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14-11
Block Diagram Theory of Operation
Block Diagram 2
necessary to do manual troubleshooting to find out if the A5 Premod Filter and NSM
assembly is correctly locking to these other clock signals. By using the service screen and
viewing the latch (NSM_PMF_CLK), it can be determined if the loop is locked. A “1” on
the latch indicates lock, while a “0” indicates no lock. If for some reason the loop is not
locked, the generator will exhibit a high frequency and phase error.
A27 DAC/Upconverter
This assembly contains the circuits necessary to create a 0.3 GMSK waveform at 13.4
MHz. The inputs to this module are the digital signals from the A5 Premod Filter and
NSM assembly, as well as a 10 MHz reference signal from the A15 Reference assembly.
These two are combined together as shown in Block Diagram 2, to produce the output
signal at 13.4 MHz.
An important characteristic of this signal is extremely low phase and frequency error. The
output from this module can be accessed using the extender boards in the service kit and
measured with either another HP/Agilent 8922 or a downconverter and HP/Agilent
11836A software. The HP/Agilent 11836A software method is recommended if it is
necessary to have a highly accurate measurement.
The diagnostics procedure for this module only checks that an RF signal is present on the
output. This output is fed into the A25 Sum Loop assembly where the signal is translated
up to the RF frequency that was selected on the front panel.
A26 Step Loop A
This assembly creates RF reference signals from 486.6 MHz to 1016.6 MHz spaced 100
kHz apart. These signals are derived from a 1 MHz output from the A15 Reference
assembly and digital inputs from the A33 Hop Controller assembly. The HP 8922 has the
ability to change RF frequencies very quickly. This is necessary because the radios and
base stations change frequencies and the HP/Agilent 8922 must be able to change along
with them. The A33 Hop Controller assembly controls which frequency the A26 Step
Loop A assembly will create. Most radio and base station testing is done at carrier
frequencies near 900 MHz. For these frequencies, the A26 Step Loop A assembly output
is approximately 13.4 MHz lower than the RF output that was selected.
A26 Step Loop A and A17 Step Loop B assemblies have exactly the same hardware and
can be interchanged if necessary.
IMPORTANT
The A25 Sum Loop assembly is adjusted to match the A22 Step Loop A
assembly. If either the A25 Sum Loop or A26 Step Loop A assembly is changed,
it is necessary to readjust the A25 Sum Loop using the instructions in chapter 7,
Adjustments and Calibration.