8.9. Electronics Noise 519
8.9.C MeasuringSystemNoise.....................
As mentioned earlier, the particular detector application dictates the design of signal
processing circuitry. However, after the design and development process, it is imper-
ative that the noise of the system is carefully measured to determine the equivalent
noise charge (ENC). The equivalent noise charge is defined as
ENC =
Qsignal
S/N(8.9.31)
=
Vnoise
VsignalQsignal (8.9.32)whereVnoiseandVsignalare the output noise and signal pulse heights andQsignalis
the input signal charge. These three parameters must be experimentally determined
to compute theENC. The best method to to this is to inject a known signal into
the circuitry and measure the relevant parameters of the output pulse. Since in such
a situation the input chargeQsignalis known therefore one needs to determine only
the other two parameters. Of course the output depends on the input as well as the
circuitry and therefore if the input signal itself has significant noise then it would be
highly unlikely that theENCof the system is accurately determined. Therefore in
general one tends to use high quality pulse generators for this purpose. There are
different ways to analyze the signal output, with use of spectrum analyzers being
the most popular choice. Most modern spectrum analyzers have the capability to
determine the spectral distribution through Fourier transformation of the input sig-
nal. They also provide a host of mathematical manipulation functions to determine
other related parameters, such as peak widths andFWHM.
The heights of the signal and the noise voltages can thus be determined from
the spectrum analyzer. These values can be substituted in the above expression to
determine the equivalent noise charge.
A point worth mentioning here is that one could, in principle, use a known
radiation source instead of a pulse generator. This would be advantageous if one
is interested in determining theENCfor the whole system including the detector.
When a pulse generator is used, the measured equivalent noise charge corresponds
to the electronic components only.
8.9.D NoiseReductionTechniques...................
Any radiation detection system contains a number of noise sources. Elimination or
reduction of these noises is, however, dependent on theacceptablesignal to noise
ratio. The acceptable S/N is of course application dependent. It is therefore a good
practice to first estimate the maximum noise level that could be afforded in the
system before deciding on the signal processing and transport modules.
The most bothersome of the noise sources in a detection system are related to
electronics components. We visited some of these sources in the previous section.
Now we will discuss how we can reduce their effect on the overall signal to noise
ratio of the system.