Radiated Immunity Testing of Integrated Circuits in Reverberation Chambers

Ralf Heinrich, Robert Bechly Teseq GmbH

Landsberger Str. 255 12623 Berlin, Germany

Ralf.Heinrich@teseq.com Robert.Bechly@teseq.com

Bernd Deutschmann Infineon Technologies AG,

Am Campeon 1-12 85579 Neubiberg, Germany

bernd.deutschmann@infineon.com

Abstract—Radiated immunity testing of integrated circuits in the higher frequency range above 1 GHz is characterized by several challenges, e.g the required high field strengths and a rising influence of the EUT orientation. The existing methods show various limitations in this respect. Due to its entirely different principle of operation the reverberation chamber provides an opportunity to overcome the drawbacks of existing methods in the higher frequency range.

Keywords- reverberation chamber, EMC, integrated circuits, emission, immunity

I. INTRODUCTION As the IC internal operating frequencies as well as data

transfer between ICs have been developed to operate approaching the 10 GHz range, the roadmap for IC level emission and immunity measurement methods has to be adapted towards higher frequency ranges [1]. The EMC testing of integrated circuits requires special test setups which are specifically adapted to IC testing. Depending on the frequency range conducted or radiated tests are used.

Below 3 GHz some standardized test methods already exist for conducted and radiated tests. For the higher frequency range some proposals for improved test techniques exist, but they are not standardized yet. The gigahertz TEM cell (GTEM) is one possibility for characterizing ICs radiated emission and immunity up to 18 GHz.

A new approach to expand the frequency range for direct power injection (DPI) testing of ICs above 1 GHz was proposed in 2011 by J. Catrysse et al. in [2]. The alternative methodology for DPI immunity testing of IC’s is based on a directional coupler to overcome problems of parasitic effects of the DC blocking capacitor. Although with this approach the frequency range of the injected RF signal can be increased, decoupling the RF signal from e.g. a DC supply that is used to supply the IC under test will still remain a challenge.

Especially when high field strengths at high frequencies are required the existing methods show various limitations, which may be overcome using a reverberation chamber (RC). The

potential application range of reverberation chambers is illustrated in fig. 1.

A reverberation chamber basically consists of a shielded room and a stirrer which changes the electromagnetic field inside the chamber [3]. The chamber itself behaves like a multi mode resonator. Thus high field strengths can be achieved with relatively low input power requirements [4]. The stirrer has the task to move the natural resonances of the chamber in such a way, that a time-averaged spatially homogenous field distribution inside the chamber is achieved.

Figure 1. IC immunity test methods

Reverberation chambers can be used for emission and immunity tests. Important advantages for radiated immunity are the generation of high field strengths with very low input power requirements and the illumination of the EUT from all directions. Hence no turning of the EUT is necessary, which is a common drawback of existing methods. This becomes important in 2 aspects. Looking at the example layout of a lead frame shown in fig. 2 the disturbance quantity can be fully coupled into the horizontal and vertical bond wires if 4 different orientations of the test board are used. However, the coupling into diagonal bond wires is typically reduced due to their orientation to the septum of the cell, which will lead to undertesting.

Figure 2. Lead frame

Looking at the entire IC as a receive antenna the radiation/susceptibility pattern depends also on the frequency. This becomes especially important in the higher frequency range where the radiation becomes more and more directional and the commonly used 4 orientations may not be sufficient to capture the maximum emission or to find the orientation of maximum susceptibility. This phenomenon is displayed in fig. 3 based on the radiation pattern of a 3 cm loop antenna in free space simulated with the method of moments for different frequencies.

0,5 GHz 1 GHz 3 GHz

5 GHz 9 GHz 10 GHz

Figure 3. Radiation pattern of a 3 cm loop in free space at 0,5GHz, 1GHz, 3GHz, 5GHz, 9GHz, 10GHz

At low frequencies the maximum of the radiation pattern can be easily found by turning the EUT. However, with rising frequency the pattern changes significantly. More and more side lobes appear, the main beam of the maximum radiation is getting narrower and finding the maximum susceptibility is getting more and more difficult. This can be easily solved with a reverberation chamber where the test result is almost independent from the orientation of the EUT.

II. TEST OBJECTS AND PERFORMANCE CRITERIA Two different ICs under test with wireless data

transmission were investigated. One system operates at 315 MHz, the other at 433 MHz. The following performance

criteria were used to evaluate the RF immunity of the test objects:

• Shift in carrier frequency less than 10 kHz

• Loss in power level less than 4 dB

• Spurious at 200 kHz from the carrier less than -8 dBc

III. TEST SETUP AND PROCEDURE

A. GTEM 250 The tests in a GTEM 250 were intended to serve as a

reference for comparison and evaluation of the tests in the reverberation chamber. Three different orientations (see fig. 4) of the test object were used to determine the most susceptible position for reference.

Figure 4. Orientations X, Y and Z of the test object in the GTEM-cell.

The disturbance field strength was generated using a signal generator and an amplifier. Fig. 5 shows the test setup.

Figure 5. Test setup GTEM cell

The test was done in the frequency range from 10 MHz to 3 GHz in steps of 500 kHz. During the test the disturbance field strength was increased up to the maximum achievable field strength and the EUT performance was monitored using a spectrum analyzer connected to an antenna next to the EUT. The main EUT reactions were a shift in carrier frequency, loss in power level and spurious. For each EUT reaction the frequency was tuned to maximum susceptibility and the inception field strength was determined.

B. Reverberation Chamber The reverberation chamber used for the investigations has a

dimension of 1.5 x 0.8 x 1 m³. Due to the nature of cavity-resonators, the RC is limited by a lowest usable frequency (LUF), which mainly depends on the RC’s dimensions. The

LUF for this chamber is about 800 MHz. Lower LUFs require larger chambers. Therefore it is not reasonable to apply RCs for IC testing in the lower frequency range (below 1 GHz). The reverberation chamber has a special fixture for a 10 x 10 cm IC test board [5]. For this investigation the IC under test was part of a battery operated module with no other connections which could be placed in the working volume of the reverberation chamber in a similar way as in the GTEM cell shown in fig. 4.

Figure 6. Test setup reverberation chamber

The test setup in the reverberation chamber is shown in fig. 6. The test frequencies in the reverberation chamber were limited to the most susceptible ones determined in the GTEM cell. For each frequency and EUT orientation the stirrer was tuned in steps to the maximum field strength at the EUT position and the corresponding field strength was determined based on the calibration data of the reverberation chamber.

IV. TEST RESULTS

A. GTEM 250 The 433 MHz IC showed different disturbance phenomena

at different frequencies, e.g. loss in carrier level at 414.85 MHz, loss in carrier level and spurious at 415.75 MHz and spurious at 451.95 MHz. These examples are shown in figs. 7 to 9. The magnitude of the disturbed signals is in each case normalized to the undisturbed carrier level. The test field strength was 128 V/m for the EUT orientation of maximum susceptibility.

Figure 7. Loss in carrier level of more than 4 dB at a disturbance frequency

of 414.84 MHz

Figure 8. Loss in carrier level of more than 4 dB and spurious at a

disturbance frequency of 415.75 MHz

Figure 9. Carrier shift and spurious at a disturbance frequency of

451.95 MHz

Above 1 GHz the 315 MHz IC showed a higher susceptibility than the 433 MHz IC, mainly resulting in carrier shift. Fig. 10 shows the shift in carrier frequency for 79 V/m and 154 V/m at a disturbance frequency of 1354 MHz.

Figure 10. Shift in carrier frequency depending on the disturbance power

level at 1354 MHz

B. Reverberation Chamber Because of the LUF of the reverberation chamber

(800 MHz) the 315 MHz IC was used for these investigations. It was only tested at the most susceptible frequencies that were found out during the GTEM cell test, i.e. 1153 MHz, 1354 MHz and 2292 MHz. The tests were performed with the ICs placed in three different orientations (1, 2, and 3) which exactly correspond to the orientations X, Y and Z that were used in the GTEM cell test. The test results for the disturbance phenomenon of 10 kHz carrier shift are summarized in table I.

TABLE I. TEST RESULTS IN THE RC FOR 10KHZ CARRIER SHIFT

Frequeny in MHz Position Stirrer angle Pf in dBm

1153 1 80 24.7 1153 2 30 23.5 1153 3 300 26.8 1354 1 100 18.9 1354 2 200 21.8 1354 3 170 20.8 2292 1 220 36.9 2292 2 300 35 2292 3 90 35.4

It can be seen that the deviation in forward power to

achieve the failure criterion in the different EUT orientations is about 3dB or less, which confirms that for each test-frequency the test result in the reverberation chamber depends only very little from the EUT orientation. With rising frequency the effect of different EUT orientations will further reduce due to smaller standard deviation of the field strength in the chamber.

V. COMPARISON OF THE RESULTS IN GTEM AND RC

A. Comparison to one Position of the test object in the RC Table II shows the comparison of the different input power

requirements for GTEM and RC to provoke the disturbance phenomenon. The required input power for the GTEM is about 14-17 dB higher, while the field strengths are in fairly good agreement in spite of the difference in field generation (homogeneous field / statistical field).

TABLE II. COMPARISON OF INPUT POWER REQUIREMENTS AND FIELD STRENGTH OF GTEM AND REVERBERATION CHAMBER

Frequency in MHz

Carrier shift in kHz

PGTEM / PRC in dB

| EGTEM/ERC| in dB

1153 -20 14 3.2

1354 -10 -85

16.2 17.1

2 1.25

2292 -10 n.a. *) n.a.*) *)Field strength in RC of 940 V/m could not be generated in the GTEM cell.

B. Comparison for Three Positions in the RC Considering all three positions in the reverberation chamber

the spread between the positions is about 3 dB. The spread of the test results of different EUT orientations in the GTEM cell was more than 12 dB and will further increase with rising frequency as discussed in the introduction.

Based on the disturbance phenomenon of 10 kHz carrier shift at 1354 MHz as an example the difference in E-field strength between the GTEM cell and RC ranges between 0.7 and 3.6 dB depending on the position on the EUT in the RC as shown in table III.

TABLE III. COMPARISON OF FIELD STRENGTH DIFFERENCE OF GTEM AND RC CONSIDERING THREE POSITIONS IN THE RC

Reference GTEM PRC in dBm ERC in V/m | EGTEM/ERC|

in dB

Pin: 37.9 dBm E: 79 V/m

Pmin: 18.8

Emin: 52

3.6

Pmax: 21.7 Emax: 73 0.7

CONCLUSIONS The test in the reverberation chamber can be done in real

operating conditions e.g. of a battery operated module as presented in this paper or also directly on the IC-level using a special test board [5]. Compared to the GTEM cell or microstrip line the reverberation chamber offers a minor dependency of the EUT orientation. This can be seen as a big advantage, which normally allows neglecting turning of the EUT. The test results in the GTEM cell could be confirmed in the reverberation chamber with much less input power requirements. Thus also the generation of field strengths of 1000 V/m or more is feasible. With these important advantages the reverberation chamber may become a valuable addition to the IC test methods for the higher frequency range above 1 GHz.

REFERENCES [1] M. Ramdani, et al. “The Electromagnetic Compatibility of Integrated

Circuits—Past, Present, and Future”, IEEE Transactions on Electromagnetic Compatibility, Vol. 51, No. 1, pp. 78-100, February 2009

[2] J. Catrysse, D. Pissoort, and F. Vanhee, “ Expanding the Frequency Range for DPI Testing of IC’s above 1 GHz: An Alternative Proposal”, Proc. of the 10th Int. Symposium on Electromagnetic Compatibility (EMC Europe 2011), pp. 400- 404, York, UK, September 26-30, 2011

[3] IEC 61000-4-21, Electromagnetic compatibility (EMC) – Part 4-21:Testing and measurement techniques – Reverberation Chamber Test Methods, 2003-08.

[4] N. Eulig, A. Enders, H.G. Krauthäuser and J. Nitsch, “Achievable field strength in reverberation chambers” in Advances in Radio Science (2003), pp. 53–56

[5] R. Heinrich, R. Bechly, “Investigations on the Suitability of Reverberation Chambers for Radiated EMC Testing of Integrated Circuits”, Asia-Pacific EMC Symposium (APEMC 2011), Jeju Island, Korea, May 16-19, 2011