Message 20313 in TECH.WW
From: G8MNY
Date: Sun, 05 Apr 26 08:12:00 Z
Newsgroups: TECH.WW
Subject: Marconi Distortion Meter TF2337
Message-ID: <56467_GB7CIP>
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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
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 and no return springs. One
coil is fed with rectified DC from the input level detector 1 and acts as the
return spring and 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.
It just uses simple RC transistor Amps and 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
Distorted Signal Filter Filtered Signal
The high pass filter must completely remove the fundamental and all LF hum 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
and 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.
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
MODERN ANSWER
Of course with a good quality PC sound card and a Furrier analysis program that
can add up the harmonics distortion measurement should be easy.
See my bul "Marconi 2331A Distortion Factor" & "Simple 1kHz AF osc".
Why don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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