A test platform solution that effectively characterizes the amplifier's interference sensitivity

This article explains how to build a test platform using standard equipment used in most high-frequency simulation laboratories. This test platform is suitable for testing and characterizing radio frequency interference (RFI) radiated in low-frequency audio circuits. This test platform was originally used to check Bluetooth applications with excessive noise issues at the output of the headphone amplifier. Although the noise caused by the Bluetooth transmitter is easily observed in the amplifier output, the combination of frequency hopping RF and the complex code modulation of the noise signal will generate an interference signal, and this signal will be particularly complex and difficult to analyze .
The test platform mentioned in this article solves this problem by creating an interference environment, which uses an RF scanning frequency modulated by a 1kHz modulation signal to replace the complex Bluetooth signal. The 1kHz modulated signal is used to track the RF input and the signal path leading to the audio amplifier output. This allows users to observe interference under controlled conditions close to the RF carrier magnetic field strength, carrier frequency, and modulation frequency.
RF signal demodulation
The radio transceiver transmits voice and data signals by modulating a high-frequency RF carrier. The sensitive circuit design near the antenna must have anti-interference characteristics, or adopt special protection to prevent RF signal demodulation in the audio circuit. The layout and proximity to the transmitter may result in several different receiving points on the circuit board, which can cause interference at different frequencies.
Several studies, experiments, and calculations have shown that the operational amplifier tends to demodulate the RF signal mainly at the emitter base junction of the input differential pair. Even if the amplifier bandwidth is much lower than the RF out-of-band signal, demodulation will occur.

Figure 1 shows how to strip the RF carrier and leave the low frequency signal. Since the frequency of the RF carrier is many times higher than the bandwidth of the audio amplifier in the test, the amplifier acts as a demodulator and filter, resulting in a carrier-free low-frequency copy of the modulated signal that appears at the output of the amplifier.
  testing platform
The platform can generate a swept carrier frequency from 100kHz to 6GHz with an external modulation signal. The test platform for RF modulation is shown in Figure 2. The platform uses the hp8753d network analyzer as a variable radio frequency carrier frequency spring and function generator to adjust the 1 kHz sine wave carrier frequency. The function generator (HP3310A) is inserted into the EXT AM BNC next to HP8753D. The output of the network analyzer is connected to the sweep modulation carrier frequency of the simple antenna shown in Figure 3. The output power of the carrier frequency is adjusted to 0dbm to match the standard Bluetooth signal. The synchronization between the frequency of the carrier and the time base of the scope is achieved by setting the scan time and scope of the network analyzer to 20 seconds and 2 seconds/zone respectively. This creates a time base for the network analyzer and the scope of 2 seconds/zone. The direct frequency reading related to the output voltage of the scope is achieved by simultaneously triggering a single sweep on the network analyzer and the scope (refer to Figure 5).
How to strip RF carrier and leave low frequency signal
interference test
By selecting the individual modulation frequency and the ability of RF carrier frequency and carrier signal level, various control experiments can be carried out to divide the interference source. By scanning the carrier frequency and measuring the modulation signal in the output of the audio amplifier circuit, a strict detection of the circuit condition can be achieved. Figure 4 shows the circuit schematic of the test board.

The output of the network analyzer is connected to the sweep modulation carrier frequency of the simple antenna shown in Figure 3

Figure 5 shows the frequency sweep result of a bipolar unity gain dual channel operational amplifier. Radio frequency interference generally exists at 3.9GHz.
Interference on the output of channel A and channel B, using 3.9GHz and a fixed carrier frequency of 1kHz 100% modulation
Figure 6 shows the interference on the output of channel A and channel B, using a fixed carrier frequency of 3.9GHz and a 1kHz 100% modulation. The reason for choosing 3.9GHz is that interference is reached at this frequency. The signal shown is a 1kHz demodulated signal with a unit gain dual operational amplifier. The asymmetry signal at 1kHz is the modulated signal generated from the laboratory equipment, not from the amplifier.