Re: Noob question about distortion

On Feb 29, 5:05 am, nos...@xxxxxxxxxx (Don Pearce) wrote:
On Fri, 29 Feb 2008 03:37:58 -0800 (PST), distort...@xxxxxxxxx wrote:
On Feb 29, 2:41 am, nos...@xxxxxxxxxx (Don Pearce) wrote:
On Fri, 29 Feb 2008 01:28:40 -0800 (PST), distort...@xxxxxxxxx wrote:
Hello All,

I am dealing with a conundrum about distortion measurements in an
amplifier which is serving as a DUT.  The architecture consists of a
two op-amp configuration that allows a single-ended input and BTL
output.
The issue arose with the difficulty of using the Audio Prescision
System II in developing a PSRR curve. Instead, the setup with the AP
consisted of two signal sources:

Input signal source: 1Vp@1 kHz
Source connected to the power supply rail: 100mVp@ sweeping 10 Hz - 20
kHz

The AP was configured to measure THD+N.  So the idea is to see how the
THD+N of the amplifier is affected by power supply ripple rather than
your traditional PSSR curve.

My conerns about this test is that it is really a poor Intermodulation
Distortion test.  SMPTE, DIN, CCIF from what I have researched do not
use a sweeping frequency; rather 2 tones of specified frequency.  So
the test methodology is fundamentally in error: source frequencies
used, and the test setup (THD+N measurement rather than a setup to
measure Intermod).  Am I "correct" in my thinking here?

I would also like to know if you can predict THD+N from an Intermod
measurement and vice versa.

Now, I will probably be told go Google, but I have been having a hard
time researching this topic.  I am very green to audio.  I hope this
is the right group to ask!

Thanks in advance.

You appear to have got the whole thing confused.

First the PSRR test. Connect a single source in series with the power
supply. A 1:1 transformer is a good way to do this. Simply connecting
it "to" the power supply just shorts out the source oscillator. You
don't measure distortion, just how much signal appears at the output
of the amplifier. The PSRR is the ratio of the source signal level to
the output signal level. Don't feed any signal to the input of the
amplifier.

Yes, I agree.  I should have been more clear.  There have been several
methods used for PSRR: (1) using the AP as the generator connected via
a capacitor or (2) a transformer, or (3) a simple power amplifier as
the generator and it is connected in a summer configuration (AC+DC) to
the power supply of the DUT.  In all three methods the AP was used as
the input analyzer.  No input was applied to the DUT; the inputs were
AC coupled to ground.  A DC bias to Vcc/2 ensures a DC return path so
this AC coupling to ground is no problem.
However, the results were not yielding expected values so another
approach was used, and that approach was to inject a ripple onto the
power supply rail with a fixed (1Vp@1kHz) input signal and measure the
corresponding THD+N.  I am curious to know if measurements taken at
the output would be a valid THD+N measurement or rather it is a
(poorly implemted) intermod test since there are two sources being
mixed together.

It depends on the amplifier, the frequency and all sorts of stuff.
What should happen is that the ripple signal you put on the power
supply simply appears at the output in diluted form. If the amplifier
is not designed well, it is possible that the ripple will instead
amplitude modulate the audio you feed in. If that happens you will see
the signal frequency plus two more sidebands at signal +/- ripple
frequency. But that has nothing to do with the actual intermod spec of
the amplifier per se. It is really better to keep these measurements
apart.

When you say "if the amplifier is not designed well [...] the ripple
will instead amplitude modulate the audio..." are you impling the
specification at fault here would be the PSRR of the amplifier?
Also, if I see the signal frequencies applied to the DUT and the
sidebands, is this not intermod?
I am not disagreeing here; rather, I want to ensure I understand.


Now for distortion. Both harmonic distortion and intermodulation are
simply manifestations of the same thing. Since there are two signals
involved in the intermod test you can expect the measured result to be
a little different to THD, but not very. In general an amplifier that
has good THD specs will also be good for IMD. The big advantage of the
IMD test is that it yields useful results all the way up to the top of
the measurable band.

Well, what I wanted to know is my test setup was as follows:

Source 1 on the input = 1Vp@1kHz
Source 2 on the supply rail = 100mVp@(10Hz to 20kHz)

The resulting output is measured using a typical THD+N distortion
analyzer.  IIRC this consists of a notch filter tuned to the
fundamental, then a bandpass limiting filter, and a true RMS
detector.  As above is this:

1) A valid THD+N measurement?  I am being told that it is.

No. Remove the power supply signal if you want a valid THD+N
measurement.

Ok, I did not think such a setup was a valid THD+N measurement either;
however, can you elaborate a little on why it is not?
My answer would be that what is being measured are not harmonic
components of the input signal. With the Audio Precision, I believe
you can move the frequency of the notch filter but only one signal
would be removed. Meaning the fundamental of the input signal or the
power supply signal would be removed. Since one signal frequency
remains then this is not a THD+N.

2) The above setup is *similar* to several Intermod standards, but not
exact.  I look at the above setup as a poorly implemented
Intermodulation Distortion test.  So when I have my Audio Precision
setup to measure THD+N then I suspect the measurements are
meaningless.

To measure intermod characteristics, both signals should be applied to
the input. Only put signals on the power supply if you are
specifically doing PSRR tests.

You say that *in general* if THD+N is good, then so is intermod.  Ok,
so it is a Rule of Thumb.  But can one be converted to another?

Probably, but I haven't done the maths so I can't say for sure.


Not a problem.

Let me give you a little bit more background on this issue. The
reason I am asking these questions deals with the difficult I have had
with a PSRR test. I completely understand that the current set up
(input signal + signal on power supply) is *NOT* a PSRR test. Rather
it is a substitute, and some of the senior apps say that seeing a "THD
+N" vs. Power Supply Frequency curve would serve customer's better.
Simply put, "We cannot give you a PSRR curve, but there is this THD+N
curve vs. Power Supply Frequency" which we think is better. I, the
junior engineer, am having trouble justifying this hence this
discussion thread.

.

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