sábado, 17 de julio de 2010

A general measurement technique for determining RF immunity

The presence of the radio-frequency (RF) environment is steadily progressing due to the ubiquitous usage of cell phones. An electronic circuit under such RF environments can give distorted results owing to the circuit's poor RF rejection capability. In order to have the electronic circuits working satisfactorily it becomes imperative to test for its RF immunity.

This article describes a generalized technique to measure the RF immunity of a circuit. It defines a standard and structured test methodology aimed at establishing adequate repeatability of the test results for qualitative analysis. The test results thus obtained aids in astute selection of ICs and developing circuits that are less prone to RF noise.

The RF susceptibility can be tested by placing the DUT near the cell phone. But to have accurate, comparable and efficacious test results the DUT needs to be tested in consistent and repeatable RF fields.The RF anechoic test chamber produces such RF fields that are accurately controlled and comparable to that generated by a typical mobile phone. The RF immunity test procedure was carried on MAX4232 and competitor's parts (Part X) and its results were compared.

The circuit diagram in Figure 1 shows the circuit board connections to the dual op-amp under test in the RF setup. The op-amps are configured as an ac amplifier. With no ac signal, the output sits at 1.5 Vdc (with a supply voltage of 3 V). The inverting input is shorted to ground using 1.5-inch loop of wire to emulate the actual trace of wire to the input signal. This loop incorporates the effects of the actual trace, which could probably be acting as an antenna at the working frequency, collecting the RF signal and demodulating it. The RF noise immunity of the op-amp is measured and quantified by connecting a dBV meter at the outputs of the op-amp.
Figure 1

Figure 2 shows the RF anechoic test setup system that emulates the RF field environment necessary for RF immunity testing. This test chamber is similar to a "Faraday's Cage" and has a shielded body. The chamber has access ports for connecting supply voltages and output monitors. The setup is formed by concatenation of the following equipment:
  • Signal generator: SML-03, 9 kHz to 3.3 GHz (Rhode & Schwarz) 
  • RF power amplifier: 800 MHz-1 GHz/20 W (OPHIR 5124) 
  • Power meter: 25 MHz to 1 GHz (Rhode & Schwarz); 
  • Parallel wired cell (Anechoic chamber); 
  • Electric field sensor 
  • Computer (PC); and 
  • dBV meter.
Figure 2
The signal generator generates the RF signal of the desired frequency and modulation and is fed to the power amplifier. The amplifier output is measured and monitored with a directional coupler in conjunction with a power meter. The computer controls the range of frequencies applied from the output of the signal generator, its modulation type, modulation percentage and its power from power amplifier output so as to generate the desired RF field. This field is radiated inside the chamber using an antenna (planer).

To perform the immunity test on MAX4232 vs. Part X, the DUT is placed inside the shielded anechoic chamber, which serves best to produce uniform, accurately calibrated and consistently repeatable electric fields.

The RF field experienced by a DUT placed near a typical cell phone is around 60 V/m at about 4 cm from the radiating antenna of the phone and decreases as one moves the DUT away from the phone (around 25 V/m at a distance of 10 cm from the phone). A uniform field strength of 60 V/m is generated to emulate the actual RF environment experienced by a DUT. Also, 60 V/m is low enough to keep the receiving devices below the clipping level and avoid measurement errors. A RF sine wave whose frequency is varied between the cell phone frequencies of 800 MHz to 1 GHz is modulated with an audio frequency of 1000 Hz with 100% modulation. Modulation with 217 Hz would have produced similar results but a more common 1000 Hz audio frequency is chosen. The access ports on the side of the chamber serve to provide power to the DUT and also to connect the dBV meter, which is set to give dBV (dB's relative to 1 V) readings. Furthermore, the RF field can be accurately calibrated by locating the position of the DUT using the field sensor.

Figure 3 depicts an average output of MAX4232 and Part X. Under the RF frequency variation from 800 MHz to 1 GHz with a uniform electric field of 60 V/m, MAX 4232 shows -66 dBV (500 µV rms with respect to 1 V) and that of Part X is -18 dBV (125 mV rms with respect to 1 V). In the absence of any RF signal, the dBV meter shows -86 dBV.
 Figure 3

Thus, MAX4232 output changes by only -20 dB [(-86 dBV) — (-66 dBV)] or goes from 50 µV rms to 500 µV rms under the influence of RF environment. We can say that the output of MAX4232 changes by only a factor of 10 under the selected RF environment. Hence, it can be concluded that MAX4232 has excellent RF immunity of -66 dBV and would not produce any major noticeable distortion at the output.

However, the average reading of Part X is only -18 dBV, which means that this part under RF influence shows 125 mV rms with respect to 1 V rms, a major perceptible increase by 2500 times than the normal expected 50 µV rms. Thus, part X can be said to have a poor RF immunity of -18 dBV and is more likely to cause problems in close proximity to cell phones and other RF sources.

Hence for applications that need the processing of audio signals such as headphone amplifiers, mic amplifiers, op-amps with high RF immunity are better suited.

Publicado por: Jahir Alonzo Linares Mora C.I: 19769430 CRF
Bibliografia: http://rfdesign.com/mag/510RFD33.pdf

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