Audio filter test & measurement with the 2204A - PART 1

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Glovisol
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Audio filter test & measurement with the 2204A - PART 1

Post by Glovisol » Thu Jun 14, 2018 4:17 pm

Pass band log to 24k.jpg
Audio filter test & measurement with Picoscope & 2204A – PART 1

This post describes tests and measurements on a speech processor design in the audio band 300 Hz to 3 KHz.

The classic (and best) contemporary filter test bench is a Spectrum Analyser plus a Tracking Generator. I have been wondering for a long time why Picotec does not better the Picoscope software with the addition of a Tracking Generator function: it should not be too difficult, considering that all Picotech scopes have a Function Generator built in already.

However the 2204A can still provide all that is necessary for filter measurement at audio frequencies, if one uses the next best, a wideband noise generator: if this is not available, the 2204A will help you to quickly build one. The Tracking Generator provides just one variable sine signal while following the Spectrum Analiser’s sweep; the Noise Generator instead provides all frequencies at once and the Spectrum Analiser “paints” the filter response on screen.

The noise is generated by an underbiased zener diode: the noisiest diodes are at voltages 8.2 to 9.2 V. Probably 12V zeners are even better, but they cannot be used with a 12 V supply. I tested a batch of 18 glass enclosed 8.2 V zeners and all delivered a wideband noise level of between 30 and 40 mVp.p. with a 56 to 100 KOhm resistor. While testing the generator, I noticed an intermittent output instability that almost drove me crazy, until I discovered, quite by accident, that the zeners are light sensitive. If you switch a lamp on nearby, the noise output will decrease in proportion to the light intensity, in some cases going down by 6/8 dB! Not to speak of 50Hz hum, also picked up from the lamps in the lab. Better encapsulate the little devil in epoxy.

Looking at the schematic, IC1A and B amplify the noise. The CA3240 has a gain – bandwith product of 4.5 MHz, good to amplify noise at least to 50 KHz. IC1A provides the required high impedance load to the zener and R6 adjusts the output level. Circuit gain is in excess and at maximum the noise clips at 4Vp.p. approx. The emitter follower Q1 isolates the output of IC1B, ensuring distortion free output.

Test procedure in the next post.
Attachments
Schema filtro audio (8).jpg
Complete schematics Noise Generator + Filter
HP-LP WB Noise 24K small.jpg
Noise generator wideband output 0 - 24 KHz
HP-LP WB Noise 24K small.jpg (30.17 KiB) Viewed 869 times
Test setup).JPG
Complete test setup
Last edited by Glovisol on Sat Jun 16, 2018 8:19 am, edited 1 time in total.

Gerry
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Re: Audio filter test & measurement with the 2204A - PART 1

Post by Gerry » Fri Jun 15, 2018 8:59 am

Hi Glovisol,

One of our customers has created a piece of software for applications such as filter characterization, which has a step generator function to sweep across a frequency range. The details are here: https://bitbucket.org/hexamer/fra4picoscope/wiki/Home.

Regards,

Gerry
Gerry
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Glovisol
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Re: Audio filter test & measurement with the 2204A - PART 1

Post by Glovisol » Fri Jun 15, 2018 12:05 pm

Hi Gerry,

Many thanks for pointing out the commendable work of Examer to me. Of course my work is at a much simpler level and surely cannot compete with Examer's complete network analysis solution.

On the other hand what I propose allows quick and effortless frequency response testing and development with such simple means as spending one hour building an audio frequency noise generator or spending £15 for a HF/VHF/UHF/SHF noise generator generally available on Ebay, employing easy to use Picotech software.

Kind regards,

glovisol

Gerry
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Re: Audio filter test & measurement with the 2204A - PART 1

Post by Gerry » Thu Jul 05, 2018 3:36 pm

Hi Glovisol,

You have created a number of very useful, cost-effective, measurement applications, with surprising accuracy (considering the equipment used) and with intuitive, in-depth descriptions of how to perform the measurement, typically with the science behind the measurement. So, many thanks for all the effort you have put in to make things easier for some of our forum users.

In case you haven't already done so, and if you have the links to your application posts, have you ever considered creating an index to them, so that potential readers can find information that may otherwise be buried in obscure search results?

Regards,

Gerry
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Glovisol
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Re: Audio filter test & measurement with the 2204A - PART 1

Post by Glovisol » Thu Jul 05, 2018 6:25 pm

Dear Gerry,

Please accept my thanks for your kind words about my posts. The work described in many of them has been stimulated by actual needs springing from my interests and hobbies, from saling/cruising to wine making. You would be surprised at a DrDAQ/1012 application for distilling very high quality Grappa liquor, which I did not dare to post, lest Picotech should drive me away from this site....

I surely could apply myself to an index, but to tell you the truth, I did not understand what you really mean with an index and the form which could be most useful to other Forum participants: perhaps a more detailed explanation of what you foresee would be of help.

Kind regards,

glovisol

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Re: Audio filter test & measurement with the 2204A - PART 1

Post by Glovisol » Wed Jul 11, 2018 9:47 am

Gerry, Hi again,

Please explain the index to applications issue. I do have all links to the posts, as well as stored copy in my Hd's.

Kind regards,

glovisol

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Re: Audio filter test & measurement with the 2204A - PART 1

Post by Martyn » Wed Jul 11, 2018 1:16 pm

No need for any Index as any application notes should be in the correct Forum

As and when we access older posts we can move them to here if appropriate.
Martyn
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