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

The A19 DAC provides the tuning voltage to the A20 YTO Main Coil Driver used to coarse
tune the YTO Start Frequency and set the proper amount of sweep. This occurs at the
beginning of each sweep and at the beginning of each harmonic band within a sweep.

Frequency information is loaded into pretune registers U5 and U6 from the Instrument Bus.
This is coded information from the Instrument Bus. This information is clocked into the
registers by LDAS (Low

 DAC Strobe). The Pretune DAC, in conjunction with U2, provides

an output of 0.0 to 

 V dc in approximately 3 

 increments. The exact voltage is

dependent on the coded information input. This voltage is applied to Summing Amplifier U4
through R3.

Coded span information from the Al2 Front-Panel Interface is loaded into register U7 from
the Instrument Bus and clocked out to switch drivers U8 and 

 by LSAS (Low = Scan

Attenuator Strobe). These switch drivers turn on (close) switches 

 through Q6 according to

the coded information input. The SCAN DAC IN signal (attenuated sweep ramp) from the
Al6 Scan Generator is then applied to the proper scaling resistors R26 through R30, R38, and
R58, to provide the sweep for the YTO. This output current is applied to Summing Amplifier

The fixed dc current from the Pretune DAC, corresponding to the desired YTO Start
Frequency, and the ramp current from the Scaling Resistors, corresponding to the desired
Frequency Span, are summed at the input of U4. U4 amplifies this combined signal and
applies it to both the YTO and YTX at a sensitivity of -3 

 to coarse tune the

analyzer to the selected Start Frequency and Frequency Span.

U3 and associated circuitry   provide a nominal -12.6 V reference for U3, the pretune DAC
in function block 

 

 and associated circuitry   provide a nominal 

 V reference for

summing amplifier U4 in function block