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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Correction Sep 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Level Meter Distortion Meter
┌──────────────────────────────┐
│--┌───────────┐┌───────────┐--│ I have been working on one of these
│ │· ││· │ │ old AF automatic distortion meters.
│ │ '. ││ '. │ │ It is quite different to the usual │--└───────────┘└───────────┘--│ fiddly variable Wien Bridge deep
│ │ notch type. So it is ideally suited
│ 0 │ for setting up tape machines, where
│ Function Distort │ the playback level & frequency will
│ [][][][] [][][][] │ never be that steady.
│ O o │
│ [][][][][][][][] │
│ O ø │
└──────────────────────────────┘
In Level Range Mains

Level Ranges:- 30V, 10V, 3V, 1V, 300mV, 100mV, 30mV, 10mV.
:- 30, 20, 10, 0, -10, -20, -30, -40 dBV


Response :- 10Hz - 250kHz @ -3dB

Distortion Filters 400Hz, 1kHz & External.

Distortion Ranges: 30%, 10%, 3%, 1% min to 0.2%
& dBs -10 -20 -40 -40 min to -55dB

Harmonic Response: 1.5x Filter to 130kHz @ -3dB

Mains 230V. odd 3 pin 1cm square socket.

The automatic refers to the distortion measurement does not need the zero level reference level to be set first. (But is must be reading 30-100% on the level meter scale.) This is done not by a very low distortion AGC system as you might think, but a cleaver meter movement with 2 coils & no return springs. One coil is fed with rectified DC from the input level detector 1 & acts as the return spring & the other is fed from an 2nd detector from a high pass filter that removes all of the fundamental, so it just the harmonics. The resultant meter deflection is the distortion fraction.

In ┌─────┐ ┌─────┐ AC┌─────┐ ┌───────────┐ o──┤Level├─┤Input├─┬────┤Level├──────────────┤LEVEL METER├─────┐
│Atten│ │ Amp │ │ │Det 1│ DC └───────────┘ │
└─────┘ └─────┘ │ │ ├──────────────────────────────┐ │
│ └─────┘ ┌────┴─┴─────┐
│ ┌─────────┐ │ DISTORTION │
│ o─┤400Hz HPF├─o ┌─────┐ ┌─────┐ │2 COIL METER│
│ ├─────────┤ │ Set │ │Level│ └────┬─┬─────┘
└>o─┤ 1kHz HPF├─o<─┤Gain ├──┤Det 2├───────┘ │
└─────────┘ │ Amp │ │ ├─────────┘
o─ External ──o └┬┬┬┬┬┘ └─────┘ DC
^──┘Dist Range

It just uses simple RC transistor Amps & bridge diodes for the electronics. The notches are sealed LC high Q units. (M derived?)

There are only 3 presets:- Level gain, Distortion gain, & input amp Bias.

DISTORTION MEASURING PRINCIPLES
f1 f2
│ f1 0dB─┤ | ┌┴────── │ f1
Level│ │ f2 │ | │ High │ f2
│ │ │ f3 │ | │ Pass │ │ f3
│ │ │ │ f4 f5 -60dB_│___|_/ Filter │ │ │ f4 f5
└───┴───┴───┴───┴───┴ └────────────── └───────┴───┴───┴───┴
Distorted Signal Filter Filtered Signal

The high pass filter must completely remove the fundamental & all LF hums etc. but leave all the harmonics intact. The detectors (especially the distortion one) should be a true RMS type, as the waveform will generally be very complex & nothing like a sine wave. The ratio of f1 to all the harmonics f2+++ is the distortion factor.

Calibration is done by adding in a 2nd tone (e.g. 3-10x freq) via high value Rs at say 10% of the level to indicate 10% distortion. The distortion meter bandwidth can also be checked with this circuit.

1V 1kHz ─o-o───10k──┬───┬──────┐
Osc 1 o │ │ │
_│_ │ │ Distortion
│ DVM Meter
100mV │ _│_ _│_
2-20kHz ─o-o───10k──┘
Osc 2 o
_│_

To check the HPF filter is working OK a very pure sine wave is needed, so an LC tuned circuit after a signal generator is used. But I did find that > 0.5V @ resonance on the meter did make the inductor distortion rise > 0.01% !

Osc 88mH 500mV
50mV 1kHz ───((((()──┬─────┐ Hi Z
low Z │ Distortion
│ 0.28uF === Meter
_│________________│_____│

MODERN ANSWER
Of course with a good quality PC sound card & a Furrier analysis program that can add up the harmonics distortion measurement should be easy.


See my bul "Simple 1kHz AF osc".


Why don't U send an interesting bul?

73 de John, G8MNY @ GB7CIP



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