HP 54540 Series - Page 92

Chapter 5: Troubleshooting
Troubleshooting attenuator failures
The attenuators consist of the following:
e A solenoid selected input resistance
e Two solenoid selected passive attenuators (1:1/5:1 and 1:1/25:1) which can be cascaded to
provide attenuation ranges of 1:1, 5:1, 25:1, 125:1
e An FET input circuit
e A programmable preamplifier
Defective attenuators can cause a variety of symptoms.
e Wrong input resistance
e Low bandwidth/slow rise time
e Signal distortion
e Calibration failures
© Self-test failures
The combination of attenuator and PC assembly affects the pulse response. You must check
or adjust the pulse response after making any of the following changes.
e After installing a new attenuator. Adjust the channel with the new attenuator.
e After installing anew PC board. Both channels must be adjusted.
e Permanently swapping the attenuators, as when troubleshooting. Adjust the affected
channels.
For pulse adjustment, of any affected channels, see "To adjust high-frequency pulse response"
in chapter 4, "Calibrating and Adjusting.”
Firmware Calibration should also be done after attenuator replacement (see chapter 4,
"Calibrating and Adjusting.")
Attenuator Click Test
The solenoids for the passive attenuators can be heard switching when the vertical sensitivity
is changed. The fine mode of the RPG will give the most accurate indication of when a
solenoid switches. However, the gain calibration will give different switching points to
different attenuator assemblies. Individual attenuator assemblies will not necessarily switch
at the same sensitivities. Also, there are different sets of calibration factors for certain
time/div ranges so the passive attenuator changeover may depend on the sweep speed. They
usually switch near the following:
e 45 to 50 mV/div
e 235 to 250 mV/div
e 1.15 to 1.25 V/div
You can hear solenoid switching when going either direction through the transitions.
You can hear the input resistance solenoid when the input resistance is changed (CHAN
menu).
Attenuator swapping is the best method of finding a faulty attenuator. Swap suspected and
good attenuators, and r-run the tests.