2.1.1 The Racal-Dana Wideband Level Meter
light, portable, but extremely versatile, microprocessor
controlled instrument. It permits measurements to be made
with exceptional accuracy on waveforms having high crest
factors. Special automatic zeroing and noise cancelling
circuits permit measurements to be made over a range of
signal amplitudes extending from 316 V r.m.s. down to 30 uV
r.m.s. The instrument has an auto ranging facility, but
provision is made for manual range selection.
2.1.2 Measurements may be made on DC signals, on AC signals
in the frequency range from 5 Hz to 20 MHz and on signals
having both DC and AC components. A low pass filter can be
switched in to restrict the operating bandwidth to 200 kHz
2.1.3 The instrument measures true r.m.s., mean, positive
peak or negative peak voltages of the applied signal.
Operator set values of resistance, voltage and power may be
stored in the instrument, and, in addition to display of the
measured value in voltage units, displays of the following
computed values can be obtained:
(a) Average or peak power, computed from V^/R.
(b) The ratio of the computed power to the stored power level.
(c) The ratio of the measured voltage to the stored voltage
(d) The difference between the measured and stored voltage
(e) The difference between the computed and stored power levels.
(f) The difference as in (d) expressed as a percentage of
the stored voltage level.
(g) The difference as in (e) expressed as a percentage of
the stored power level.
The instrument will also compute, and display, mean value
scaled to r.m.s., peak·-to-peak value, form factor and crest
2.1.4 The input common line is isolated from chassis earth
by back-to-back diodes, which permit a voltage difference of
±0.5 V. The common line can be connected to chassis earth,
if required, by means of an isolation override switch.
2.1.5 The instrument features a large, four digit, liquid
crystal display (LCD). Coarse and fine LCD bar and dot
displays form part of the display, and provide a
pseudo-analogue form of indication. Units annunciators, GPIB
status indicators and instrument status indicators are also
2.2 CREST FACTOR
2.2.1 The maximum permitted crest factor on any range
increases in inverse proportion to the displayed
measurement. Errors will arise if the measured waveform has
a significant proportion of its energy in harmonics which
lie outside the instrument's measurement bandwidth.
2.3 OPERATING PRINCIPLES
2.3.1 Following a common input attenuator the instrument has
separate AC and DC measurement channels. The AC channel
features separate detectors for r.m.s., mean, positive peak
and negative peak measurements, so that these waveform
parameters are all measured simultaneously.
2.3.2 The detector outputs are connected in turn to an
analogue to digital Converter
. The microprocessor scans the
output every 0.1 second, reading each detector
output in turn, One detector output, determined by the
measurement function selected, is either converted directly
to a display drive signal or processed, together with the
contents of various internal stores, to provide power,
difference or percentage difference display signals.
2.4 REDUCTION OF DISPLAY JITTER
2.4.1 The instrument has a variable averaging time, which
may be set by the operator to ensure minimum jitter of the
measurement indication when measurements are made on
waveforms of high crest factor. The period over which
measurements are averaged may be varied in steps of 0.1
second between 0.1 second and 99.9 seconds (nominal).
2.4.2 The display updating rate is set by the averaging time
in use. To avoid the necessity for unacceptably low updating
rates, continuous averaging of the measurements can be
introduced by means of a special function. In this mode the
display is updated every 0.1 second, the new displayed value
being formed by adding Ν3ί of the current displayed value to
(100-N%) of the new measurement. The effect of this is
similar to filtering successive measurements in a single
pole RC filter. The value of N is related to the time
constant of the effective filter, which can be set by the
2.5 STORAGE OF FRONT PANEL SETTINGS
2.5.1 Provision is made for the storage of up to twelve
complete sets of front panel control settings, including the
values set in the computed function stores, in a
non-volatile memory. Each setting is allocated a number, and
is retrieved and set by recalling that number. The front
panel control settings in use when the instrument is
switched off are automatically stored, and may be recalled
when the instrument is switched on again.
2.6 CALIBRATION TO AN EXTERNAL SOURCE
2.6.1 The instrument display may be set to a reference
value, when measurements are made on a reference signal
source, by means of an operator selected calibration factor.
The calibration factor, which is a divisor, is stored in the
instrument, and may be enabled and disabled as required when
making measurements on other signal sources.
2.7 GPIB Interface
2.7.1 An internally mounted Interface
board permits the
instrument to be controlled from, and communicate with, the
IEEE 488 GPIB. The instrument may be used in the addressed
mode or in the talk only mode. An adaptor to permit use with
the I EC 625-1 GPIB is also available.
2.8.1 It is recommended that customers should take advantage
of the servicing and calibration service offered by
Racal-Dana Instruments Ltd., and their agents. For customers
wishing to carry out their own servicing, a comprehensive
Maintenance Manual is available from Racal-Dana Instruments
Ltd. When ordering a manual, the serial number of the
instrument for which the manual is required should be quoted.