© 2019 Thorlabs
3 Operating Instruction
9
3.4 Mounting
The PDB440x and PDB450x detectors is housed in a rugged shielded aluminum enclosure.
For post mounting an adapter can be attached to the bottom or side surface using four M2x8
screws (see below). This adapter supports #8-32 as well as M4 post mounts. The M4 tread is
marked.
Attention
Please note that for PDB440X the INPUT- is on top and INPUT+ is on the bottom.
3.5 AC Coupling of the Outputs
Beside the standard DC coupling of the RF OUTPUT, AC coupled versions for any model of
PDB440x and PDB450x detectors are available on request. AC coupling blocks the CW com-
ponent (the unmodulated part) of the optical input signal. However, large CW components of
the optical input signal will decrease linearity of the detectors.
AC coupling helps to improve the measurement capabilities in applications, where a compar-
ably weak frequency modulated signal shall be measured on a strong CW background signal,
which could saturate the amplifier. With AC coupling, equalizing of CW power levels on both in-
puts is not mandatory for noise cancellation. However, for optimal noise suppression the signal
of interest (e.g. the modulated part) should be well balanced. AC coupling is also recommen-
ded when using the balanced detector in combination with a chopper and lock-in amplifier to
further decrease noise level.
Please note, that AC coupling slightly increases noise figures at lower frequencies. The mea-
surement bandwidth of the RF OUTPUT is not affected by AC coupling.
The figure below shows the comparison of AC and
DC coupled RF Output signals when modulating
the input signal with a mechanical chopper at a fre-
quency of 500 Hz.
Note : The input signal for AC coupling was in-
creased by factor 2 to allow direct waveform com-
parison.
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