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Passive Filter for Class D Amp Testing
Passive Filter for Class D Amp Testing
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Old 19th July 2015, 01:49 PM   #21
JohnW is offline JohnW  Czech Republic
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Lukas,

Interesting links, I not sure if the OP is trying to measure a Half Bridge or Full Bridge AMP design - and what sound card he's using as I ponder if he also needs a Diff to SE conversion as well as a LPF.

The Elektor kit uses ferrite cores, from my experience ferrite core based inductors bottom out at about 0.001% THD - maybe they have found a better core material - they don't mention the use of airgaps to improve linearity.
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Old 19th July 2015, 02:31 PM   #22
JohnW is offline JohnW  Czech Republic
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Quote:
Originally Posted by udok View Post
Hi John,

Yes, I referred to the Audio Precision usage of the AES-17 filter. A simple R-C filter gives about 20 dB attenuation and should be ok for "normal" Class-D amps.
Interesting note you made about the AD797. Did'nt think of the degeneration resistors. AD claim that the AD797 holds its superb noise performance up to very high frequencies due to the single stage design.
Can you post a graph of the R&S THD+N over frequency if you find time? Would be
interesing to see the comparison to AP Sys2.
We have at work a lot of UPV and UPD but none with the low distortion generator option :-/
But i can post a graph of the newest AP 555 machine :-)

Greetings,
Udo
Udo,

If you ever get a chance to visit me here in Czech then we can run through some Opamp RF IMD tests. I have to perform the test manually, based upon a concept developed by Paul miller.

I did Email you awhile back to see if you would be interested in for a fee writing a test procedure for me using the GPIB interface BUS to automate the tests:-

https://dl.dropboxusercontent.com/u/...ercussions.pdf

Here is more on the subject, ignore my mention of the Labview environment as apparently not even LabView's own Mother loves it:-

http://www.diyaudio.com/forums/equip...i-labview.html

I meant to mention earlier that the fact the the AES17 filter improves a 20KHz BW limited measurement just highlights the RF non linearity of the test equipment.

If the test equipment had perfect behavior then the filter should have zero impact on the measurement results (presuming the same measurement BW).

It sound not be considered that the filter it artificially "Improving the measurement figures" rather its give more accurate measurement results by reducing the non linearity of the test equipment, and not somehow improving the linearity of the UUT.

I'll post A THD+N plot later - yesterday my UPD started puffing smoke from its side vents, a Ceramic Cap popped making a real mess of the PCB - what a nice Birthday present

https://dl.dropboxusercontent.com/u/...1/IMG_7352.JPG
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Old 23rd July 2015, 05:19 PM   #23
1audio is offline 1audio  United States
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Passive Filter for Class D Amp Testing
Ferrites have a reputation for nonlinearity but the Radiometer CLT-1 uses them for both the filters and the matching transformer and it has a residual of less than -165 dB so I think its not that simple. I reverse engineered the AP passive filter some time ago. It also used ferrite inductors as I remember.

Most digital amps seem to bottom out at .01% so it may not be an issue on the other side. If you are working with Bruno's amps (Hypex) you probably are not obsessing about THD, since he did the work for you.

I would argue that a differential input is required to test amps. The return current from the load to the amp ground getting mixed with the test signal from having a common ground for both can at a minimum alter the actual measurement and could lead to all sorts of mischief and instabilities. The one aspect missing from the eBay filter is an attenuator. Of course an attenuator could be part of a load resistor anyway. The LinearX version has an attenuator but the steps would drive me nuts for the scaling.
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Old 23rd July 2015, 05:41 PM   #24
1audio is offline 1audio  United States
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Passive Filter for Class D Amp Testing
If I understand the Miller test linked, its using a pseudorandom modulation of an RF carrier and plotting the resulting audio spectrum. It should be pretty easy to do. I'm not sure why the modulation needs to be pseudo-random. I would think two RF carriers with a sub 20 KHz difference would accomplish about the same and be easier to detect? Is there something about the RF sensitivity that is greater to random noise? Maybe its very frequency specific? It does not look to be in the plots. A wideband RF noise generator could also be used it seems. High pass filter it at say 100 KHz or 1 MHz and gradually increase the level and plot the noise spectrum on the output of the device.
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Old 23rd July 2015, 06:16 PM   #25
JohnW is offline JohnW  Czech Republic
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Its hard to say as each situation is different - but I guess Pauls thinking was to modulate the whole 20KHz baseband "in a single go" - if you look at the Power amplifier plots you can see their sensitivity to RF gets worst towards 20KHz (at certain RF frequencies) - spot modulation frequency's would not clearly represent whats really happening across the Baseband.

The bottom the graph is scaled 20Hz to 20KHz, so a 20KHz BW limited noise source is the perfect stimulus.

Really today it would be better to extend the baseband to say 100KHz.
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Old 23rd July 2015, 06:23 PM   #26
JohnW is offline JohnW  Czech Republic
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Quote:
Originally Posted by 1audio View Post
I reverse engineered the AP passive filter some time ago. It also used ferrite inductors as I remember.
I've a couple of the AUX0025 sitting stacked up on the bench in front of me, I'm pretty sure they are aircores - although I've not looked inside one for 10 years or so...

Quote:
Originally Posted by 1audio View Post
Most digital amps seem to bottom out at .01% so it may not be an issue on the other side.
Thats typical, but does not have to be the case, I've built ClassD amplifiers that are more linear then the AP SYS2.


Quote:
Originally Posted by 1audio View Post
If you are working with Bruno's amps (Hypex) you probably are not obsessing about THD, since he did the work for you..
UCD uses post filter feedback so your not left at the mercy of the inductors linearity.
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Old 23rd July 2015, 07:09 PM   #27
1audio is offline 1audio  United States
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Quote:
Originally Posted by JohnW View Post
Its hard to say as each situation is different - but I guess Pauls thinking was to modulate the whole 20KHz baseband "in a single go" - if you look at the Power amplifier plots you can see their sensitivity to RF gets worst towards 20KHz (at certain RF frequencies) - spot modulation frequency's would not clearly represent whats really happening across the Baseband.

The bottom the graph is scaled 20Hz to 20KHz, so a 20KHz BW limited noise source is the perfect stimulus.

Really today it would be better to extend the baseband to say 100KHz.
I'm just looking for a simpler way to test for the same issue. I think the following would be a possibility- Use a sweepable RF generator with AM modulation capability. AM modulate it with 10 KHz at 100% witch will have two spectral sidebands at +/- 10 KHz from the now missing carrier or in effect two carriers 20 KHz apart. Then slowly sweep it from say 200 KHz (if possible) to 200 MHz and plot the output at 20 KHz (demodulated difference IM). This should be a good proxy for RF nonlinearity and spot specific frequencies where problems exist. It would be much easier to sense the specific 20 KHz even in noise than trying to spot pseudorandom noise in random noise. Quicker as well. Is there some aspect this would miss?
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Old 23rd July 2015, 10:16 PM   #28
JohnW is offline JohnW  Czech Republic
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Here's the odd thing, lets say when an opamp is subjected to RF, you don't see discrete sidebands or fixed spurie (atleast within the audio Baseband) but a wideband elevated noise floor - it might only be elevated by 2dB to 3dB but its elevated non the less.

A slightly different scenario, but same result is that on numerous occasions I’ve caught a high speed Opamp oscillating due to its elevated baseband noise floor. Typically the noise floor would be elevated by no more then say 10dB, so you need to have an idea where the noise floor should be otherwise you would miss it.

If you look at Paul's graphs you will see that the noise floor is scaled from 0dB to +15dB, this matches my own experience where I would typically see around 10dB noise floor modulation on a bad day…

This is the process I'd like to automate:-

1. Perform FFT of UUT noise floor across 20KHz or 100KHz span (With no injected RF stimulus). Store this noisefloor result as the Reference baseline noise-floor (the UUT will be an amplifier or some form of analogue gain stage etc).

2. Sweep a RF signal generator from say 20KHz to 3GHz, or a user set frequency span. The RF Signal Gen set to 20mV RF carrier level with AM 100% modulation via an external random noise source B/W limited to 20KHz.

3. At frequency points along the RF sweep, perform a Analogue FFT and plot result on a colour graded waterfall showing any noise floor modulation as a result of the external RF carrier injected stimulus (any RF induced noise floor modulation when compared to the earlier stored Reference noise floor without the external RF signal present).
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Old 24th July 2015, 01:02 AM   #29
1audio is offline 1audio  United States
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I follow the noise floor increase and have seen that myself. 3 GHz RF generators are not common in DIY but the basics are available. Maybe it would be possible to link something with an FFT package that can be linked, Excel (which can drive GPIB with the right bit of software) and someone who has done some of this.

100% modulation by noise is a misnomer since noise has a crest factor that can be pretty high. However I understand what you are after. The NI stuff is a major commitment to get into which is why I have not gone near it. Using a soundcard for the FFT would be obvious. Excel? or Sigview spectrum analyzer - FFT based signal analysis software Sigview may be able to do the whole task. It would be worth an e-mail. If so I'll get a copy as well. And then scrounge a GPIB equipped RF generator.
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Old 24th July 2015, 02:14 AM   #30
RNMarsh is offline RNMarsh  United States
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Passive Filter for Class D Amp Testing
T&M junky here........

Demian, I have several RF/uWave analyzers with tracking generators here [to 6GHz]. If you are still interested in this approach when i return from Asia at end of summer, we can arrange to get one to you.

The HP is ultra/uber with apc connectors (i have type n adapters for it) See HP 8753D. or a more portable but limited to 1GHz - Instek GSP810. A couple of Anritzu's are lower freq (HF) but dual-channel or diff input.

Would these be of any use?

[PS.. I have a wide band true random (white) noise source.... made by General Radio.]

THx-RNMarsh

Last edited by RNMarsh; 24th July 2015 at 02:21 AM.
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