Chapter 10 565
Synthesizer Section
Sweep Generator Circuit (for Spectrum Analyzers with 100 s max. Sweep Time)
Sweep Generator Circuit (for Spectrum
Analyzers with 100 s max. Sweep Time)
The sweep generator operates by feeding a constant current from DAC
U307 into an integrator, U320B. See function block A of A14 frequency
control schematic. This current is scaled by resistors R20 through R24
and U312B/C/D. See Figure 10-9 on page 566. The capacitors used in
the integrator depend on the sweeptime 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 +10 Vdc.
The analyzer ADC monitors the scan ramp at U325A pin 1 via the scan
ramp attenuator U320B pin 7. When the ramp reaches +10 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 off diodes CR1, CR2 and turns on
transistors Q1 and Q2. The integrating current has a maximum value
of 236 µA.
During retrace, HSCAN is low, closing U306B and U312A. See Figure
10-10 on page 567. 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. For more information, refer to "First-LO Span Problems
(Multiband Sweeps)" in this chapter.
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