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
µ
s to
2,000 seconds. The sweep times are generated in two different ranges, a
50
µ
s to 30 ms range and a 50 ms to 2,000 second range. The 50
µ
s 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
µ
A, therefore the emitter
current of Q3A is 200
µ
A 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
µ
F and a current of 200
µ
A, 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
µ
s to 30 ms, Q1 is opened putting C16
in series with C14. This changes the effective capacitance from 1
µ
F 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 ..................................................... 300 MHz
Span ......................................................................... 100 MHz
Sweep time .................................................................... 50 ms
sweeptime
capaci
ce C
14
(
)
∆
V
current
-------------------
×
tan
=
Содержание 8564EC
Страница 17: ...25 1 General Information ...
Страница 37: ...47 2 Adjustment Diagnostic Software ...
Страница 55: ...Chapter 2 65 Adjustment Diagnostic Software 2 LO Frequency Figure 2 2 LO Frequency Adjustment Locations ...
Страница 57: ...Chapter 2 67 Adjustment Diagnostic Software 3 YTO FM Coil Figure 2 3 YTO FM Coil Adjustment Locations ...
Страница 77: ...89 3 Manual Adjustment Procedures ...
Страница 129: ...161 3a Manual Adjustment Procedures 3335A Source not Available ...
Страница 142: ...175 4 Assembly Replacement ...
Страница 151: ...184 Chapter4 Assembly Replacement Procedure 2 A1 Front Frame A18 LCD Figure 4 2 LCD Assembly Exploded View ...
Страница 163: ...Chapter 4 205 Assembly Replacement Procedure 5 A2 A3 A4 and A5 Assemblies Figure 4 10 Assembly Cables 2 of 3 ...
Страница 166: ...208 Chapter4 Assembly Replacement Procedure 5 A2 A3 A4 and A5 Assemblies Figure 4 14 GPIB and A1A1 W1 Cable Placement ...
Страница 168: ...210 Chapter4 Assembly Replacement Procedure 6 A6 Power Supply Assembly Figure 4 15 Power Supply Cover ...
Страница 176: ...226 Chapter4 Assembly Replacement A9 Input Attenuator Figure 4 22 A9 and A10 A12 Mounting Screws ...
Страница 179: ...Chapter 4 229 Assembly Replacement A11 YTO Figure 4 23 A11 Mounting Screws at Right Side Frame ...
Страница 183: ...Chapter 4 233 Assembly Replacement Procedure 8 A14 and A15 Assemblies Figure 4 25 A14 and A15 Assembly Cables ...
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Страница 234: ...315 7 General Troubleshooting ...
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Страница 239: ...320 Chapter7 General Troubleshooting Introduction Figure 7 3 Ribbon Cable Connections 2 of 3 ...
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Страница 358: ...Chapter 9 463 IF Section A5 IF Assembly Figure 9 14 Noisy Signature Figure 9 15 Noise with Correct Shape ...
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Страница 369: ...474 Chapter9 IF Section Cal Oscillator P O A4 Assembly Figure 9 21 Output Waveform 10 kHz Resolution Bandwidth ...
Страница 370: ...Chapter 9 475 IF Section Cal Oscillator P O A4 Assembly Figure 9 22 Output Waveform 3 kHz Resolution Bandwidth ...
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