Cessna EME Lab Reverb Chamber Emissions

1. Cessna EME Lab Reverb Chamber Emissions Matthew Wills EME Lab Supervisor December 7th, 2004 1

2. Goals • This effort is directed at developing a better test method than the current method. We should not expect to reduce the errors to the last .1 dB. Nor will we eliminate the current technique. • A 4dB standard deviation is achievable. What is the standard deviation of the current method? EUT emissions may vary 5 or 6 dB. • Repeatability between test facilities will be improved, this may allow a relaxation of the test requirement. Good news if we can do it. • This new method must be simple to execute and understand.

3. The Force is with Us! • Reverb chambers are very good at finding peak signals. That’s what we are looking for during radiated emissions measurements. • Reverb chambers are very repeatable, usually within 3 dB, or in many cases even better. • The power measured by the antenna in the reverb chamber is the total power radiated by the EUT. • The EUT loses it’s directivity inside the aircraft fuselage, again, a total power condition.

4. Chamber Uniformity • Inject a known power level into the chamber, 0 dBm as an example. • Use an electrically short dipole (i.e. less than 0.1 m) • Orient the dipole in three mutually perpendicular orientations for each of 9 measurement locations that define the working volume of the chamber. • Care should be taken to ensure minimal influence by the interconnecting cable. A fiber optic coupling system is recommended.

5. Uniformity Measurements The uniformity may be measured with a tracking generator and a spectrum analyzer in Peak Hold mode. This is the preferred method. You need to sweep the analyzer at least 100 times per tuner rotation to get the standard deviation below 4dB using a tracking generator. Alternatively it may be measured using a separate signal generator and a spectrum analyzer. If a separate signal generator and spectrum analyzer are used, a 200:1 ratio gets the standard deviation to 4dB.

6. Uniformity 20’ x 25’ x 15’ Chamber with 5” dipole – 5 positions / tables installed, stirred.

7. Uniformity Measurements

8. How much power is in there anyway? • The power measured depends on the efficiency of the receive antenna and the insertion loss of the chamber. We need a way to measure both. • The simplest method is to inject a known power level into the chamber and measure the power out. • The power into the chamber times the xmit antenna efficiency minus the xmit coax loss = power in the chamber. • Section 20 assumes a .75 efficiency for a log-periodic antenna and .9 for a horn.

9. How much power is in there anyway? (cont) • By knowing the total power into the chamber we can wash out the numbers on the receive side. We now know the insertion loss of the chamber over a given frequency range. • We can add the total insertion loss of the chamber to the power received by the spectrum analyzer to get the total radiated power emitted from the EUT.

10. Measuring Non-CW Emissions • Keep in mind that the current test method does not address EUT’s that emit a pulsed type of emission. CW measurements using the following techniques will have much lower variations and standard deviations. • We should address the shortcomings of the current technique but not at the expense of making the new technique so difficult that no one will use it. • At Cessna we measured the output from a signal generator set up with varying parameters to see what we could measure without increasing test time beyond reason. • The RBW and VBW were both kept at the widths specified in the Section 21. The analyzer was in peak hold using peak detect mode.

11. Analyzer Sweeps vs. One Tuner Rotation50 uS PW, 200 Hz Rep Rate

12. Analyzer Sweeps vs. One Tuner Rotation50 uS PW, 200 Hz Rep Rate

13. Pulse Width Variation200 Hz Rep Rate, 200 Sweeps / Tuner Rotation

14. Pulse Width Variation200 Hz Rep Rate, 200 Sweeps / Tuner Rotation

15. Rep Rate Variation 50 uS Pulse Width, 200 Sweeps / Tuner Rotation

16. Rep Rate Variation 50 uS Pulse Width, 200 Sweeps / Tuner Rotation

17. Std Dev 10% Duty Cycle

18. Summary • Pulsed emissions are readily measured inside the Reverb Chamber with reasonable accuracy. • Uniformity can be measured pretty well using commonly available test equipment. • The technique has a lot of potential and should reduce errors between test facilities. • Absorber is not required for the chamber. • Testing can be done using the same chamber and setup as radiated susceptibility.

19. Future Efforts • We need to verify the techniques presented at several other labs. • A standard emitter would be very useful. • Limit lines need to be developed • Once we have a technique and limit lines we can discuss how to get it in the document.