HP (Hewlett-Packard) HP 85660B ユーザーズマニュアル

The YTX Current Driver has three functions:

1. Provides the output stage for the 

 YTX coil.

2. Provides filter circuitry for the YTX.

3. Provides control circuitry for the YTX heater.

The YTX drive output consists of Q5, 

 R14 to R17, VR4, and CR3. Q5 and Q6 form a

darlington transistor driven by the 

 YTX Driver. R14 to R17 are four 

 ten-watt

resistors connected in parallel to form a 

 resistor which is used to sense the current in

the YTX coil. This sense signal is fed back to the YTX Driver. Four resistors are used for
power handling capability and temperature stability. VR4 and CR3 limit the 

 voltage

produced across the YTX coil during scan reset and prevent breakdown of Q5 and Q6.

When the spectrum analyzer is set to narrow spans and narrow resolution bandwidth, hum
and noise in the YTX drive circuitry which 

 the YTX can introduce phase noise and

line frequency sidebands onto a displayed signal. To prevent this occurrence, a large filter
capacitor, 

 is connected across the YTX coil by switch 

 whenever the resolution

bandwidth is 100 Hz or less.

 is controlled from 

 Slope Generator through Q2, Q3, and Q4. When the resolution

bandwidth is 100 Hz or less, the filter capacitor is connected before the start of a scan and
disconnected at the end of a scan so that the scan reset and hysteresis correction can be
performed.

The YTX heater control is a feedback circuit designed to maintain the center support of the
YTX at a constant 

 as the ambient temperature ranges from 0“ to 55°C. The green

LED, 

 indicates that current is flowing properly in the heater. Red LED, DS2, indicates

the circuit is saturated, putting maximum power into the heater.

The heater ring of the YTX has a 

 thin-film heater resistor, and a thermal sensor with

a positive temperature coefficient. The resistance versus temperature characteristics of the
thermal sensor are shown in Figure 1.