MODEL 8559A

SERVICE

Sweep Ramp HighlLowLimit Comparator (C)

Operational amplifiers U l a and U l b are connected to form a comparator circuit. A voltage divider, comprising resistors R6, R7, and R8, establishes a high voltage reference at U l a pin 2 and a low voltage reference at U l b pin 5. The switching limits are approximately + 5V and +0.7V, respectively. The signal applied to other inputs of the comparator is the YIG tuning voltage, the same signal that drives the YIG main coil. It consists of the analog tuning voltage and the sweep ramp (S + T). The tuning voltage is proportional to the instantaneous frequency to which the analyzer is tuned; the ramp sweeps from + 1.2V to 4.8V.

As the YIG tuning voltage at U l a pin 3 rises above the reference at U l a pin 2 (+4.95V), the output of U l a rises to about + 14V. This turns on 4 4 in the blanking driver and blanks the display. If the YIG tuning voltage goes below the lower reference limit (+0.7V), the output of U l b goes to about +14V and again blanks the display. The upper and lower blanking limits correspond to 50 MHz below and 100 MHz above the ends of each band being swept.

VerticallBaseline Comparator (D)

The Vertical/Baseline Comparator consists of 416 and 4 8 . The baseline clipping reference voltage is set by the BL CLIP control A2AlR2, which varies the base voltage of 416. The Vertical Preamplifier output signal is applied to the base of 4 8 and compared to the dc reference voltage at the base of 416. If the signal becomes more negative than the reference, Q8 turns on. This turns 4 4 on and blanks the display.

Blanking Driver (F)

The Blanking Driver comprises transistors 4 4 and Q9 (see Figure 8-72). Normally, 4 4 is off, placing a low level at the base of Q9 and causing Q9 to be turned on. For Q9 to be turned off and provide a positive going blanking

SWEEP RAMPH I / L O L I M I TCOMPARATOR

i 1 2 I N BYPASS MODE. - 1 2 I N MODE

CRT

BLANKING

- 1 1 V TRAC E

FIGURE8-72,BLANKING CIRCUIT, SIMPLIFIED SCHEMATIC