Circuit Notebook 106 - Video Amplitude Test Box

Introduction

Click to view full size in a separate window To measure the amplitude of a video signal, it is usual to feed the signal (appearing at the end of a coax cable which has been terminated by a 75Ω load resistor) into the vertical 'Y' input of an oscilloscope.

The 'Y' input sensitivity would be set to '0.2V/division' and the time-base to '10µs/division'. With the time-base triggered correctly the display will show a conveniently sized image of one line (64µs) and a bit of the next line. A signal of 1V amplitude would occupy 5 vertical divisions of 0.2V on the screen.

Using the vertical 'Y' shift control, the bottom of the sync pulse (or alternatively black level) would be set to a line on the oscilloscope graticule and the amplitude of the desired part of the waveform read off against the graticule divisions. This may mean estimating the position of part of the waveform where this does not fall on an exact line.

If you have one of the latest wiz-bang oscilloscopes, it will probably have two movable cursor lines to set on the parts of the waveform you want to measure, and the result would be shown directly in volts on a digital display.

Operating Principle

The operating principle of the Amplitude Test Box, shown in Fig.2, is that of a calibrated voltage off-set control. The incoming video signal is terminated in 75Ω and the signal passes through a variable DC voltage supply to the 'Y' input on an oscilloscope. The variable supply is monitored by a voltmeter, in practise a Digital Multi Meter (DMM) is used.

Circuit

In the practical circuit, shown in Fig.3, the video input has a switched 75R terminating resistor, for use when required. The video input is also 'linked through' to the video output for in-line measurements The variable voltage (in series with the video signal) is supplied by a battery and a potentiometer, the output of which is monitored by the DMM. This voltage is decoupled by C1 to maintain the HF response into the oscilloscope. As all the components and the DMM are floating live, they form about 60pF of stray capacitance to ground. This is in addition to the oscilloscope input capacitance and that of the short connecting cable, the total being less than 100pF.

Construction

Click to view full size in a separate window The circuits of the Test Box are built into a simple two part aluminium enclosure, shown in Fig.1, although any similar sized enclosure could be used. The position of the front panel items is shown in Fig.5. The drilling positions and sizes are not stated, as switches, terminals and connectors may vary, especially if they are rescued from scrap equipment. The labelling of the front panel is done on a PC using a drafting programme. It is then printed and laminated. The front panel area is cut out and fixed to the enclosure using thin double-sided adhesive tape and the holes punched through. The internal wiring is point-to-point, as shown in Fig.4, and is positioned to minimise stray capacitance to the metal enclosure.

Using the Test Box

A typical set-up is shown in Fig.6. The incoming video signal is connected to the input and is terminated by the switched 75R load. The 'scope' output is connected to the oscilloscope 'Y' input by a short coax cable. The oscilloscope input is switched to DC and the 'Y' shift control adjusted to align the bottom of (say) the sync pulse to the centre horizontal line on the graticule.

Click to view full size in a separate window The variable voltage is switched on and then adjusted, causing the image to move downwards, until (say) the top of the waveform (peak white) aligns with the centre line. The voltage displayed by the meter is then equal to the difference between the points being measured i.e. sync bottom to peak white, (980mV in Fig.6). By referring each time to the centre of the screen, any non-linearity in the oscilloscope 'Y' amplifier system is eliminated and any parallax error minimised. The Test Box could also be used for staircase waveform measurement and other pulse and sine wave peak-to-peak measurements.

Parts List

C1 10µf 16V electro
R1 75R 1% 250mW
R2 220R 5% 250mW
R3 1k0 5% 250mW
RV1 1k0 linear pot
D1 LED green
SW1 switch SPST 'TERM'
SW2 switch SPST 'BATT'
2 - 4mm Terminals
3 - 75R BNC panel sockets
2 - AA Batteries
1 - Battery holder (2-AA)
1 - 2-part Enclosure Maplin LF08 (Box AB7)

References

  1. Television measurements - PAL Systems Margaret Craig, Tektronix Television Division, Beaverton, Oregon, USA.
  2. Television video transmission measurements L. E. Weaver, Marconi Instruments Ltd.

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