Manual - 8500A Series Peak Power Meter - Giga-tronics
Manual - 8500A Series Peak Power Meter - Giga-tronics
Manual - 8500A Series Peak Power Meter - Giga-tronics
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<strong>Series</strong> <strong>8500A</strong> <strong>Peak</strong> <strong>Power</strong> <strong>Meter</strong>s<br />
D.3 Electrical Description<br />
D.3.1<br />
Preamplifier PC Assembly<br />
Refer to Figure D-2<br />
The circuitry on this PC board is used to amplify the voltage produced when the detected RF power has<br />
been converted to dc. The circuit is similar to a two stage operational amplifier with the non-inverting<br />
input at the base of Q2 and the inverting input at the base of Q1 (see Figure D-2).<br />
The overall gain of the circuit is set by the ratio of R1 to R5 and R2 to R3. C9 is the compensation<br />
capacitor. R26 provides the match to the cable impedance. The output for the circuit can be measured<br />
between TP2 (DET Pulse), and TP1 (ground).<br />
The first stage of the amplifier is formed by transistors Q1 through Q6. Q3 and Q4 form a constant<br />
current source which sets the current that is drawn through the first stage. C1 and C2 are used to AC<br />
couple the signal to Q1 and Q2, the input devices of the amplifier. Q5 and Q6 provide high impedances<br />
which are driven by Q1 and Q2. The gain of the first stage is set by the ratio of R34 and R33 to R11. R9<br />
and C3 provide adjustments to optimize the step response of the amplifier. U1 is a low offset and low<br />
bandwidth op-amp that provides the DC signal path by controlling the current through Q6.<br />
Figure D-1. Op-Amp Equivalent of Pre-Amp Circuit<br />
The second stage of the amplifier is formed by Q7 through Q10. Q9 and Q10 provide a constant current<br />
source for the second stage of the amplifier. Q7 and Q8 form a differential amplifier. The gain of the<br />
second stage is set by the ratio of R18 and R19 to R24.<br />
Q11 is an emitter follower to drive the necessary high currents required by the circuit.<br />
The difference between the low speed and high speed detectors is diode element. The high speed detector<br />
has a very small output capacitance, which produces a rise time of 15 ns while the low speed detector has<br />
a large output capacitance to produce a 750 ns rise time. This causes a change in the minimum frequency<br />
at which the detector can accurately convert RF power to dc voltage.<br />
D-4 <strong>Manual</strong> No. 20790, Rev C, November 1998<br />
Superceded by Revision D, March 2009
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