14-13
Block Diagram Theory of Operation
Block Diagram 2
A12 Pulse Attenuator
In addition to 0.3 GMSK modulation, the RF signals must also be pulse modulated
because the GSM system uses TDMA (time division multiplexing). The function of the
A12 Pulse Attenuator assembly is to pass the RF output signal with 0 dB, 30 dB or > 80
dBof attenuation. The A12 Pulse Attenuator assembly allows “straight through” operation
to simulate the RF carrier ON or it provides >80 dB of attenuation to turn the RF carrier
OFF.In addition to these two functions, the A12 Pulse Attenuator assembly can provide a
calibrated 30 dB of attenuation. This is used to test a radios ability to recover a weak
signal with other high power signals in adjacent time slots.
The diagnostics procedures check this assembly by using the internal RF spectrum
analyzer. The pulse attenuator itself is solid state and highly reliable. Diagnosing the
attenuator requires many other assemblies in the HP/Agilent 8922. Diagnostic failures of
the Pulse Attenuator could also be caused by the A23 Input, A11 Receiver Mixer, A16
Receiver,A17 Step Loop B, or A18 Spectrum Analyzer assemblies, or a missing LO cable
on the rear panel (early instruments).
A23 InputA24 High Power Attenutor
The A23 Input assembly is both the input for the Signal Analyzer section and the final
output from the Signal Generator section. For additional information on how the A23
Input assembly is used in the signal analyzer, refer to the Block Diagram 1 discussion.
TheRF output signal is received from the A12 Pulse Attenuator assembly. The A23 Input
assembly has a step attenuator (5 dB/step) that can attenuate the RF signal up to 125 dB.
The A23 Input assembly also contains a switch to select the proper output port. For high
output levels, the port AUX RF OUT is available. For most operations, the RF signal is
routed to the RF IN/OUT port and connected directly to a radio or transmitter. The radios
are “duplex”, meaning they simultaneously transmit and receive at different frequencies.
Themost common setup is to have the HP/Agilent 8922 Signal Generator routed to the RF
IN/OUTconnector to simulate a base station and is “transmitting” to the radio-under-test.
At the same time, the radio-under-test is “transmitting” to the HP/Agilent 8922 Signal
Analyzer at a frequency offset by 45 MHz. This signal comes in the HP/Agilent 8922 RF
IN/OUT connector and is routed to the Signal Analyzer section.
The 14 dB (8 dB with the HP/Agilent 8922F,H,M,S) A24 High Power Attenuator
assembly is shown on Block Diagram 1 inside the A23 Input block. It is actually external
tothe Input Module and provides 14 dB attenuation of all signals going into or coming out
of the RF IN/OUT connector on the front panel.
Diagnostic procedures individually check that all the step attenuator switches provide
attenuation,although the accuracy of this measurement is limited. A connectivity check is
provided with the diagnostics to verify the connections going into and out of the A23
Input assembly. This section is the most likely cause of output level accuracy problems,
especially “hard failures” where the output is incorrect by 5 to 20 dB (indicating an
attenuator pad has failed).