Chapter 11 549
Synthesizer Section
Unlocked Offset Lock Loop (Sampling Oscillator)
11.If the feedback signal is not near the indicated power, measure the
signals at the following test points on the feedback path. Refer to
function blocks AD, AG, and AH of A15 RF schematic (sheet 3 of 4).
12.Measure the 4 MHz loop-IF signal at the mixer output. The
frequency of the IF is the same as the reference frequency and can be
found in Table 11-4 on page 548.
13.If the IF signal is not near the indicated power, troubleshoot the loop
mixer (function block AI).
Check path to phase/frequency detector (steps 14-19)
14.Measure the loop IF signal at the input to the IF amplifier/limiter
(function block AK):
15.Confirm the presence of a 4 MHz square-wave reference frequency
signal at U406 pin 3. The square wave is TTL and should be less
than +0.6 V and greater than +2.2 V.
16.Disconnect the jumper from X201 pins 1 and 5. Disconnect the dc
power supply which is connected to A15J200 pin 16.
17.Set the spectrum analyzer to the following settings:
Center frequency ........................................................ 300MHz
Span ................................................................................... 0Hz
18.Use an oscilloscope to confirm the presence of a 4 MHz TTL-level
reference frequency signal at U406 pin 11.
19.Connect a short across A15R425. Connect A15U406 pins 3 and 11
together. This puts the same signal on both the phase/frequency
detector inputs.
20.Observe the phase/frequency detector outputs, U406 pins 6 and 9,
with an oscilloscope. Narrow TTL pulses should be present. Pin 9 is
normally low, pulsing high, and pin 6 is normally high, pulsing low.
21.Check the end of L417 (nearest C445) with an oscilloscope. With the
oscilloscope input ac-coupled, a triangle waveform approximately
20 mVp-p should be present.
A15TP200 +4 dBm
A15TP201 +9 dBm
A15TP202 +5.5 dBm
A15R447 (end nearest L414) −6 dBm
A15L428 (end nearest U411) 4 MHz (approximately −6 dBm)
To Next Page
Loading...
Need help?
General
