Chapter 11 579
Synthesizer Section
Sweep Generator Circuit
Sweep Generator Circuit
The sweep generator circuitry generates a ramp from 0 to 10 volts
during the sweep time. The available sweep times range from 50 µs to
2,000 seconds. The sweep times are generated in two different ranges, a
50 µs to 30 ms range and a 50 ms to 2,000 second range. The 50 µs to
30 ms range is only needed for analog zero span sweeps.
The sweep generator is controlled with an 8-bit latch and the control
signal HSCAN. The latch, U308, controls the sweep rate. HSCAN
determines when to reset the scan ramp and when to let it sweep.
Operation of the 50 ms to 2,000 second range will be described using a
50 ms sweep time as the example. For a 50 ms sweep time, Q1 shorts
out C16. The D to A converter, U307, has zero output current. U334A is
a buffer with zero offset, because there is no current coming out of
U307. The buffering of U334 makes the base-emitter voltages on Q3A
and Q3B the same. These two transistors are matched, so their collector
currents should be identical when their base-emitter voltages are
identical. The emitter current of Q3B is 200 µA, therefore the emitter
current of Q3A is 200 µA and the sweep ramp is generated by C14. The
sweep time is given by the formula:
Where ∆V is equal to 10 Volts.
With a capacitance of 1 µF and a current of 200 µA, the sweep time
should be 50 ms. The DAC setting is increased for longer sweep times.
This increases the current sunk by the DAC output U307 pin 4, which
increases the emitter voltage on Q3A, decreasing the base-emitter
voltage drop. Q3A acts as an exponentiator and reduces its collector
current, creating a slower sweep ramp.
For the shorter sweep times, 50 µs to 30 ms, Q1 is opened putting C16
in series with C14. This changes the effective capacitance from 1 µF to
1,000 pF, or a reduction of 1,000 to 1.
The HSCAN signal uses Q2 to reset the ramp. Q2 shorts the integrator
and sets its output nominally to ground.
Check the sweep
generator circuit
Center frequency ..................................................... 300MHz
Span ......................................................................... 100MHz
Sweep time .................................................................... 50ms
sweeptime capaci ce C14()
∆V
current
-------------------
×tan=
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