Download University of LeicesterPLUMERef: PLM-PAY-LabElecsGain-014

University of Leicester
PLUME
Ref: PLM-PAY-LabElecsGain-014-1
Date: 02/10/2008
Finding the gain of the electronics chain
P. peterson
Date
Updated Reference Number
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02/10/2008
PLM-PAY-LabElecsGain-014-1
first version issued
Gain is defined as the ratio of an input voltage to an output voltage. It is very important to
know the gain produced by the electronics chain under different conditions both for use of
the multichannel analyser (MCA) and to calculate the size of the charge pulse when using
the microchannel plate (MCP) detector body.
The equipment used for this experiment is given in the “Equipment Configuration” document.
This series of experiments calculates the gain for every component in the chain individually,
but for the actual experimental setup with the MCP we transferred a lot of the signal
processing electronics to a large NIM rack and significantly changed the chain components.
Because this will likely interfere with the gain, the chain will need to be recalibrated after the
initial testing of the MCP, and next time we will calibrate the chain as a whole, not
component by component, to reduce error.
Basically, we set the electronics chain up and use the pulse generator to send pulses
through it, then connect one channel of the oscilloscope to one side of a component an the
other channel to the other side. Then, we just compare the maximum height of the voltage
spike on one side with its height on the other to get the gain.
Preamp
This is a charge-sensitive preamp, not a voltage amplifier, but we can simulate the MCP’s
charge pulses by using the test connection of the preamp. This connection has a test
capacitor which is used to convert the voltage pulses from the pulse generator into charge
pulses which are then fed into the rest of the amplifier circuitry. [1] The preamp is also an
inverting amplifier, which means that an influx of negative charges (electrons from the MCP)
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University of Leicester
PLUME
Ref: PLM-PAY-LabElecsGain-014-1
Date: 02/10/2008
will produce a positive voltage spike. The oscilloscope readings taken from either side of the
preamp are shown in figure 1.
Figure 1 (Left): Pulser signal, (Right): Preamp signal
The ratio of voltages were 3:0.9, so the voltage gain of the preamp is -0.3, for a voltage
pulse with rise time 0.02µs and fall time 0.05µs. For longer voltage pulse times (1µs rise,
10µs fall) gain fell only slightly to around 0.28, but the amplified pulse duration was almost
unchanged.
The charge gain is determined in this setup by the test capacitor. For an ORTEC 118A
preamp, the internal test capacitor has a value of about half a picofarad [2]. The 3V incoming
signal was thus converted into a 1.5pC charge injection for the amplifier circuitry. This 1.5pC
produced an output voltage spike with maximum magnitude 0.9V, and so the
voltage/coulomb ratio of the amplifier is 1V out = 1.666pC in.
Shaping amp
The central component of the shaping amp is a CR-RC circuit. A detailed discussion of the
characteristics of this circuit is referenced from the Payload wiki page. [3] The most
important setting of the shaping amp is the rise time - a gradual rise is essential to allow the
MCA to accurately analyse the peak voltage. The MCA can handle a rise time of half a
microsecond, which is as low as the shaper will go. This is a good thing, as a longer rise
time will lead to a lower gain.
Another feature of the shaping amp's CR-RC circuit is the pole-zero effect. This manifests
itself as a negative voltage peak that occurs after the initial positive peak. This phenomenon
doesn't affect the ability of the MCA to record the height of the positive voltage peak, and
can be ignored.
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University of Leicester
PLUME
Ref: PLM-PAY-LabElecsGain-014-1
Date: 02/10/2008
The shaping amp has an internal signal amplifier, too. There’s a fine gain amplifier that can
be set anywhere between 0.5x and 1.5x, and a coarse amplifier with discrete values of 20,
50, 100, 200, 500, and 1000x. The equipment is pretty old, and there’s a possibility that the
actual gain of the amplifier has drifted away from its factory settings. A small experiment was
done to examine this, described below.
As before, the oscilloscope was connected on either side of the shaping amplifier. The
readings are shown in figure 3. The attenuation of the pulse generator was changed to give
a 3V output signal from the preamp, for maximum precision. The gain of the shaping
amplifier was set to 10x.
Figure 3 (Left): Preamp signal, (Right): Shaped signal
The voltage gain of this configuration is 3:0.45, or 0.15x
By examining the voltage gain produced by the different coarse gain settings on the shaping
amp, we were able to determine the actual gain of each setting. Three runs were done to
increase accuracy, as the oscilloscope used had significant ‘signal drift’ where the trace
would distort over time. As you can see in table 1, they’re actually not that different.
Listed
Average
Actual
gain
voltage (V)
gain
1000
6 (reference)
1000
500
3.1
516.6667
200
1.263333
210.5556
100
0.603333
100.5556
50
0.310667
51.77778
20
0.131
21.83333
Table 1: Shaping amp gain
Conclusion
We now have enough data to determine the gain of the entire electronics chain for both
voltage signals and charge pulses.
Vout = -0.3 x 0.15 x 0.1 x Gshaper x Vin = 0.0045 x Gshaper x Vin
(1)
Vout = -6x10^11 x 0.15 x 0.1 x Gshaper x Qin = 9x10^9 x Gshaper x Qin
Where Gshaper is the actual gain of the shaping amp given in table 1.
Page 3 of 4
(2)
University of Leicester
PLUME
Ref: PLM-PAY-LabElecsGain-014-1
Date: 02/10/2008
[1] Data recording and analysis - Pearson, Lees and Brunton (2003)
[2] http://prola.aps.org/pdf/v29/i6/p3500_1
[3] http://solarwww.mtk.nao.ac.jp/kobayash/thesis/node37.html
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