Prev: Class A error
Next: Garage Door Opener - Electric Eye
From: Robert Baer on 18 May 2010 04:10
John Larkin wrote:
> On Mon, 17 May 2010 14:17:09 -0500, Vladimir Vassilevsky
> <nospam(a)nowhere.com> wrote:
>
>>
>> John Larkin wrote:
>>
>>> I wonder how one could ever verify low distortion. I guess you could
>>> trust Audio Precision or Agilent or somebody, at their guaranteed THD
>>> floor.
>> The direct measurement with AP provides for the THD floor slightly below
>> 0.001%. I don't know if it is limited by generator or analyser, but this
>> is only -100 dB.
>>
>>> But is there any fundamental way to demonstrate the THD of a
>>> source or analyzer, or to make a sinewave source that's known to have
>>> below, say, 1 PPM distortion?
>>> How do you build a lowpass or highpass or bandpass filter that's known
>>> to have < 1 PPM distortion? Vacuum caps and toroidal inductors wound
>>> on quartz forms? Would there be mechanical issues that create
>>> distortion?
>> You just have to operate at small voltages and currents. Noise can be
>> eliminated by sufficient averaging.
>>
>>> An RC highpass filter could be switched into the signal chain and its
>>> effect on distortion noted. If it were a very good filter, and the
>>> switch/connectors were to be trusted.
>> Unfortunately, there is very little data on distortion in the passive
>> components. I did some experimentation with high voltage film capacitors
>> and good quality resistors; it is feasible to make an active filter with
>> 0.0001% distortion provided the opamp is good enough.
>>
>>
>
> I was thinking of using a rigid vacuum capacitor into a TIA as a
> differentiator/high pass filter. If some sort of analyzer reports the
> amplitudes of the fundamental and the harmonics, then you boost the
> harmonic content with a known-good highpass, that sort of crosschecks
> the analyzer. Or make a really good lowpass, ditto.
>
> We've occasionally used a passive notch filter to remove the
> fundamental of a sine wave before going into a spectrum analyzer. If
> the reported harmonics change with/without the notch, one suspects the
> analyzer.
>
>
Well, at minimum without filter, one can suspect overload distortion
in the front end (if difference is seen).
Noting that, there is no guarantee that all is OK if the filter is used..
...only that things are a bit better..
From: Robert Baer on 18 May 2010 04:18
George Herold wrote:
> On May 17, 4:27 pm, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
>>> On May 17, 11:35 am, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
>>>> I need to make a test source with THD ~ 0.0001% at the frequencies below
>>>> 1kHz. Leaving many details aside, the biggest problem seems to be an opamp.
>> George Herold wrote:
>>> How about a different opamp? The opa134 seems to have enough zero's
>>> in the THD+noise spec. (0.00008%)
>
> " That's what I planned on using initially. Unfortunately, the
> availability is an issue."
>
> Geesh, you scared me there for a moment, (All my favorite TI/BB opamps
> seem to have 6 month delivery times these days.) but TI reports stock
> almost everywhere. Still I'd better order a couple of tubes.
* Now we know why there is a stocking problem..

>
> George H.
>
>> David Eather wrote:
>>> This might be better:
>>> http://www.national.com/pf/LM/LME49990.html#Overview
>> I thought about bipolar opamps from National also. One drawback is high
>> power consumption, the other problem is high noise current. I am not
>> sure if it would be possible to make them work right with ~100k
>> impedances in the circuit.
>>
>> VLV
>
From: whit3rd on 19 May 2010 18:39
On May 17, 8:35 am, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
> I need to make a test source with THD ~ 0.0001% at the frequencies below
> 1kHz. Leaving many details aside, the biggest problem seems to be an opamp.

At the lowest frequencies, the op amp temperature is an issue, but at
1 kHz
the big problem will be in the output stage (usually class AB).
Crossover distortion cannot be eliminated by feedback to the desired
extent.
You can apply a bias current to the output pin to move the bias to
class A,
which should improve most op amps. Preamp on one chip and
power stage on a separate chip is a possible strategy, and heatsinking
the power stage might help a lot.

Transformers and emitter followers are fairly safe, but resistors
aren't
as good as you'd want. Self-heating is completely nonnegligible
at low frequencies, and the best resistors aren't specified for their
thermocouple voltages AT ALL. Resistors will make second-harmonic
distortion.

Lotsa luck verifying your distortion figures. I'm thinking there
will need to be some tuned amplification.


From: Tim Williams on 19 May 2010 19:00
"whit3rd" <whit3rd(a)gmail.com> wrote in message
news:8ddff6a1-fd6c-4186-9bef-99347114cda2(a)b21g2000vbh.googlegroups.com...
> Self-heating is completely nonnegligible
> at low frequencies, and the best resistors aren't specified for their
> thermocouple voltages AT ALL. Resistors will make second-harmonic
> distortion.

Tektronix called this effect "thermals", although that was regarding
transistor self-heating specifically.

Curiously, this effect has been recognized since the vacuum tube days.
Wonder if that's due to plate/grid heating, exapnding and distorting the
electrode structure.

Of course, a resistor with AC across it will cause zero 2nd, but it will
cause 3rd and other odds.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: David Eather on 19 May 2010 19:30
On 18/05/2010 4:28 AM, Mike wrote:
> Vladimir Vassilevsky<nospam(a)nowhere.com> wrote:
>> Would it be possible to make a cascaded amp to boost
>> the open loop gain and improve distortion?
>>
>> Vladimir Vassilevsky
>> DSP and Mixed Signal Design Consultant
>> http://www.abvolt.com
>
> Yes. See AN67, Linear Technology Magazine Circuit Collection, Volume III
>
> An Ultrapure Oscillator By Dale Eagar.
>
> Http://cds.linear.com/docs/Application%20Note/an67f.pdf
>
> Article starts on page 62
>
> Schematic is Figure 96 on page 65

This is just my curiosity..

Having a look at this circuit they are using an lt1228 OTA for amplitude
control. Does anyone have any experience in using a light dependent
resistor in this role and what sort of distortion did they experience?



First  |  Prev  |  Next  |  Last
Pages: 1 2 3 4 5
Prev: Class A error
Next: Garage Door Opener - Electric Eye