Manual FCKL 1528 page - 19
12 Meter and Meter Reading
12.1. Basics
We consider an EMI-receiver as a frequency selective voltage meter.
Frequency selective means, that only a. c.-voltages of the tuned frequency are measured
(within the specified bandwidth). Using the FCKL-receiver in the frequency range from
150 kHz to 30 MHz the bandwidth is 9 kHz (-6 dB).
A common analogue or digital multimeter also measures a. c.-voltages, but the
measurement is not frequency selective in the useful frequency range.
This frequency range is limited by the low (some Hz) and high (some kHz) frequency limit,
depending on the qualification of the meter in use.
The meter measures the sum, because all voltages within the range are fed to one
rectifier.
One single high voltage dominates the measurement.
Weaker voltages on other frequencies don’t influence the reading significantly.
The frequency selective EMI-receiver in contrast will show a multitude of voltages with
different frequencies separately.
A common multimeter has a basic dynamic range in which different voltages can be
measured without changing the range.
To extend the voltage range dividers are used to divide high voltages down to the basic
range.
Common multimeters for example have switch positions for 200 mV, 2 V, 20 V, 200 V,
1000 V.
The smallest voltage to measure in the 200 mV-range is 0,1 mV, the highest 199,9 mV.
Voltages < 0,1 mV are ignored, voltages > 199,9 mV show overflow.
In contrast to common multimeters which are scaled in V, EMI-receivers use a scaling in
dB
µ
V, which means dB over 1
µ
V.
The logarithmic dB-scaling is widely used in signal generators, pulse generators and
receivers.
It is for this reason that the attenuator steps of the FCKL are also in dB.
Just like the multimeter the receiver has a basic dynamic range and an attenuator.
If the attenuator is switched to zero attenuation, a noise floor of less than -10 dB
µ
V is
measured in the frequency range 150 kHz-30 MHz using the CISPR/Quasi-Peak detector.
0 dB (centre of the meter scaling) is 0 dB
µ
V/1
µ
V.
The right edge of the linear scale is +6 dB
µ
V/2
µ
V.
The right edge of the logarithmic scale is +26 dB
µ
V/17,8
µ
V.
The attenuator shifts this dB-range into higher voltages, the attenuation has to be added
to the meter reading.
This shift can also be made using the low noise position.
This attenuation is not added on the input, but near the output of the receiver and therefor
reduces noise.
This makes measurement easier because of reduced noise errors, but it increases
receiver loading which can result in errors caused by overload (saturation, compression,
intermodulation).