eenvoudig zelfbouw notchfilter voor IMD metingen

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nicolaas
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Lid geworden op: vr 04 nov 2016, 11:46

eenvoudig zelfbouw notchfilter voor IMD metingen

Berichtdoor nicolaas » vr 04 nov 2016, 14:35

Kort geleden heb ik een notchfilter gebouwd voor IMD metingen en gepubliceerd. Ik wil het dit forum niet onhouden, maar het is in het Engels, hopenlijk geen probleem.
In order to measure the THD of tube amplifiers (adjusting Bias, cathode resistors etc.) I needed a simple notchfilter that can measure THD down to approx. 0,01% U. My generator is an old LEVELL that should be better than 0,05%, good enough for these mesurements of tube circuits.
because I wanted to tune automatically, I used a circuit that is not critical for parts value's. The meter was designed and build in a few days and does not pretend to be an instrument to measure down to unbelievable low value's, it is just a practical tool for the design of my amps.
The THD meter is finished and works as expected. I use 700 Hz in order to have all components and expected harmonics well in the audio band.
First of all, the diagram
The input goes via a 500K pot to an opamp in order to have a high input impedance. The input level should be adjusted to 200 mV eff, a meter circuit is included wich should be calibrated . Screem the input from the meter circuit, the signal at the meter may introduce some capacitively coupled distorsion into the input. The input to the filter is also led to a comparator in order to have a reference signal for the nulling loops.
The filter is a variable state filter connected such that there is little dependence of component value's. Normal E12 resistors and caps can be used. The filter has a notch output (via an amplifier), a band pass output (BP) and a 90-degrees output, used in the nulling loops.
When build as such, the filter will give you approx a notch with a factor 50 - 100 attenuation, wich is not sufficient for most THD measurements.
But the Notch output is in counter phase with the BP output, so when a small amount of BP signal is added to the NOTCH output, one can get a very deep null. But the frequency becomes extremely sharp ( a part of a Hz) and tuning becomes almost impossible.
Therefore, I added a control loop for the frequency by adjusting the drive resistor of the second integrator with a LDR driven by a red LED. Both are mounted together in a piece of shrink sleeve. The tuning information comes from the phase of the REF signal and the 90 degrees signal. These are 90 degrees out of phase when tuned correctly. I used the pahse comparator 1 of CD4046 PLL circuits for simplicity
In order to test, first test the filter without the LDR for the frequency loop and the nulling loop connected. One should get a Notch at approx 6500 Hz. Set the DC NULL so that there is no DC at the NOTCH output.
Now adjust the loop gain for the freq. loop to midrange and connect the LDR for the freq. loop. The tuning should go to apprix 700 Hz and the loop will have much less frequency dependency. Now increase the loop gain until the loop starts oscillating and back-off a little.
There should be a remainder of the 700 Hz visible at the NOTCH output.
This remainder can be nulled by a variable resistor of 100 K between the NOTCH and the BP to add a sample of BP signal to the NOTCH signal. You should adjust between both the FREQ adjustment and this resistor to get the deepest null, but again it is a very sharp null. So the NULLING loop is connected also using a LDR wich is connected between the BP and the NOTCH. (instead of the proposed variable resistor.)
The Null should inmedeately become very deep. Now adjust the FREQ ADJ trim pot for minimum NOTCH signal and only the harmonics of the 700 Hz frequency source are now visible. Adjust the nulling loop gain at the max gain possible without oscillatory effects.
Calibration
Use a low distorsion generator at 700 Hz and adjust the level.
Let the meter warm up for 10 minutes (always do before use) and fine adjust the freq. adjust to have the deepest null.
The output meter should be calibrated for 0,1% FS, 1%FS and 10% FS, but calibratio at 1% FS is sufficient.
The null should be less than 0,05% THD. Mine is 0,03% for my LEVELL generator.
Now define the generator output impedance (mine is 600 Ohms ) and connect a second generator via a resistor of 100X the generator impedance (so e.g. 56 kOhm) to the generator output. Adjust the second generator for the same output level as the 700 Hz generator but at a frequency between 1,4 and 10 kHz. This should add 1% distorsion to the 700 Hz signal and adjust the NOTCH GAIN until the meter shows 1%
BUILDING
I did build at PCB using islands glued to the surface with cyano glue. The IC's are glued upside down. Please note that the pincount at the bottom view is CW. mark where the pin 1 is with a felt pen. Building this way gives a very good ground plane and one can build very fast just following the diagram.
The circuit uses cheap aliexpress opamps with good results.
The power should be between +/- 6VDC and +/- 7,5 VDC, well stabilsed and low impedance. I used 78L05 and 79L05 with a red LED in the common in order to raise the voltage to 6,6VDC
Bijlagen
THD meter schema 1.jpg
THD meter schema 2.jpg
THD meter 001.jpg
THD meter 002.jpg
THD meter 003.jpg

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Henkjan
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Re: eenvoudig zelfbouw notchfilter voor IMD metingen

Berichtdoor Henkjan » vr 04 nov 2016, 16:58

da's een leuke binnenkomer! welkom hier Nicolaas.

nicolaas
Berichten: 3
Lid geworden op: vr 04 nov 2016, 11:46

Re: eenvoudig zelfbouw notchfilter voor IMD metingen

Berichtdoor nicolaas » za 05 nov 2016, 21:23

Met deze een fototje van de vervorming van mijn LEVELL generator. De vervorming is 0,025% en wordt hoofdzakelijk door de compressie van de negatieve helft van het signaal veroorzaakt. De lijn dikte van de vervormings signaal is wat groot door brom en ruis hoofdzakelijk vanuit de generator.

nicolaas
Berichten: 3
Lid geworden op: vr 04 nov 2016, 11:46

Re: eenvoudig zelfbouw notchfilter voor IMD metingen

Berichtdoor nicolaas » za 05 nov 2016, 21:30

Sorry, het lukt niet het fototje te plaatsen. Weet niet waarom. Probeer het nogmaals
THD meter, 0,025% LEVELL generator.jpg


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