570 Chapter11
Synthesizer Section
Frequency Span Accuracy Problems
YTO Main Coil Span Problems (LO Spans >20 MHz)
For YTO main coil spans, the YTO is locked at the beginning of the
sweep and the sweep ramp is summed into the main coil tune driver.
1. Perform the “7. YTO Adjustment” in Chapter 3 . If the YTO
adjustments cannot be performed, continue with step 2.
2. Set the spectrum analyzer to the following settings:
Start frequency .......................................................... 10MHz
Stop frequency ........................................................... 2.9GHz
3. Verify that a −1.2 V to −4.8 V ramp (approximately) is present at
A14U331 pin 2.
4. If this ramp is not present, troubleshoot the main/FM sweep switch.
See function block H of A14 frequency control schematic (sheet 2 of
5).
5. Measure the output of the main coil tune DAC at A14J18 pin 3. At
the frequency settings of step 2, this should be −2.48 V.
If the voltage is not −2.48 V, troubleshoot the main coil tune DAC.
See function block E of A14 frequency control schematic (sheet 2 of
5).
YTO FM Coil Span Problems (LO Spans 2.01 MHz to
20 MHz)
In YTO FM coil spans, the YTO loop is locked and then opened while
the sweep ramp is summed into the FM coil. The FM coil sensitivity is
corrected by changing the sensitivity of the FM coil driver.
1. Perform the “7. YTO Adjustment” in Chapter 3 . If the YTO
adjustments cannot be performed, continue with this procedure.
2. Set the spectrum analyzer to the following settings:
Center frequency ..................................................... 300MHz
Span ........................................................................... 20MHz
Sweep time ................................................................... 50ms
3. Check for the presence of a 0 V to −10 V sweep ramp at A14J15 pin
14 (input to the main/FM sweep switch). Refer to function block H of
A14 frequency control schematic (sheet 2 of 5).
4. Check for the presence of a 0 V to +5 V sweep ramp at A14U405 pin
6 (YTO FM coil driver). Refer to function block M of A14 frequency
control schematic (sheet 2 of 5).
5. Check the state of the Main/FM sweep switches as indicated in
Table 11-14 on page 571.
To Next Page
Loading...
Need help?
General
