EMC Test System Power Amplifier Checks

Fixing faults in EMC test set-ups, especially those in test chambers, can tie test-staff down for an inordinate amount of time. In this article, we describe the process of how to carry out EMC Test System Power Amplifier Checks.

This test procedure describes how the small signal performance data of an amplifier is gathered and checked against previously recorded data. This is a fault finding step that establishes the health of the preamplifier. Data is gathered over 5 predetermined spot frequencies and is then compared with the data of previously recorded power amplifier performances. The antenna in the chamber is used as the high power termination. The spot frequencies chosen are where the antenna / chamber termination is a good match (generally away from the start of the antenna band).

Required Equipment:

• Power Amplifier being tested
• Signal Generator (set the signal generator output level to -30 dBm BEFORE connecting it to the input of the power amplifier)
• Directional Coupler
• Attenuator Type N - 20 dB, 5 watts (the attenuator value is established as ‘Staying below the ‘No Damage Level’ input power of the Analyzer’)
• Connecting Cables
• Spectrum Analyzer

Figure A: Power Amplifier Small Signal Test Setup

Test Procedure:

For an 80-1000 MHz 250 W amplifier, the five spot frequencies are 200 MHz, 300 MHz, 500 MHz, 700 MHz and 900 MHz. (In all cases, the input signal is un-modulated, that is, the 80% AM modulation is OFF). Now with the signal generator output set at -30 dBm and the output set to OFF, set up the test equipment as shown in figure above.

Set the frequency to the first spot frequency on the measured data fill-in sheet. Adjust the signal generator output to -10 dBm. Set the signal generator output to ON. Now measure and record the preamplifier output power on the spectrum analyzer (in dBm) (allow for the attenuation in the signal path by putting in the necessary offset) Repeat this test for all spot frequencies.

Compare the data to the previously recorded data. If the data is within +/- 1dB, the preamplifier is healthy.

Additional Information:

Staying Below the ‘No Damage Level’ Input Power of the Analyzer

How to select the test system attenuation so the spectrum analyzer is not damaged and how to establish the input power to the amplifier for small signal operation?

In these examples an amplifier with the following specifications is being tested (the data is taken from the amplifier data sheet)

Establishing the ‘No Damage’ Attenuation

Referring to Figure A, note that in the absence of a suitable high power termination, the ‘good match’ can be the antenna / empty chamber combo. If the combo at a proposed spot frequency does not present a good match, replace the frequency with a nearby one that does. The power at the reflected power connector of the coupler should be measured to ensure a good match exists. A good match for these health check purposes is a VSWR of 3:1 or less (25% reflected power or less). To protect the input of the spectrum analyzer the total attenuation must permit no more than +10 dBm through (or whatever maximum limit a particular analyzer / instrument specifies) when the amplifier is producing maximum power.

So basically, the coupling factor of the coupler and the attenuator between the coupler and the analyzer input must reduce +55 dBm (amplifier maximum power) to no more than +10 dBm. Therefore the combined attenuation must be +55 dBm minus +10 dB = 45 dB. In the case of a coupler with 30 dB coupling factor, a further 15 dB of attenuation must be provided by the attenuator in Figure A. The specified saturated power is a minimum, so as a safety factor a 20 dB attenuator is used.

Establishing the Small Signal Operation Input Level

A rule of thumb is that the amplifier should be running at its P1dB level minus 10 dB. To establish the approximate signal generator output power that achieves this, the specified small signal gain is subtracted from the P1dB level in the datasheet. That is subtracting 55 dB 1 from +54 dBm and getting a signal generator level of -1 dBm. Now subtract the rule of thumb 10 dB to get -11dBm. This is used as the test level for all 5 spot frequencies.