HP 8560A, Service Manual

Page: 264 / 450

Flatness Control (RF Gain
See function block M of A3 Interface Assembly Schematic Diagram (sheet 4 of 6).
RF Gain control the Al5 assembly’s flatness compensation amplifiers. The RF Gain
are arranged so that the output of one DAC is the voltage reference for the other DAC.
This results in an RF GAIN voltage which is exponentially proportional to the DAC settings.
Each DAC is set to the same value. The Al5 RF assembly converts the RF GAIN signal to
a current for driving the PIN diode attenuators in the Flatness Compensation Amplifiers.
The exponentially-varying voltage compensates for the nonlinear resistance-versus-current
characteristic of the PIN diodes.
1. Place the WR PROT/WR jumper on the A2 Controller assembly in the WR
position.
Press MORE OF 2 ,
SERVICE CAL DATA, FLATNESS, FLATNESS DATA. Press
NEXT BAND until “FLATNESS BAND # is displayed.
3. Press the key until “DATA 300 MHz” is displayed. Note the number directly below
“DATA 300 MHz”; this is the RF Gain DAC value.
4. Connect a positive DVM lead to pin 13 and the negative DVM lead to
5. Check that the DVM reading increases from near 0 Vdc to between and Vdc as
the RF Gain DAC setting is increased from 0 to 255.
6. If the DVM readings are incorrect, press MORE 1 OF 2,
SERVICE CAL DATA, FLATNESS FLATNESS DATA. Press NEXT BAND until
“FLATNESS BAND # is displayed. Press the key until “DATA 300 MHz” is
displayed. Proceed as follows:
a. Check the V reference.
b. Check for pulses at pin 16 (LWRCLK).
c. While rotating the front-panel knob, check for pulses at pin 15
d. While rotating the front-panel knob, check for pulses at U417 pin 6 (IAO).
7. The LWRCLK and LDACl are incorrect, refer to the Interface Strobe Select block in this
chapter.
8. Place the WR WR jumper on the A2 Controller assembly in the WR PROT
position.
Section