GBS Elektronik MCA-166 Influence of very long detector cables on spectrum measurements - Manual

Application Note
Influence of very long detector cables on spectrum
measurement with MCA166
12.4.2000
Jörg Brutscher
GBS Elektronik GmbH
Problem:
For some applications, it is desirable to use very long detector cables. Normally, only a short detector cable
comes with a detector and it is not recommended to use longer cables.
This paper shall point out the problems which occur using very long detector signal cables, how they can be
overcome and what compromises have to be made in terms of system performance.
Effects and problems are not specific to MCA 166, but concern interaction of any MCA with a detector.
Experiment
A Co60 source was measured with a GL0310 HPGe detector. At first, the spectrum was measured with the
standard cable (approx. 4m length). Then the spectrum was measured again with a 250 m coaxial cable
(impedance 50Ω) introduced between detector signal out and MCA signal input.
Measuring the Co60 spectrum caused 90% of the high energy part of the spectrum to be cut off! Examining the
preamplifier signal with a scope showed why:
As the MCA 166 has an input impedance of 1 kOhm, the input is not matched to the cable impedance and the
signal is reflected. Also the detector preamplifier has a poor match to the cable and the signal is reflected again.
This can be seen as an attenuated version of the original signal with a delay which corresponds to 2 times the
cable signal traveling time. The cable length of 250 m corresponds here to a delay of 2.7 µs.
0 2 4 6 8 10
Time (µs)
-2
-1
0
P
re
a
m
p
S
ig
n
a
l
(V
)
Preamp Signal with short cable
Preamp Signal with 250m cable
Preamp signal with 250 m cable, terminated by 50 Ohm
Signal
Echo
Fig. 1: Preamplifier signal with short cable, with long cable (250 m) and with long cable and a 50 Ohm
terminating resistor parallel to the MCA input.