The ubiquitous presence of GSM handsets is leading to an continual increase in unwanted RF signals that can cause distortion in the results produced by the circuit if the electronic circuitry does not have sufficient RF rejection. In order to ensure reliable operation of electronic circuits, the measurement of RF suppression capability of electronic circuits has become an indispensable part of product design. This paper introduces a general RF suppression capability measurement technology-RF anechoic chamber measurement device, describes its composition and operation mode, and gives examples of actual measurement results.
The wireless technology used in most cell phones today is Time Division Multiple Access (TDMA), which multiplexes high frequency carriers with on/off pulses at a frequency of 217 Hz. An IC that is susceptible to RF interference demodulates the carrier signal and reproduces a signal of 217 Hz and its harmonic components. Since the vast majority of these spectral components fall into the audio range, they produce unwanted audible clicks. It can be seen that the circuit with poor RF suppression capability demodulates the RF signal of the cellular phone and generates low frequency noise that is not desired to be heard. In order to measure the quality of the product, the circuit needs to be measured in an RF environment that is comparable to the environment encountered by the circuit during normal operation.
This paper describes a general-purpose integrated circuit RF noise suppression capability measurement technique. The RF rejection test places the board at a controllable RF signal level that represents the amount of interference that the circuit may experience when operating. This results in a standardized, structured test method that yields useful results that are repeatable in quality analysis. Such test results help the IC to be selected to obtain the circuit that is most resistant to RF noise.
The device under test (DUT) can be placed close to the working cell phone to test its RF sensitivity, but in order to obtain an accurate, repeatable test result, a fixed measurement method is required, in a repeatable The DUT is tested in the RF field. The solution is to use an RF test anechoic chamber to provide a precisely controllable RF field that is equivalent to the RF field produced by a typical mobile phone.
Below, we tested the RF suppression capability of a dual op amp (MAX4232) and a competing product (X) from Maxim.
Figure 1: RF noise suppression capability measurement circuit for dual op amps (online)
The RF suppression test circuit (Figure 1) shows the board connections to the dual op amps to be tested. Each op amp is configured as an AC amplifier. When there is no AC input, the output is set to 1.5V DC (VCC = 3V). . The inverting input is shorted to ground through a 1.5" loop (PC lead on the analog input) that is used to simulate the effect of the actual lead. The actual lead is used as an antenna at the operating frequency to collect and demodulate the RF signal. Connect a voltmeter to measure and quantify the RF noise rejection of the op amp.
Figure 2: RF noise suppression capability measuring device
Maxim's RF test set (Figure 2) produces the RF field required for RF suppression capability testing. The test anechoic chamber has a shielded chamber that acts like a shielded chamber in the Faraday chamber and has ports for connecting the power supply to the output monitor. Connect the devices listed below to form a test device:
1. Signal generator: 9kHz to 3.3GHz (Rohde & Schwarz SML-03)
2. RF Power Amplifier (PA): 800MHz to 1GHz, 20W (OPHIR 5124)
3. Power meter: 25MHz to 1GHz (Rohde & Schwarz)
4. Parallel line unit (radio darkroom)
5. Field strength detector
6. Computer
7. Voltmeter
The signal generator produces an RF modulated signal of the desired frequency and feeds it to the power amplifier, which measures and monitors the PA output through a Directional Coupler connected to the power meter. The computer establishes the required RF field by controlling the frequency range of the signal generator output, the type of modulation, the percentage of modulation, and the power output of the amplifier. The electric field is radiated through the antenna (planar type) in the shielded anechoic chamber and is precisely calibrated to produce a uniform, uniform, repeatable electric field.
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