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'clock multiplier for switched cap filter ?'
1997\04\15@084917 by Stephen Brown

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Hi,

I'm trying to build a programmable bandpass filter whose center
frequency is controlled by a PIC16C84. I need to filter audio signals
between about 200Hz up to 1500Hz under software control. I was hoping
to be able to use a switched capacitor filter, e.g. National
Semiconductor MF-8 or the equivalent Linear Tech Part. For these
devices you control the center frequency via an external clock signal
running at either 50x or 100x of the desired center frequency (which I
had hoped the PIC could provide). The PROBLEM is that 1500 Hz * 100 =
150000 which is too fast for a 4 or 10 or 20 MHz clocked PIC to do, I
think. I've tried various types of codes from the EASY PIC'N book and
from frequency generation ideas using phase accumulators by Scott
Dattalo within the MPSIM simulator and I can only get a clock to go at
about 22 kHz using any of these methods -- not nearly fast enough.
The phase accumulator method is preferred, since the application is
musical note identification and the frequencies are oddball multiples
of the PIC clock frequency.

THEREFORE -- If I'm going to continue on this tack, I need a clock
multiplier to hang on the output of the PIC to boost up the frequency
a bunch (several clock doublers ??). Any good ideas here?

By the way I did run across a programmable sine wave generator chip
good from DC to 500 kHz from MicroLinear -- I suppose this could be
squared up and used as a clock. the problem is that I can't seem to
find small quantities for sale, they want to sell tubes of 25 for
$10.00 each.

Any help or suggestions are greatly appreciated.

Steve

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Applied Research Associates
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1997\04\15@091542 by Jonathan King

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> Hi,

> had hoped the PIC could provide). The PROBLEM is that 1500 Hz * 100 =
> 150000 which is too fast for a 4 or 10 or 20 MHz clocked PIC to do, I

> THEREFORE -- If I'm going to continue on this tack, I need a clock
> multiplier to hang on the output of the PIC to boost up the frequency
> a bunch (several clock doublers ??). Any good ideas here?
>
Try a 4046 or 74hc4046.  They are cheap and relatively easy to use ( for a
phase locked loop).  These are made by Motorola, Harris, National,and I'm sure
others &  operate in the frequency range you need.

Look at Motorola AN535 for a primer on phase locked loop design.  The data
sheets for the parts also give some basic equations that will get the part
working.



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1997\04\15@091551 by David BALDWIN

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Did you really tried to generate this 150kHz signal? I think that with
a PIC running at 20MHz, this should be feasible. 200ns of internal
instruction cycle, it's 33 instructions for one period of external
clock. You have time to do other things, no?

1997\04\15@102659 by Jerry Meng

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At 08:49 AM 4/15/97 -0400, you wrote:
>THEREFORE -- If I'm going to continue on this tack, I need a clock
>multiplier to hang on the output of the PIC to boost up the frequency
>a bunch (several clock doublers ??). Any good ideas here?
>
Hi Steve,
       Why don't you change your design and use another way to generate
clock for the switched cap filter? You can use Motorola MC145162 universal
dual
PLL, you only use this chip to divide crystal clock(5MHz or others) into the
switch cap filter clock, for a 150000Hz clock, use 5MHZ/33=150KHz. since
the MC145162 is a dual PLL, so you can use it to generate two different
clock for different filter frequencies  :) The price for MC145162 is about
1.5 US$ each for QTY 25. It is SPI compatible, friendly to PIC. More info,
you can check the Motorola web, there is a PDF data sheet upon MC145162.



Jerry Meng, BA1FB

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1997\04\15@132159 by Gerhard Fiedler
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At 08:49 15/04/97 -0400, Stephen Brown wrote:
>THEREFORE -- If I'm going to continue on this tack, I need a clock
>multiplier to hang on the output of the PIC to boost up the frequency
>a bunch (several clock doublers ??). Any good ideas here?
>
>By the way I did run across a programmable sine wave generator chip
>good from DC to 500 kHz from MicroLinear -- I suppose this could be
>squared up and used as a clock.

Why don't you try a programmable divider (from 20MHz)? It probably gives
you enough resolution.

1997\04\15@193140 by John Payson

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|>By the way I did run across a programmable sine wave generator chip
|>good from DC to 500 kHz from MicroLinear -- I suppose this could be
|>squared up and used as a clock.

|Why don't you try a programmable divider (from 20MHz)? It probably gives
|you enough resolution.

[nb: Does anyone know a good way to make MS Exchange format
replies properly without having to hand-insert all the "|" or
">" characters?]

A "conventional" clock divider running from 20MHz won't have much
resolution around 500KHz.  Your steps would be:

 487,805Hz
 500,000Hz
 512,821Hz

etc.  [i.e. about 12KHz resolution].  You may get much finer
resolution if you don't mind a little bit of "jitter" in the
output.  You may get any frequency you want, to arbitrary
accuracy, with 1/2 clock-period of jitter by building a suit-
able-length accumulator (i.e. adders and latches).  If you
build, say, a 16-bit accumulator and put 2193 on the input,
then the MSB of the output will toggle at a speed (2193/65536)
times the cycle rate.

Alternatively, if you don't mind having a bit more jitter, you
may use a simpler technique as found in a 4089 [a cute but not
terribly useful part which has been discontinued; if you can
find the data sheet for it somewhere, though, you'll see what
I'm talking about].  A 22v10 fed with a 20MHz clock should be
able to produce any frequency from 39,139Hz to 10,000,000Hz
within 0.4% [if you were to use this route, you'd need 11 out-
puts from the PIC to set the desired output speed but the PIC
itself wouldn't have to do anything].  If more accuracy is need-
ed, you could change the circuit a little bit and have the PIC
"fine-tune" the output (the circuit itself works as a divide-down
counter that selectively "drops" counts; the PIC could "drop" a
few more if needed).

This technique, by the way, works best when the output frequency
is much below the input frequency, or when it will be divided down
so jitter doesn't matter too much (the device works by producing
a frequency between 1/2 and 1/4 of the input frequency and then
dividing that by a power of two; the more stages you divide it the
less jitter will remain).  If you use it for audio, you need to
divide it by at least 100 in order to eliminate the jitter comp-
onents.  For your applications, though, it sounds as though that
shouldn't be too much of a problem.

1997\04\16@014746 by Jerry Meng

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At 06:45 PM 4/15/97 -0500, you wrote:
{Quote hidden}

Hi John,
       If you need a fine resolution high speed clock source, why don't
you try
the new technology Direct Digital Synthesizer, Like AD9850, it can generate
up to 42MHz with a 125MHz clock input , output tuning resolution of 0.0291 Hz.
The AD9850 also contains a high speed comparator which can be configured
to accept the (externally) filtered output of the DAC to generate a low
jitter
square wave output. But the cost is much higher.



Jerry Meng, BA1FB

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1997\04\16@015406 by nigelg

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In message  <@spam@199704151249.IAA01745KILLspamspamterrier.ned.ara.com> KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU
writes:

Hi,

> I'm trying to build a programmable bandpass filter whose center
> frequency is controlled by a PIC16C84. I need to filter audio signals
> between about 200Hz up to 1500Hz under software control. I was hoping
> to be able to use a switched capacitor filter, e.g. National
> Semiconductor MF-8 or the equivalent Linear Tech Part.
[snipped]

You don't explain exactly what you are trying to do, but in the current
issue (May 97) of 'Everyday Practical Electronics' they have a project for
a 'Musical Instrument Tuner' using a 10MHz 16C84. It picks up the audio via
a microphone and (optionally) bandpass filters it using an LM13600
controlled by an D2A fed from PORT B, after calculation the nearest note
and percentage error is displayed on an LCD module.

The software can be downloaded from their web site,
http://www.epemag.wimborne.co.uk

Nigel.

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1997\04\16@015949 by brooke

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Stephen Brown wrote:
.......
> THEREFORE -- If I'm going to continue on this tack, I need a clock
> multiplier to hang on the output of the PIC to boost up the frequency
> a bunch (several clock doublers ??). Any good ideas here?
......
Steve
The Harris HSP45102 NCO can easily do this.  See the Jan 1995 issue
of Electronics Now for more info.  It is controlled with a serial
data and clock stream.

The Harris data book talks about using this part for a digital output
by only connecting the MSB and not using a D/A converter to get a sine
wave output.

Have Fun,
Brooke

1997\04\16@145519 by Vishram sarurkar

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On Tue, 15 Apr 1997, Jonathan King wrote:

> Try a 4046 or 74hc4046.  They are cheap and relatively easy to use ( for a
> phase locked loop).  These are made by Motorola, Harris, National,and I'm sure
>
> Look at Motorola AN535 for a primer on phase locked loop design.  The data

hello,
       is this application note available on the net. if yes could you
give the web site address ???

thanx,
vishram.

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