Agilent Technologies E4440A, Руководство по обслуживанию

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Chapter 3 83
Troubleshooting the RF Section (E4440A, E4443A, E4445A)
RF Section Description (E4440A, E4443A, E4445A)
second mixer is the second IF lowpass filter.
LO Nulling is the process of reducing the LO feedthrough signal that
appears on screen when the instrument is tuned to 0 Hz. When tuned to
0 Hz, the first LO is at 3.9214 GHz. The LO Nulling circuitry works
around this frequency and adds a signal of approximately-equal
amplitude, but 180 degrees out of phase. This signal is coupled off the
main LO path.
A faulty Lowband assembly will commonly cause low or no signal below
3 GHz, and no LO Nulling. The 3.6 GHz second LO signal is supplied by
the A9 Second LO assembly. The second LO is amplified in the
Lowband assembly before it is applied to the Second Mixer
A failure with the Lowband assembly will likely result in a problem
with signals up to 3 GHz only. First check for a signal at the input,
A20J1, the First LO at A20J2, and the 2
nd
LO at A20J5 by referring to
the signal conditions given on the overall block diagram. The multiplied
2
nd
LO can be checked at A20J6. Similarly the first IF signal can be
verified at A20J3.
With the PSA set to a center frequency below 3 GHz, and in zero span,
check that the bias voltages are present. Some of these can be accessed
on the A13J12 Test Connector and some others have test points. With
the negative lead of the DVM on A13J12 pin 6, look for values listed in
the table.
CAUTION
Use care when probing connector pins on A13, especially on A13J8.
Connector spacings are close and shorting nodes can cause damage.
Pin/ Name Nominal Voltage
A13J12 pin1/
−
5V_LB
−
5 V
A13J12 pin2/
−
5V_M1LO
−
5 V
A13J12 pin3/
−
5V_M1IF
−
5 V
A13J12 pin4/
−
5V_M2IF
−
5 V
A13J8 pins 7,8/
+
3V_LB 3 V
A13J8 pin25/
+
5.2V_LB 5.2 V
A13TP22/M1LO_ADJ 100 mV
A13TP23/M1LO_ADJ 150 mV
A13TP24/M2LO_ADJ 100 mV
A13J8 pin16/
+
10V_LB 10 V
A13TP26/LO_NULL_I_DAC
−
5 to
+
5 V
A13TP27/LO_NULL_Q_DAC
−
5 to
+
5 V