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Input Stage

The test port input of the analyzer can be damaged by a discharge of static electricity or by the application of a DC voltage or a strong RF signal. Therefore, it is always suggested observing the following precautions when operating the device:
  1. Never connect the unit to an antenna during a lightning storm and windy dry days. Lightning strikes and static discharges can damage the unit and may kill the operator.
  2. Static build-up on an antenna may cause damage to the unit when connected. As a precaution, before pluging the coax cable into the analyzer always discharge the antenna by momentarily shorting the center and outer conductors of the connecting coax cable together and then touching the coax connector shell to the analyzer to equalize the charge. After operation, always disconnect the antenna.
  3.  Maximum safe RF input is 2V peak (+16 dBm). Exceeding this RF level may damage the unit. Note that powerful active transmitters nearby may induce a high RF voltage on the antenna.
  4. Test port is DC coupled. Use an external series capacitor for measurement scenarios whereby a DC voltage is present.

In case of damage, the good news is that the failure is usually restricted to a few components that are not difficult to replace.

Troubleshooting

The Calibrator Detector setup can be very useful for identifying the damaged components. From this setup, the correct operation of the Voltage and Current detectors used in the analyzer. can be verified. The section "Reference Voltage and Current Readings" below shows the typical readings of the Voltage and Current detectors for different loads.

For the troubleshooting, it will be also needed a multimeter and calibration loads (short, 50-ohm and open).

Please follow the procedure described below:

1) In some cases the problem can be a faulty external connector or cable adapter, so test the continuity of any connector adaptors and cables you may have between the load and test port.

2) Disconnect any load you might have connected to the test port.

3) Turn on the unit and select <SETUP><Calibration><Detector>. Please do not select Continue and observe the Voltage and Current bar graphs for the different load conditions.


Figure 1, Detector Calibration setup mode

4) For the no-load condition, the bar graphs should be as in the Figure 2. Voltage bar should be almost full and Current bar should be almost empty.

a.       If both Voltage and Current bars are almost empty, this will likely be a severe problem: e.g. both mixers (U4: Voltage and U6:Current) and/or voltage bridge resistors damaged (R73 and R79), DDS faulty, etc.

b.      If  Current bar is filled more than 1/4, there could be a short circuit in the input stage; e.g. shorted D3 protection diode.

5) If previous step was correct, connect a short load and observe the bar graphs. Voltage bar should be full in half and Current bar almost full as illustrated in the Figure 3.

a.       If the bar graphs show the same values as the no-load condition, the most likely fault is a damaged R64 (open or high resistance). R63 maybe be damaged as well. These resistances can be measured in place with a multimeter. It is possible that the mixer U4 (Voltage) is damaged as well.

6) If previous step was correct, connect a 50-ohm load and observe the bar graphs. Both Voltage and Current bar should be about full in half as illustrated in the Figure 4.

a.       If the Voltage and Current bar graphs show significant different readings, it is likely that one of the mixers (U4: Current, U6: Voltage) is damaged; normally the one that displays a lower reading.

Reference Voltage and Current Readings

Screenshots below show the correct Voltage and Current readings in the Detector Calibration setup mode, for the different load conditions.


Figure 2, Open load condition


Figure 3, Short load condition


Figure 4, 50-ohm load condition

Repair

In most cases the repair will involve replacing the damaged resistors in the input stage, most likely R64; and the mixers (U4 and U6) maybe damaged as well. The resistors are very small (0402 size), so you should use a magnifier and thin twizzers for facilitating the rework operation.

In some severe cases, some components and tracks on the circuit board may have burned out, such as the example shown in the following image.


Figure 5

In these cases, the repair will be more difficult as you cannot use the original pads to solder the components, which means you can only fix it using hacks such as illustrated in the following image.
Figure 6

After repairing the unit, it will be necessary performing the Detector Calibration and then OSL calibration.

Reference


Figure 7, Input Stage Components Location


Figure 8, Input Stage Schematics

Digi-Key Part No.

Mfr Part No.

Reference

Description

RMCF0402FT100KCT-ND

RMCF0402FT100K

R68

RES 100K OHM 1/16W 1% 0402 SMD

RR05R30DCT-ND

RT0402DRE0730RL

R64

RES 30 OHM 1/16W .5% 0402 SMD

PESD0402-140CT-ND

PESD0402-140

D3

ESD PROTECTOR 0402 14VDC POLYMER

568-1204-5-ND

SA612AD/01,112

U6,U4

IC MIXER/OSC DOUBLE-BAL 8-SOIC

311-20UCT-ND

RT0402DRE0720RL

R63,R73

RES 20 OHM 1/16W .5% 0402 SMD

P80.6DCCT-ND

ERA-2AEB80R6X

R79

RES 80.6 OHM 1/16W .1% 0402 SMD

Table 1, Input Stage Bill of Materials