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'[PIC]: ETS on a PIC ??? Yes - it can be done.'
2001\02\16@144231 by Drew Vassallo

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>I've been playing around with equivalent-time sampling on a PIC,
>and have put up some info about this on the web.
>http://www.oricomtech.com/ets.htm

Good stuff Dan!  Isn't your first example sort of a non-test, since it
violates the Nyquist criterion?  In other words, isn't it already known that
you can't sample a 120KHz signal at 50KHz rate?  I.e., it requires a minimum
of 240KHz rate.  Or am I missing something?

I'm trying to do something similar, but at much lower frequencies, using the
A/D at 10KHz using a 4MHz crystal.

By the way, that World's Smallest Testbench looks amazing.  Is there any
possibility of an option to reduce the number of input channels for the
logic analyzer to increase the sampling rate?  It would be nice to get a 2-
or 4-channel analyzer running at 5-10MHz instead of the default.  I want one
when it comes out :)

--Andrew
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2001\02\16@145800 by severson

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> Is there any
> possibility of an option to reduce the number of input
> channels for the
> logic analyzer to increase the sampling rate?  It would be

Dan,

Can you reduce the number of channels to 0 and get an infinite sample rate?

;-)

OK. I just wanted to agree that the WST looks great. Now I will have
something to do with the old 486 that I have.

-Rob

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2001\02\16@153447 by Dan Michaels

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Rob Severson wrote:
>> Is there any
>> possibility of an option to reduce the number of input
>> channels for the
>> logic analyzer to increase the sampling rate?  It would be
>
>Dan,
>
>Can you reduce the number of channels to 0 and get an infinite sample rate?
>
>;-)
>

Well ........ [I'll talk to Mchp about it].
================

>OK. I just wanted to agree that the WST looks great. Now I will have
>something to do with the old 486 that I have.
>

 \
+--\-------+
|   \      |
| Win\dows |
|    9\x   |
|      \   |
+-------\--+
        \
Or your new Palm - if I ever get things done around here.

- dan

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2001\02\16@161258 by Drew Vassallo

picon face
>Sounds about right 20:4 = 5:1 = 50khz:10khz. You would need to select
>AD clock = 8 Tosc.
>=============

Yes sir, works well.  Interestingly, I'm curious as to which frequency I
would select at 20MHz.  The datasheets say that 32Tosc violates the minimum
A/D time, but the only option for 20MHz, then, is RC, in which case the Tad
is 2-6us (4us typ.)!  I'm not sure what's happening here, and I haven't
tried it at 20MHz to find out.  Any info?

>Ha - noooooo way with a PIC76, 2.5 Mhz is tops, although 18Cxxx w/10mhz

Exactly what's the limiting factor here?

>xtal with PLL enabled might do 5Mhz. Scenix will do 20 Mhz all day long.

Hmm, sounds like a project for the WFT (World's Fastest Testbench) :)

--Andrew
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2001\02\16@165136 by Dan Michaels

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Drew wrote:
>>Sounds about right 20:4 = 5:1 = 50khz:10khz. You would need to select
>>AD clock = 8 Tosc.
>>=============
>
>Yes sir, works well.  Interestingly, I'm curious as to which frequency I
>would select at 20MHz.  The datasheets say that 32Tosc violates the minimum
>A/D time, but the only option for 20MHz, then, is RC, in which case the Tad
>is 2-6us (4us typ.)!  I'm not sure what's happening here, and I haven't
>tried it at 20MHz to find out.  Any info?
>

Umm, my datasheets for both 'C76 and 'F876 show 32Tosc is ok at 20 Mhz.
8Tosc does violate min Tad. Forget RC, you can find a way to do it using
the clock.

BTW, using 'C76, you can get reliably sampling right at a 50 khz [~20
usec] rate with 20 mhz xtal - although there may be a slight LSB resolution
loss - at least according to the datasheet. Looking at it on the screen
looks fine however.

OTOH, you cannot go quite this fast on the 'F876, due to its having
a 10-bit A/D. However, you can slow down to 40 Khz [~25 usec] sampling,
and all is ok.
=============


>>Ha - noooooo way with a PIC76, 2.5 Mhz is tops, although 18Cxxx w/10mhz
>
>Exactly what's the limiting factor here?
>

How fast you can execute an instruction. You will notice, with a 20
mhz xtal, sampling at 2.5 mhz allows you "exactly" 2 instructions to
do everything --> ie, linear code.
===========

>>xtal with PLL enabled might do 5Mhz. Scenix will do 20 Mhz all day long.
>
>Hmm, sounds like a project for the WFT (World's Fastest Testbench) :)
>

WFST - World's Fastest Smallest TestBench. Been there, done that,
too :).

- dan

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2001\02\16@171247 by Dan Michaels

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Drew wrote:

>I'm trying to do something similar, but at much lower frequencies, using the
>A/D at 10KHz using a 4MHz crystal.
>

Drew, I looked at this again, and it's better than I thought.

For 4 mhz xtal and 8Tosc, Tad = (5/4)*1.6usec = 1.92 usec.
10 Tad = 19.2usec ~ 52 khz, which means the A/D can go much
faster than your original 10 khz number.

OTOH, if you jump up to an 8mhz xtal, you get killed, because
you will have to use 32Tosc. Tad will increase to 3.82usec, if
I figured it correctly.

- dan

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2001\02\16@173049 by Eisermann, Phil [Ridg/CO]

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Hi,

       At the risk of being a labeled a heretic... I've recently worked
with an ADuC812, which is an 8052 variant by Analog Devices with a 12-bit, 8
channel ADC. It's maximum conversion frequency is spec'ed at 200kHz. It has
a DMA mode that can write data to external memory. That's pretty much
required at 200k samples per second, since only 5 usec are available to
process the data. Like most classic '51s, Tosc is divided by 12, so that
doesn't leave much time for processing your data. However, the DMA mode
leaves the core free to do its normal thing: e.g. you get 200k SPS at 12
bits written to memory, and get an interrupt when the data transfer is done.

       Perhaps useful for those wishing to build a faster PC-based
oscilloscope? ETS could be done on the external memory by the uC or the host
PC. The processor was around $8.30 for 1000 (working from memory), direct
from ADI's website. I would think it's a good fit for this type of
application.

{Original Message removed}

2001\02\16@223105 by Dan Michaels

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Don Hyde wrote:
{Quote hidden}

Exactly - the 1st figure Drew was referring to illustrates this
in the time domain. Combined with the 2nd figure, the two show
that the practical limitations of normal sampling by the PIC A/D
can be overcome using the ETS mode. Basically, the normal mode
with max 50khz rate will adequately display a waveform of only
about 5 khz max, while ETS mode pushes this out to 120 Khz.

Who'd a thunk it on a PIC? 120 khz analog signal acquisition.

- dan

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2001\02\16@224125 by Don Holtz

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At 07:27 PM 2/16/01 -0800, you wrote:
{Quote hidden}

Hi,

The signal must necessarily be periodic for this to work....
Lets not forget that restriction!

Cheers,
Don

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2001\02\17@004420 by Sean H. Breheny

face picon face
Hi Don,

For certain methods of accomplishing reconstruction, you are right, but I
don't think that is true in general. You can take any signal bandlimited to
delta_f, sample it at 2*delta_f, and reconstruct it from the samples. It
doesn't matter what the center frequency is, or whether it is periodic, as
long as it is bandlimited. So, for example, you could have a signal which
filled the entire band from 0.9 to 1.1 MHz, sample it at only 0.4
Megasamples per sec, and reconstruct it from that, as long as you had the
additional piece of info that its center freq was 1 MHz.

Sean

At 07:41 PM 2/16/01 -0800, you wrote:
{Quote hidden}

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2001\02\17@011121 by Robert Rolf

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Bear in mind that the series resistance of the sampling switch (6k at
5V) and the value of the hold capacitor (51.2pf for a '76) will band
limit your signal to under 500khz.
However, with an external fast sample and hold, one could digitize video
at a 50khz rate (3 samples per video line). By sampling progressively
later in the line, one could acquire a 640x480 60Hz video frame in 3.8
seconds. (A fully analog version of this technique was used back in the
70's to convert full speed video to the 15Hz line/7.5S frame of slow
scan television used in the amateur radio bands (TV in a 3khz BW)).

"Sean H. Breheny" wrote:
{Quote hidden}

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2001\02\17@012412 by Sean H. Breheny

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Hi Robert,

Yes, you are right, I should have added the proviso that your sampling
system needs to have a high enough bandwidth (and your sampling aperture
needs to be very small and low jitter).

I'm not talking about the progressive sampling method, though. I'm talking
about just taking samples normally. As another poster pointed out, aliasing
really only occurs when two "copies" of the signal overlap (in the
frequency domain). This will only happen if you exceed the bandwidth
allowed by the Nyquist Sampling Theorem, NOT just because the center
frequency is very high. Sure, the output samples will be consistent with a
much lower center frequency, but who cares if the modulation riding on top
of them is the narrowband signal you are looking for? You really don't need
to do any special processing (such as progressive sampling), as far as I
can see. If you want to reconstruct the signal exactly, you will need to
know what the original center frequency was, which you can obtain either by
progressive sampling or by just using a frequency counter (like the
Bitscope does).

Sean

At 11:05 PM 2/16/01 -0700, you wrote:
{Quote hidden}

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2001\02\17@022438 by Robert Rolf

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Hi Sean,

The method you describe is often used in direct digital detection
receivers. The IF (or tuned RF) output is in a known band
and an appropriate choice
of sample rate achieves a downconversion to 'base band' without
additional RF stages. I believe Plessey Semiconductor and/or Thomson
CSF had digital TV chip sets and app notes using this method.

It is actually quite instructive to take two function generators
(one as signal, the other as sampler) and feed the first through
an analog switch (4066) which is driven by the 2nd as a gate for
a sample and hold, to a scope. As one sweeps the frequency of the
input signal past Fs/2 one easily sees the changing wavelength
of the signal, and the folding effect (and zero beat as the frequency
approaches Fs).

Try it. It will make aliasing a very 'intuitive' quantity.
[10Khz is a nice sample rate which gives you stable bright traces]

Robert

"Sean H. Breheny" wrote:
{Quote hidden}

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2001\02\17@181726 by Dan Michaels

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Robert Rolf wrote:
>Bear in mind that the series resistance of the sampling switch (6k at
>5V) and the value of the hold capacitor (51.2pf for a '76) will band
>limit your signal to under 500khz.


Hi Robert,

Ah good, I see someone else is familiar with the PIC A/D internals.

Based upon the published info, I had expected to see a fairly good
BW in the A/D, something somewhat commensurate with

F = 1/(2*pi*7K*51pF) = 440 khz

However, I was surprised to measure only about 120 khz BW [external
influences being taken into account] for the 'C76. And the 'F876,
which has a 120 pF sampling cap, registered only about 60 khz BW.

In addition, the A/D gain internally falls off at about -60 dB/decade,
which makes a really nice anti-aliasing filter when you are doing ETS
at 1 mhz, but I was hoping for a little more BW.

Interesting that the measured values are so far off the 1st order
approximation. Got any clue as to why?

best regards,
- dan michaels
http://www.oricomtech.com
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2001\02\17@183657 by Drew Vassallo

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>In addition, the A/D gain internally falls off at about -60 dB/decade,
>which makes a really nice anti-aliasing filter when you are doing ETS
>at 1 mhz, but I was hoping for a little more BW.

Maybe I'm barking up the wrong tree here, but my electret microphone input
on a project (single-stage amplification of 50 nominal) into the A/D results
in absolutely no amplification at all, really.  Not being very familiar with
all this audio stuff, does the above statement have any relation to this
problem?

I have the mic biased to 2.5V, but I can't get any AC swing.  What I ended
up doing, only haphazardly, was to crank the gain up to something
outrageous, in the 1300 range.  Oddly, the noise is only about 2 bits on an
8-bit A/D.  The overall range still only goes from ~20d to 225d on the A/D
result, and only saturates if I speak DIRECTLY into the mic.  If I'm about
one foot away (actually, several feet is my eventual goal), it requires me
to really shout.  Any idea why the output needs such high gain?  Is this
even possible?

--Andrew
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2001\02\18@122203 by Dan Michaels
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Andrew Vasallo wrote:
>>In addition, the A/D gain internally falls off at about -60 dB/decade,
>>which makes a really nice anti-aliasing filter when you are doing ETS
>>at 1 mhz, but I was hoping for a little more BW.
>
>Maybe I'm barking up the wrong tree here, but my electret microphone input
>on a project (single-stage amplification of 50 nominal) into the A/D results
>in absolutely no amplification at all, really.  Not being very familiar with
>all this audio stuff, does the above statement have any relation to this
>problem?
>

Nothing whatsoever. The A/D doesn't fall off until about 120 khz,
audio is only 20 khz tops.
=========

>I have the mic biased to 2.5V, but I can't get any AC swing.  What I ended
>up doing, only haphazardly, was to crank the gain up to something
>outrageous, in the 1300 range.  Oddly, the noise is only about 2 bits on an
>8-bit A/D.  The overall range still only goes from ~20d to 225d on the A/D
>result, and only saturates if I speak DIRECTLY into the mic.  If I'm about
>one foot away (actually, several feet is my eventual goal), it requires me
>to really shout.  Any idea why the output needs such high gain?  Is this
>even possible?
>

Nothing unusal, you just need a lot of amplification when using
an electret mike. The key, I believe, is to DC bias the mike, and
use AC-coupling to all successive stages: mike --> amp --> amp --> A/D.

Probably best to use at least 2 amplification stages, as a single
stage at gain = 1300 is not gonna have much BW.

LM324 has GBW = 1 Mhz. If setup with gain = 1300, the BW of the
stage is only 1mhz/1300 = 770 hz. This is not enough BW for a
typical audio signal.

- danM

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