HP 70427A User Manual page 506

Block Diagram
YIG-Tuned-Filter (YTF) and GaAs FET Amplifier Driver Assembly
The YTF current limiter acts as a current controlled switch. At turn-on, when
the heater resistance is very low, Q5 acts as a 200 mA current source which
limits the surge current. When the heater resistance rises, and the current
Ω
becomes less than 200 mA, Q5 saturates, and the limiter appears as a 10
resistance to ground.
HP 70427A/HP 70428A User's Guide 13-71