
It seems like a difficult job elegantly done. I don't know which is
harder: cataloging (and maybe learning) the elements of good design, or
deciding which of them to include in a beginning course.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by RRogers●July 2, 20052005-07-02
I did a detector noise limited design for IR detection back in the
80's. You could practically write a whole book on the sample and hold
section. Nothing is "perfect". You have to choose an imperfect
material for the capcitor and imperfect switches, and then work around
the imperfections, deal with the response time of the circuit, the
spectral content of the noise and signal. The problems and solutions
just go on and on; totally satisfying as a design project :)
In the end though we had a 100ppm precision, offset/leakage
compensation, 1kHz sampling rate, 5 channels, and detector noise
limited system.
The first (and last) thing is learning how to do fast estimates, then
you can proceed towards detailed calculations. All of the first order
errors can be cancelled then you are left with the second order errors
that typically must be esamined in detail, including interactions; then
you have a stable good system you have control of. Believe it or not
the second order errors hold surprises. It sounds hard but it is all
standard Electronic theory applied in detail.
The final sampling circuit only had about 4-5 parts; it's all in timing
and process.
Ray
> Tim Wescott wrote:
>
>>I'm writing some material about sampling for the beginner. I want to
>>include a bit about non-ideal sampling. Assuming that sampling is
>>defined as a process that takes on the value of a continuous-time signal
>>at the instant that t = (sample time) * n, the only difficulties that I
>>can attribute to the sampling process itself are:
>>
>>1. Finite Aperture, i.e. the signal will be filtered a little bit
>>before sampling.
>>
>>2. Jitter, i.e. the actual sample time will vary from the assumed
>>sample time by some amount.
>>
>>What have I missed?
>>
>>-------------------------------------------
>>Tim Wescott
>>Wescott Design Services
>>http://www.wescottdesign.com
>
>
> Might want to discuss anti-aliasing filtering topics including,
> avoiding aliasing, phase effects, and possibility of compromises
> between filtering, signal bandwidth, and sample rate.
>
> Might want to look at/discuss sigma-delta/delta-sigma converters.
>
Yes, I'm introducing the concepts of aliasing, discussing anti-alias
filters and their drawbacks, why you want your anti-alias filter to be
of lower order and wider in a control context than you would in a pure
signal processing context, etc.
I hadn't considered discussing converters at all -- perhaps if there's
room I will discuss converters in general, however.
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com