Manual - 8500A Series Peak Power Meter - Giga-tronics

Theory of Operation
and read/write lines. Control signals to activate the sound generator come through U8, which also
interfaces with the PROMs in each detector. A special 4-bit-wide data bus is connected to each detector,
and a clock signal common to both detectors sequentially counts the address from 0 to 2048.
Q1 and Q2 switch on and off the +5 V supply to the channel A detector PROM, and Q3 and Q4 do the
same for the channel B detector PROM. When the +5 V is switched on, the address counter at the
detector is reset to zero.
U8 also has an RF Blanking output signal used during a calibration routine to turn off any input from the
RF source. The sound generator is controlled by U8 through an amplifier consisting of U11A and Q8,
with the volume adjustable at R2. U7 is a frequency divider that takes a 3 MHz input and divides it by
two so that the sound generator gets the 1.5 MHz input it needs for proper operation.
A major function performed by the GPIB/Cal Control board circuitry is to provide the control signals
required to operate the automatic RF detector Calibrator.
With reference to DWG# 21015, Sheet 2, and Figure 4-2, PPI U9 performs several functions. One is the
control of switches U12A, B, & C and U11B, C & D. These switches are drivers for the pin diode
switches in the Calibrator, which switch in or out the required attenuation (as shown by the lines on the
right side of the schematic). Since the three sets of two switches each operate the same, we will look at
the action of U12A & B.
When a logic signal is generated from the B0 output of U9, the signal is applied to two inputs of
opposite polarity (U112A, pin 6, is the minus input and U12B, pin 3, is the plus input). If the logic signal
is high, this will force pin 7 of U12A to be driven negative and pin 1 of U12B will be driven positive.
This will turn on the pin diode switch in the Calibrator, which will switch in the desired attenuation (in
this case, the 10 dB attenuator). If a logic zero comes from the B0 output of U9, then the reverse
condition will be true and the attenuator will be switched out because current will be forced through the
attenuator in the opposite direction.
PPI U9 also interfaces the U10 14-bit DAC with the incoming data bus lines. The interface consists of a
byte wide data bus from U9 port A, two control lines from U9 ports C0 and C1 to U10 ports A0 and A1,
and a write enable signal from U9 port C3 to U10 WR. The CS port on U10 is not used. Signals from A0
and A1 control the operation of U10 so that a write operation can either load the data into the DAC
internal register’s least significant byte or through DB0 to DB5 to the most significant byte or to transfer
the register data to the DAC output.
U13 and U10 work together as a standard inverting operational amplifier. The loaded data connects the
V REF input through a resistive network to an internal summing junction. The data is also connected to the
feedback signal from the RFB input through an internal fixed value resistance, and to the inverting input
of U13. Since V REF has a value of -10 V, the output of U13 pin 6 will range from 0 to 10 V.
Shown to the lower right of U10 on the schematic is a summing junction which receives one signal from
U13 through R15, and potentiometer R16 (CALIB SET), which is in parallel with R74. A second signal
then comes from the thermistor bridge circuit of U16 and Q5. The bridge circuit uses the thermistor in
the Calibrator as one leg of a bridge. The bridge is self-balancing, and always seeks a voltage level that
will keep it in balance. When the system is first turned on, the thermistor will have a high resistance
because it is cold. This unbalanced condition will cause a maximum voltage level to be present at TP9.
The voltage level will stay high until the thermistor warms up to a point where the bridge is balanced.
Then the voltage level at TP9 will drop. Due to negative feedback, it will seek a level just sufficient to
maintain a balance in the bridge. The bridge voltage is fed back into the summing junction through R18.
The system has two modes of operation controlled by U17A & B which are, in turn, controlled by either
a high or low output from the B7 connection of U9. In one mode, the circuit measures the voltage of the
bridge (initiated by a low output from B7); in the other mode of operation, the circuitry controls the
bridge voltage (initiated by a high output from B7). When it is required to measure the bridge voltage,
the Calibrator must be turned off so the voltage that selects the measuring mode also turns off the
oscillator. The reason for this is that the ambient voltage of the bridge must be measured with no RF
applied. U17B has two parts, one of which serves to invert the logic signal through pin 14 which is the
Manual No. 20790, Rev C, November 1998 4-5
Superceded by Revision D, March 2009