A low range ac voltmeter suitable for connecting across a 600 ohm line and self contained except for requiring an external 3.6k ohm resistor. Calibrated for use with a 3.6k ohm resistor 4vu above the scale marking so that the reference deflection (0vu) is produced by a signal 4vu above 1mW in 600 ohms (1.228v).
The performance of the meter is based upon the requirements of BS 6840: Part 17:1991, IEC 268 - 17: 1990. The dynamic characteristics quoted, which requires the response time to 99% of reference deflection (0vu) to be 0.3 seconds +/-10% with an overswing of at least 1.0% but not more than 1.5% are not fully met.
The overswing, because damping is not so tightly controlled but allowed to spread within normal design restraints, can vary between deadbeat to 5%. To specify the dynamic characteristics in terms of time to 99% of 0vu under such circumstances would give an unrepresentative measure of the speed of response. For this reason the time to reach -3vu with a steady reading of reference deflection (0vu) has been taken as a more realistic measure.
The following comparison can be made based on a rise time to -3vu with steady deflection of 0vu:
IEC 0.13 - 0.15 seconds
AL39 0.17 - 0.21 seconds
For back of panel mounting with bezel and with only the scale and pointer visible.
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