SERVICE

MODEL 8559A

FIRST MIXER ASSEMBLY A4, TROUBLESHOOTING

Typically, a bad first mixer results in at least a 15 to 20 dB loss in sensitivity (i.e., the amplitude of displayed signals is 15 to 20 dB low). There are, however, other factors that can affect spectrum analyzer sensitivity that should be checked. The measurement of power levels along the signal path can give a good indication of where the loss is occurring. The output of the Second Converter Assembly A5 offers a convenient point to isolate the RF front-end from the IF section. If the loss appears to be in the front-end, measure the power levels of the first and second local oscillator with a second spectrum analyzer. Next, measure the supply and bias voltages at the first mixer. To access the push-on connectors of the first mixer's bias and supply lines, it is helpful to remove the instrument's bottom guide-rail.

The First Mixer Assembly A4 can be damaged by electro-static discharge. Tools and hands should be grounded before handling this assembly. It is also possible to damage the mixer diode with an ohmmeter. Damage may occur with as little as 3V open-circuit-voltage between the ohmmeter probes. Therefore, dc testing of the assembly is not recommended. If it becomes necessary to remove the rigid coaxial cable connecting the first mixer output and the second mixer input, be careful not to damage the Low Pass Filter Assembly FL1 internal to the cable. The filter assembly is very sensitive to bending.

Set HP 8559A controls as follows:

 

FREQUENCYBANDGHz ..................................................

.01-3

TUNING ...............................................................

.035GHz

FREQSPAN/DIV ..........................................................

]MHz

RESOLUTIONBW .........................................................

lMHz

INPUT A'ITEN .............................................................

10 dB

REFERENCELEVEL .....................................................

-10dB

REFLEVELFINE ..............................................................

0

Amplitudescale .........................................................

ldB/DIV

SWEEP TIME/DIV ........................................................

AUTO

SWEEPTRIGGER .....................................................

FREERUN

VIDEOFILTER .............................................................

OFF

BLCLIP ....................................................................

OFF

SIGIDENT .................................................................

OFF

ALTIF .....................................................................

OFF

Connect the CAL OUTPUT signal to the RF INPUT

 

NOTE

Before making the following adjustments, measure and note the first mixer bias voltage (A16TP1). This permits the instrument to be returned to calibration if the first mixer is good.

Adjust the V1 potentiometer (A12R72 on the Step Gain Amplifier Assembly A12) through its range and observe the changes in the displayed signal peak and the bias voltage. With a good mixer, two changes are observed: the displayed signal peaks at some point in the adjustment (usually with about - 5V or - 6V of bias voltage) and the bias voltage (A16TPI) ranges from - 9V to +2 k 0.5Y If all of these characteristics are not present, the mixer is probably damaged.