Rev: 2.4, 16-Jul-2021 © 2021 Thorlabs Page 7
3 Operating Instructions
PDB2xx Series
voltage swing of the MONITOR output is +10 V for high impedance loads (+1.5 V into 50 W).
Saturation of MONITOR output will occur at optical input power level greater than 1 mW, de-
pending on the detector wavelength response.
MONITOR output can be used to roughly adjust equal input power levels on each detector for
balanced operation. MONITOR outputs of the unit are also convenient to use for free-space
beam alignment.
The amplifier offset voltage is factory set to zero at 23°C ambient temperature. A small drift dur-
ing a short warm-up period (~5min) may occur. For exact DC light level measurements a con-
stant temperature environment is recommended.
3.5 Mounting
As shown in the chapter Operating Element , the PDB2xx Series is housed in a rugged shiel-
ded aluminum enclosure. This slim line housing has three tapped holes for convenient mount-
ing to the optical setup. PDB210A, PDB201C and PDB220A2 can be purchased as imperial or
metric versions (/M) with the tapped mounting holes 8/32 or M4, respectively. PDB230A and
PDB230C feature tapped holes which are compatible for both imperial and metric threads.
The input aperture is fitted with both an external SM1 and an internal SM05 thread that is com-
patible with any number of Thorlabs' SM1- and SM05-threaded accessories. The included
SM1T1 allows externally SM1-threaded accessories (e.g. external optics, filters, apertures, or
fiber adapters) to be mounted to the input aperture.
The PDB2xx Series has three tapped mounting holes to mount the unit to a imperial or met-
ric post or pedestal, depending on the PDB2xx model.
3.6 AC Coupling of the Output Signals
Beside the standard DC coupling of the RF OUTPUT, AC coupled versions for any model of
PDB2xx Series are available on request. AC coupling blocks the CW component (the unmodu-
lated 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 is measured on a strong CW background signal, which
could saturate the amplifier. With AC coupling, equalizing of CW power levels of both input sig-
nals 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.
The lower cut-off frequency of the AC coupled versions is typically below 5 Hz.
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 modu-
lating the input signal with a mechanical chopper at a frequency of 500 Hz.
Note Input signal for AC coupling was increased by factor 2 to allow direct waveform compar-
ison
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