Input Amplifier Block Circuit Description (Channel A) - HP 5334B Service Manual

8-108. Input Amplifier Block Circuit Description (Channel A)
8-109. SIGNAL CONDITIONING STAGE. The first stage, Signal Conditioning has the relay switches for set-
ting ACDC coupling, 50RJ1 MR Impedance, XlJX10 Attenuation, Common Meparate signal path, and Chan-
nel A 100 kHz Filter odoff selection. The relays are driven by 'ITL driver U37, either directly or through in-
verter U38. The driver receives instructions from the Measurement MCU over the Measurement Data Bus, and
sets the signal conditioning parameters according to the front panel setup or HP-IB commands.
8-110. Immediately after the Channel A input connector, relay
K5
selects either straight-through DC coupling
or AC coupling through capacitor C95. Relay K6, when closed, provides the 50R input impedance via parallel
100 ohm resistors R120 and R121. The combination of these resistors and C95 forms a 200 kHz High-Pass filter.
8-111. When K6 is open, the input impedance equals 1 Megohm. Input impedance is set by two different com-
binations of resistors, depending on which input attenuation is being used. In the X1 attenuation setting, the
1 MR impedance is set by series resistors R108, R109, R110, and R95. In the XI0 attenuation setting, the im-
pedance is set by resistors R116 and R93. A constant 1 Megohm load is presented to the input signal regardless
of the attenuator setting.
8-112. The attenuation function is set by relay K1. In the X1 attenuation setting, K1 shorts out resistor R116. In
the XI0 attenuation setting, R93 is placed in parallel with R108, R109, R110, and R95. This parallel combination
forms a voltage-divider with R116 so that the signal level after R116 equals 10% of the input amplitude. Variable
capacitor C87 provides frequency compensation for any stray capacitance after the divider.
8-113. The series resistors after the attenuator relay perform other functions in addition to the ones described.
R108 provides high frequency line matching and transient input protection. R109 and clamping diodes CR28 and
CR29 provide over-voltage protection to the following stage. To reduce distortion from high amplitude signals,
R109, R110, and R95 form a voltage-divider such that the signal level after R109 equals approximately 76% of
the signal before R109. This prevents Q6 and Q10 from going into saturation with a 5V peak-to-peak (p-p) input
signal. Capacitor C88 compensates the divider.
8-114. The Channel A filter function is set by relay K2. When the Filter mode is off, K2 is closed, bypassing
resistor R94. With the Filter mode switched on, K2 is open and the combination of R94 with the shunt
capacitance of the Q8 FET gate-source line of -5.2V forms the 100kHz Low-Pass filter.
8-115. The last signal conditioning function is the SeparateICommon A switching between the two input chan-
nels. The Separate or Common signal paths are set by relays K7 and K8. In the Separate mode, K7 is open and
K8 is closed, and each channel is connected to its own input connector. In the Common A mode, K7 is closed
and K8 is open, so the Channel B circuit after K8 is connected to the Channel A input.
8-116. HIGH AND LOW FREQUENCY AMPLIFIER STAGE. The High and Low frequency amplifier stage
consists of parallel high and low frequency buffering circuits. The high frequency path contains an impedance
converter circuit with an AC gain of approximately 0.9; the bandwidth of the circuit is from approximately 100
Hz to greater than 100 MHz. The low frequency path provides DC trigger level control with an AC gain of ap-
proximately 0.9; the bandwidth of the circuit is from DC to approximately 100 kHz.
8-117. In the
high
frequency path, the input signal is AC coupled, via C74, to the
FET
Q8 gate. Q8 converts the
high impedance at the circuit input to a low impedance at the FET source. Resistor R78 biases the FET to
saturation and resistor R77 is a buffer to prevent Q10 from oscillating. Additional current is provided via pull-up
resistor R60 to speed-up the positive voltage swings at the FET source with instantaneous trigger level voltage
changes. Resistor R76 sinks current from emitter-follower Q10. The signal from the output of Q10 is fed through
the low frequency circuit and input back to the base of Q10.
HP
5334B -
Service Manual
8-18