Chapter 10 559
Synthesizer Section
Frequency Span Accuracy Problems
Fractional N Span Problems (LO spans ≤2 MHz)
If the fractional N spans are inaccurate or nonexistent, but the
fractional N PLL is locked to the correct frequency and other spans are
correct, there may be a problem with the HSCAN signal. Check that
HSCAN is present at the fractional divider, U113 pin 41 in function
block AS. HSCAN comes from the A3 interface assembly and goes to the
sweep generator circuitry in function block A and to Fractional N.
First LO Span Problems (All Spans)
1. Set the HP 8560E/EC to the following settings:
Center frequency .................................................. 300MHz
Span .......................................................................... 2MHz
Resolution BW .......................................................... 1MHz
Video BW .................................................................. 1MHz
Sweep time ................................................................. 50ms
2. Check that there is 0 V to +10 V ramp of 50 ms duration at A14J15
pin 15.
3. If a scan ramp is not present, refer to "Sweep Generator" in this
chapter.
4. If there is a 0 to −10 V ramp at A14J15 pin 14, the fault is probably
in the Main/FM sweep switch. See function block H of A14 frequency
control schematic.
5. Check that there is a 0 V to +10 V ramp at U325 pin 1. The spectrum
analyzer ADC obtains information about the sweep from this node.
Check span attenuator (steps 6-13)
1. Continue with step 7 to check the span attenuator. See function
block L of A14 frequency control schematic (sheet 2 of 5).
2. With the spectrum analyzer set to the settings in step 1, monitor
A14U323 pin 6 with an oscilloscope. A 0 V to −10 V ramp should be
present.
3. Change the spectrum analyzer span to 10 MHz and check for a 0 V to
−5 V ramp at U323 pin 6.
4. Change the spectrum analyzer span to 2.01 MHz and check fora0V
to −1 V ramp at U323 pin 6.
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