HP 70908A, Service Manual

Page: 109 / 335

Troubleshooting the A13 ALC/Bias
The A13 ALC/bias provides the drive signals for the A1 leveling amplier and the
A2 LO multiplier . The A2 LO multiplier gate bias is a voltage divider adjustment. The
A2 LO multiplier block takes the detected output information from the A2 LO multiplier and
compares it with a A2 LO multiplier voltage reference . This information is integrated and
sent back to the A2 LO multiplier attenuator . Multiplier band switching takes decoded band
information from A10 power supply/driver (BC1,BC2,BC4) and enables the appropriate paths
inside the A2 LO multiplier to achieve the LO frequency necessary for fundamental mixing
inside the A3 SYTFMD . Refer to A2 LO multiplier troubleshooting for switching logic per band.
LOLA Output Switch/Leveling
The LOLA output switch takes band information from the A10 power supply/driver (BC0) and
sends the pin switch biasing to the A1 leveling amplier for switching the LO output to either
the A15 RF 1st converter (.0000001 to 2.9 GHz) or the A3 SYTFMD (2.9 to 22 GHz Input).
The LOLA leveling circuitry takes the detected output from the A1 leveling amplier and
compares it to a reference voltage from A1. This information is integrated and sent to the
A1 leveling. When the LO input power to the A1 is low , PIN diodes 1 and 2 are at minimum
attenuation. As the input power increases , A13Q9 gradually biases PIN diode 2 to maximum
attenuation, leaving PIN diode 1 to respond to small changes in power . LO gate bias is a voltage
divider bias adjustment scheme .
Multiplier Leveling Circuit
Tune the HP 70000 Series modular measurement system to a center frequency of 8 GHz with a
span of 100 MHz. Connect a DVM to the A TTEN pin of the A2 LO multiplier . Connect one side
of a jumper wire to ground. T ouch the other side of the jumper wire to the REF pin of the A2.
The voltage at the A TTEN pin should be less than +1.5 V . Remove the jumper from the REF
pin and connect it to the DET pin of the A2. The voltage at A TTEN should be greater than +11
V . If either of the voltages is incorrect, troubleshoot the A13 ALC/bias .
LO Leveling Amplier Leveling Circuit
Tune the HP 70000 Series modular measurement system to a center frequency of 8 GHz with a
span of 100 MHz. Connect one side of a jumper wire to ground. Connect the other side of the
jumper to the REF pin of the A1 leveling amplier . The voltage at A1 PIN 1 should be +10.5 V ,
and PIN 2 +1.3 V . Remove the jumper from the REF pin and connect it to the DET pin of the
A1 leveling amplier . The voltage at PIN 1 should be 3.5 V and the voltage at PIN 2 should be
1.75 V. If any voltages are incorrect, troubleshoot the A13 ALC/bias.
4-48 T roubleshooting