Sweep Generator Circuit

The Sweep Generator operates by feeding a constant current from DAC U307 into an integrator, U320B. See function block K of Al4 Frequency Control Schematic (sheet 2 of 5). This current is scaled by resistors R20 through R24 and U312B/C/D. See Figure 10-9. The capacitors used in the integrator depend on the sweep-time range; smaller-value capacitors provide faster sweep times.

The integration is initiated by HSCAN going high. This opens U312A which places the output of U320A near -15 Vdc, turning CR6 off and allowing the output of integrator U320B to ramp from 0 V to +lO Vdc. The analyzer’s ADC ( via the scan ramp attenuator U320B pin

7)monitors the scan ramp at U325A pin 1. When the ramp reaches +lO V (for single-band sweeps), HSCAN is brought low and the integration ends. During normal non-fast-zero spans (sweep times >30 ms), comparators U319A and B are high. This turns on diodes CRl, CR2, CR3, and CR4. Note that the integrating current (maximum value 236 PA) actually flows backwards through diodes CR3 and CR4.

During retrace, HSCAN is low, closing U306B and U312A. See Figure 10-10. The output of U320A tries to go high, turning CR6 on and sourcing current through R26. This current discharges the capacitors in the integrator, forcing U320B pin 7 toward 0 Vdc. Ultimately, the output of U320B will be brought and held to 0 V by U320A supplying a current equal to that which is sunk by the current source.

C U R R E N T S O U R C E

+1ov

 

U308, R20-R24

U312B/C/D

7 5 0 R R26

+15VF

1 iOOK

U 3 1 12A

R65

5 2 6 K

IK

SK1 1 0 9

Figure 10-9. Simplified Sweep Generator

Synthesizer Section lo-47