Nanosecond Photodetectors
Operation • 9
Supplying your own load resistor
At high frequencies, typical resistors begin to exhibit
parasitic inductance and capacitance. For instance, if
you want to operate on the Open setting and supply
your own 100-
Ω
resistor, you might be tempted to use
a standard carbon-film resistor soldered across the ter-
minals of a BNC tee. The problem with this approach is
that the parasitic inductance of such a home-made
resistor will cause a significant change in the imped-
ance at high frequencies. So, for best results you may
need to buy a precision load resistor that is optimized
for high-frequency use.
You must also watch out for impedance mismatch
problems. The internal transmission line from the
photodiode to the BNC output has a characteristic
impedance of 50
Ω
. This has been chosen for best
compatibility with 50-
Ω
impedance equipment. If you
terminate this transmission line with a non-50-
Ω
load,
you should expect degraded frequency response.
When you use 50-
Ω
impedance equipment, you
should switch the nanosecond photodetector into the
Open
position. If you leave the internal 50-
Ω
resistor
switched in, the photocurrent will be divided between
the two loads, cutting your signal in half. If you leave
the internal 10-k
Ω
resistor switched in, it will have lit-
tle effect, as the 50-
Ω
load will draw the majority of the
photocurrent.
DC blocking devices are a potential source of confu-
sion. Some sensitive high frequency equipment
requires 0 V at DC input. In this case, we recommend
that you put a high-frequency DC blocking load
between the nanosecond detector and your equip-
ment. If so, you must supply a DC path to ground for
your photocurrent. This is easiest to do by switching in
the photodetector’s 50-
Ω
internal load resistor. If you
switch in the 10-k
Ω
load resistor, you must be certain
that the circuit does not saturate itself on the DC level.
162x nanosec revB.fm Page 9 Friday, August 17, 2001 5:51 PM
