RF anti-interference ability test device

RF anti-interference ability test device

Most cell phones today use the Time Division Multiple Access (TDMA) standard, a multiplexing technique that modulates a high-frequency carrier wave with on/off pulses at 217 Hz. An IC that is susceptible to RF interference will demodulate this carrier signal and regenerate the 217 Hz and its harmonic components. Since most of these spectral components fall into the audio range, they produce an unpleasant "buzzing" sound. As a result, circuits with poor RF immunity will demodulate the RF signal from a cell phone and produce undesirable low-frequency noise. As a quality assurance test, the circuit needs to be placed in an RF environment that is equivalent to the circuit's operating environment during normal operation.

This article describes a general technique for measuring the RF noise immunity of integrated circuits. The RF immunity test subjects the circuit board to a controlled RF signal level that represents the interference intensity that the circuit may experience during operation. The result is a standardized, structured test method that can produce repeatable test results in quality analysis. Such test results can help select ICs to obtain circuits that are immune to RF noise.

The device under test (DUT) can be tested for RF susceptibility by bringing it close to an operating cell phone. However, to obtain accurate and repeatable test results, a fixed measurement method is required to test the DUT in a repeatable RF field. The solution is to use an RF test anechoic chamber, which provides a precisely controlled RF field, equivalent to that generated by a typical mobile phone.

RF Immunity Test Setup
Below we compare the RF immunity test results of the MAX4232 dual op amp and a competitive product X. The RF Immunity Test Circuit (Figure 1) shows the circuit board connections for the dual op amp under test. Each op amp is configured as an AC amplifier. In the absence of an AC input signal, the output is set to a 1.5V DC level (VCC = 3V). The inverting input is shorted to ground through a 1.5-inch loop of the PC board lead at the analog input. This loop is used to simulate the effect of the actual lead, which acts as an antenna at the operating frequency to receive and demodulate the RF signal. A voltmeter (dBV) is connected to the output to measure and quantify the RF noise rejection capability of the op amp. Figure 1. RF Noise Rejection Test Circuit for the MAX4232 Dual Op Amp

Maxim's RF test setup (Figure 2) generates RF fields for testing RF immunity. The test anechoic chamber has a shielded room that functions similarly to a Faraday cavity, with ports for connecting a power supply and an output monitor. The test setup is constructed by connecting the following equipment:

a.Signal generator: 9kHz to 3.3GHz (Rhode & Schwarz SML-03)

b.RF power amplifier: 800MHz to 1GHz, 20W (OPHIR 5124)

c. Power meter: 25MHz to 1GHz (Rhode & Schwarz)

d. Parallel line unit (anechoic chamber)

e. Field strength detector

f. Computer (PC)

g. Voltmeter (dBV) Figure 2. RF noise suppression capability test device

The signal generator generates an RF modulated signal of the required frequency and feeds it to the power amplifier. The power amplifier (PA) output is measured and monitored through a directional coupler connected to a power meter. The computer establishes the required RF field by controlling the frequency range, modulation type, modulation percentage of the signal generator output and the power output of the PA. The electric field is radiated in a shielded anechoic chamber through an antenna (planar type) and is precisely calibrated to produce a uniform, consistent and repeatable electric field.

The RF field strength near a typical cell phone is approximately 60V/m (4cm from the phone antenna), and the field strength decreases as you move away from the phone. At 10cm from the phone, the field strength drops to 25V/m. Therefore, a uniform 60V/m field strength is generated in the anechoic chamber to simulate the RF environment in which the DUT is located (the 60V/m radiation intensity ensures that the device under test will not be level clamped, avoiding measurement errors). The RF signal used is an RF sine wave that varies in the 800MHz to 1GHz cell phone frequency range, modulated with an audio frequency of 1000Hz and a modulation depth of 100%. Similar results can be obtained when modulating with a frequency of 217Hz, but 1000Hz is a more common audio frequency and was chosen here for ease of evaluation. Power is supplied to the DUT through the access port of the anechoic chamber, and a voltmeter is connected through the access port to read the dBV value (dB relative to 1V). The RF field can be accurately calibrated by adjusting the position of the DUT in the anechoic chamber and using a field strength tester. Figure 3. RF noise rejection test results for two dual op amps using the test setup of Figure 2.

Test Results
The test results of the two dual op amps (MAX4232 and Competitor X) are shown in Figure 3, with the measured values ​​being the average output dBV. When the RF frequency varies from 800MHz to 1GHz, in a uniform 60V/m electric field, the average output of the MAX4232 is about -66dBV (500µVRMS relative to 1V); the average output of Competitor X is about -18dBV (125mVRMS, relative to 1V). In the absence of an RF signal, the voltmeter reads -86dBV.

Therefore, the output of the MAX4232 changes by only -20dB (-86dBV to -66dBV), which means that the RF interference causes the output of the MAX4232 to change from 50µVRMS to 500µVRMS. In the presence of RF interference, the MAX4232 changes by a factor of 10. Therefore, it can be inferred that the MAX4232 has excellent RF immunity (-66dBV) and does not produce significant output distortion. The noise rejection average reading for device X is only -18dBV, which means that the output changes by 125mVRMS (relative to 1V) under RF influence. This is a large increase, 2500 times the normal value of 50µVRMS. Therefore, device X has poor RF immunity (-18dBV) and may not work properly when close to a cell phone or other RF source. Clearly, the MAX4232 is a better choice for audio processing applications such as headphone amplifiers and microphone amplifiers.

Summary
In short, in order to ensure the working quality of the product in the RF environment, the measurement of RF anti-interference ability is a step that circuit board and IC manufacturers must consider. The RF anechoic chamber measurement device provides an economical, flexible and accurate RF anti-interference ability measurement method.