The proliferation of wireless transceivers in portable applications has created a need for increased attention to an electronic circuit's ability to operate in the vicinity of high frequency radio transmitters.
By Donald Lafontaine, Intersil

The proliferation of wireless transceivers in portable applications has created a need for increased attention to an electronic circuit's ability to operate in the vicinity of high frequency radio transmitters. In gigahertz radio systems, the close proximity of the radio antenna to low frequency amplifier sub-assemblies can result in the demodulation of the radio signal causing a disruptive interference in the receiving circuit.

This article describes how to build a test platform using standard equipment found in most high frequency analog labs. This test platform is suitable for testing and characterizing radiated Radio Frequency Interference (RFI) in low frequency audio circuits. The test platform was built to troubleshoot a Bluetooth application with an excessive noise problem at the output of the headset amplifier. Although the noise caused by the Bluetooth transmitter was readily observed at the amplifier output, the frequency hopping RF combined with the complex encoded modulation of the voice signal produced an interference signal that was too complex to analyze.

The test platform presented in this article solves this problem by creating an interference environment that replaces the complex Bluetooth signal with a swept RF frequency source modulated by a 1 kHz modulation signal. The 1 kHz modulation signal is used to track the source of RF entry and the signal path to the audio amplifier’s output. This enables the user to observe the interference under closely controlled conditions of the RF carrier field strength, carrier frequency and modulation frequency.

Demodulation of the RF Signal
Radio transceivers transmit voice and data signals by modulating the high frequency RF carrier. Sensitive circuits located near the antenna must be designed to reject interference or have special shielding put in place to prevent demodulation of the RF signal in the audio circuits. Layout and proximity to the transmitter can result in several different receiving sites on the circuit board that can cause interference at different frequencies.

Several studies, experiments and calculations have shown the propensity for operational amplifiers to demodulate RF signals principally at the emitter-base junction of the input differential pair. Demodulation occurs even though the amplifiers bandwidth is much lower than the RF’s out-of-band signal.

Figure 1 illustrates how the RF carrier is stripped off leaving behind the low frequency signal. Because the frequency of the RF carrier is many times higher than the bandwidth of the audio amplifier under test, the amplifier acts as the demodulator and filter, resulting in a carrier-free, low-frequency replica of the modulating signal appearing at the output of the amplifier.

Test Platform (Modulated Sweep Test Hardware)
This platform is capable of generating a swept carrier frequency from 100 kHz to 6 GHz with an external modulated signal. The RF modulated test platform is built using a HP8753D network analyzer as a variable RF carrier frequency source and a function generator to modulate the carrier frequency with a 1 kHz sine wave. The function generator (HP3310A) is plugged into the EXT AM BNC on the back of the HP8753D. The output of the network analyzer is now a swept modulated carrier frequency that is connected to a simple antenna as shown in Figure 2. The output power of the carrier frequency was adjusted to 0 dbm to match the standard Bluetooth signal.

Synchronization between the frequency of the carrier and the time base of the scope is done by setting the sweep time of the network analyzer to 20 seconds and the scope to two seconds per division. This results in a time base for both the network analyzer and the scope of two seconds per division. A direct frequency reading correlated to output voltage from the scope is made by simultaneously triggering single sweep on both network analyzer and scope reference Figure 4.

Interference Testing
With the ability to select a single modulating frequency and the RF carrier frequency and level of the carrier signal, a variety of controlled experiments to partition the interference sources is capable. Close inspection of circuit behavior is possible by sweeping the carrier frequency and measuring the modulating signal at the output of the audio amplifier circuit. Figure 3 shows the circuit schematic of the board under test.

Figure 4 shows the result of a frequency sweep of a bipolar unity gain dual channel Opamp. RF Interference was prevalent at 3.9 GHz.

Figure 5 shows the interference on channel A and channel B outputs with a fixed carrier frequency of 3.9 GHz and a 1 kHz 100% modulation. The frequency of 3.9 GHz was chosen because of the interference peak at this frequency. The signals shown are the 1 kHz demodulated signal at the outputs of the unity gain dual Opamp. The non-symmetry in the 1kHz signal is from our lab equipment generating the modulated signal and not the amplifiers.

References [1] Joseph G. Tront, James J. Whalen, Curtis E. Larson, James M. Roe “Computer-aided Analysis of RFI Effects in Operational Amplifiers. IEEE Transaction on Electromagnetic Compatibility, Vol.EMC-21, NO.4, November 1979 [2] Muhammad Taher Abuelma’atti “Radio interference by Demodulation Mechanisms Present in Bipolar Operational Amplifiers IEEE Transactions on Electromagnetic Compatibility, Vol 37. NO.2, May 1995. [3] Robert E. Richardson, Jr. Modeling of Low-Level Rectification RFI in Bipolar Circuitry. IEEE Transactions on electromagnetic Compatibility, Vol.EMC-21, NO4, November 1979. [4] Robert E. Richardson, Vincent G. Puglielli and Robert A. Amadori. “Microwave Interference Effects in Bipolar Transistors” IEEE Transaction on Electromagnetic Compatibility, Vol. .EMC-17, NO.4, November 1975.