THORLABS PDB440A, Operation Manual, Page: 12 / 29

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PDB440x and PDB450x detectors
3.6 CMRR and Frequency Response
An important specification for balanced amplifiers is the Common Mode Rejection Ratio
(CMRR) that reflects the ability to suppress common mode noise.
In the setup as described below, the Device under Test (DuT) - here a PDB440x and PDB450x
detectors balanced detector - is tested for CMRR. A common mode signal is generated, which
is canceled out when the amplifier is in balanced mode.
A network analyzer is used as signal generator (output A) and receiver (input B) The receiver is
synchronized with the signal generator and measures selectively at the same frequency. A
laser light source is modulated by the signal generator (port A) and acts as transmitter. To the
laser output a 3 dB fusion coupler is connected, splitting the modulated light signal into two
paths.Depending on the measurement task, one or both coupler outputs are connected to the
inputs of the DuT, for example using S120-FC adapters. One of the DuT's outputs is connected
to the network analyzers Port B.
Frequency response measurements
The frequency response of each signal path can be measured by connecting only one coupler
output to the appropriate input. This way, the frequency response curves of the RF OUTPUT
from INPUT + and INPUT- can be measured, as well as the frequency responses of the
MONITOR outputs, as shown in the individual technical data.
CMRR measurement
For Common Mode Rejection measurement, both outputs of the fusion coupler are connected
to both inputs of the DuT. The optical power level at both inputs must be well matched ("bal-
anced") in order to achieve the optimal common mode suppression. Now the common mode re-
jection can be measured as a function of frequency. The frequency response of the RF
OUTPUT must be considered when calculating the CMRR - it is the difference between the RF
OUTPUT signal at a given frequency and the measured common mode or balanced output sig-
nal - at the same frequency. Typical measurement curves can be found in the individual tech-
nical data.