Fluke 5790A Service Manual page 68

5790A
Service Manual
2-87. Chopper Oscillator
This block provides a pair of square waves (50 % duty cycle, 180 degrees out of phase)
that clock the chopper switches. A 4047 (U8) is configured as an astable multivibrator.
Resistor R2 and C4 control the 31.5 Hz rate except when wideband operation is selected,
where U13 switches R15 in parallel with R2 to change the frequency to 80 Hz. Lines
OSC-SET and OSC-RESET are normally low. Setting OSC-SET high stops the chopper
in the inverting state. Setting OSC-RESET high stops the chopper in the non-inverting
state.
2-88. Chopper Switches
This block provides a symmetrical square wave equal in RMS value to the DC input
voltage (0.7 to 7 V). A square wave is used instead of DC for making the transfers for
two main reasons:
Op-amp U10 buffers the positive reference and Q3 increases current capability. Resistor
R11 biases Q3 and R10 provides current limiting. IC U6 switches the output and sense
alternately to a 20 dB divider (Z6) or a dummy load (R14). Similarly, the components
U11, Q4, R12, R13, and U7 switch the negative reference between the dummy load and
divider.
2-89. Chopper Attenuator
This block selects the output level of the chopper by switching in 20 and 40 dB
attenuators. Total attenuation is:
Relay K1 selects either 0 or 20 dB output from Z6 pin 3 or 1. Most of the current from
this attenuator is cancelled by an opposite current in the dummy load, R14. The
remaining current returns to CH-COM through the mecca point at TP1. IC U13 pins 1-3
or pins 14-16 select the 0 or 40 dB output from Z2 pin 1 or 3. Capacitors C10 and C11
form a 40 dB capacitive divider to reduce the output impedance at high frequencies.
Relay K2 routes the output to the A6 Wideband assembly for wideband operation, or to
the A10 Transfer assembly for all other modes.
The following chart shows division ratio and attenuation for the various ranges:
2-38
The square wave passes through the AC-coupled amplifiers on the wideband
board, while DC would be blocked by the coupling caps.
Errors caused by DC offsets which add directly to a DC reference tend to average
out with a dual polarity input.
0 dB for the 2.2 V, 22 V, 220 V, and 1000 V ranges (neither attenuator in)
20 dB for 220 mV, 700 mV, 7 V, 70 V, and 700 V ranges
40 dB for 22 mV and 70 mV ranges
60 dB for 2.2 mV and 7 mV ranges (both attenuators in)
Input range 2.2 mV: divide by 5, 20 40 dB attenuators
Input range 7 mV: no division by by 5, 20
Input range 22 mV: divide by 5, 40 dB attenuator
Input range 70 mV: no division by by 5, 40 dB attenuators
Input range 220 mV: divide by 5, 20 dB attenuator
Input range 700 mV, 7 V, 70 V, and 700 V: no division by by 5, 20 dB attenuator
Input range 2.2 V, 22 V, 220 V, and 1000 V: divide by 5, no attenuators
40 dB attenuators