Rev: 2.4, 16-Jul-2021 © 2021 Thorlabs Page 9
3 Operating Instructions
PDB2xx Series
For Common Mode Rejection measurement, both output ports of the fusion coupler are con-
nected to both input ports of the DuT. The optical power level at both inputs must be well
matched ("balanced") in order to achieve the optimal common mode suppression. Now the
common mode rejection can be measured as a function of the frequency. The frequency re-
sponse of the RF OUTPUT must be considered when calculating the CMRR - it is the differ-
ence between the RF OUTPUT signal at a given frequency and the measured common mode
or balanced output signal - at the same frequency. Typical measurement curves can be found
in the respective Performance Plots .
3.8 Recommendations
Thorlabs PDB2xx Series Balanced Amplified Photodetectors can eliminate noise sources to al-
low precise measurements. The PDB2xx Series is designed for use in a dual beam setup: one
optical path for measurement and one invariant reference path. If set up properly, the PDB2xx
Series can reduce common mode noise for more than 35 dB over the specified frequency
range. Please find recommendations to achieve an optimal common mode suppression below:
· Equal power levels on each photodetector are essential. Any power imbalance will be ampli-
fied and hence decrease the possible noise reduction
· Equal power densities on both detectors are important to obtain maximum possible common
mode rejection. Always try to illuminate the whole active area of the detectors to prevent non-
linearities. Focused high power beams may lead to frequency response degradation, result-
ing in dramatically reduced common mode rejection.
· Equal optical path lengths are critical for common mode noise suppression especially at
higher frequencies. Any path length difference will introduce a phase difference between the
two signals, which will decrease the noise reduction capability of the balanced detector. The
figure below shows the maximum allowed path length difference in air to obtain a desired
CMRR. For fiber based application the maximum path length difference must divided by 1.5.
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