Spurious Emissions and Intermodulation Distortions: Will Your Wireless Receiver Transmit Signals?
2019-09-05 | 6 min read
Various combinations of signals in a transmitter and receiver can cause spurious signals. Spurious responses may be a spur from a LO, a harmonic of a power supply, distortion products, or a harmonic of the system clock. They impact not only its receiver sensitivity, but the receiver sensitivity of other wireless devices.
This post will discuss unwanted signals that a “receiver” generates and test setups for the signals, including spurious emissions and intermodulation distortions.
Receiver Spurious Emissions
Spurious emissions are unwanted emissions that emanate from the devices under test. Receiver spurious emissions are generated internally by the receiver or result from the interaction of the receiver with the coupling transmitter’s signal. A receiver spurious emissions power measures the power of emissions generated, or amplified in a receiver that appears at the antenna connector. The purpose of the test is to limit the interference caused by receiver spurious emissions to other devices or systems.
To measure receiver spurious emissions, you need to connect the receiver’s output to a signal analyzer with a transmitter notch filter and terminate all transmitters and untested receivers as shown in Figure 1. For example, 3GPP Release 15 specifies a user equipment (UE) receiver spurious emissions test case in the technical specification (TS) 36.101 subclause 7.9. Enable the transmitter to output specified maximum power level, and measure the receiver port’s emission level with 100 kHz or 1 MHz resolution bandwidth (RBW) for specified frequency bands. The power of any spurious emission should not exceed the specified power level.
Figure 1. Measuring receiver spurious emission
FDD vs. TDD System
For frequency division duplex (FDD) systems, you need to perform the spurious emission measurements when transmit and receive are ON. For time division duplex (TDD) systems, the test requirements apply during the transmitter OFF period.
Regulatory agencies such as the Federal Communications Commission (FCC) in the United States and the European Telecommunications Standards Institute (ETSI) in Europe also specify spurious emissions measurements. Some spurious emissions tests require a quasi-peak detector. You need to select a right detector in your signal analyzer in order to comply with the test standard.
Receiver Intermodulation Distortions
Third- and higher-order mixing of the two interfering RF signals can produce intermodulation signals in the band of the desired channel at a receiver. The intermodulation signals may degrade the receiver’s sensitivity performance. The frequencies of the interfering signals are set such that one of the third-order intermodulation products (frx1 = 2f1 – f2 and frx2 = 2f2 – f1) falls within the passband of the receiver as shown in Figure 2 below.
Figure 2. Receiver intermodulation products
For receiver intermodulation measurement setups, you need one signal generator for the wanted signal, and two signal generators as interfering signals as shown in Figure 3. The two interfering signals are defined using a continuous wave (CW) carrier and a modulated signal. In the presence of the two interfering signals, a receiver receives the wanted signal and needs to meet the bit error rate (BER), packet error rate (PER), or throughput requirement.
Figure 3. Receiver intermodulation measurement setup
Tips to Make Accurate Measurements
When the two interfering signals input to a combiner, you need to watch out for the signal routes from one signal generator to the other. The reverse signal will cause the signal generator’s automatic leveling control (ALC) to malfunction and create intermodulation products. To minimize these impacts, apply the techniques below:
- Maintain a frequency separation between the interfering signals that is greater than the bandwidth of the ALC of the signal generators
- Add attenuators to the outputs of the signal generators; theoretically, reduce the intermodulation products 3 dB for every 1 dB of attenuation
- Use hybrid combiners (with high isolation and low insertion loss)
- Use isolators
- Turn off the ALC of the signal generators to prevent conflict in power level control between the two signal generators
A receiver is designed to receive signals, but it transmits unwanted signals under certain conditions and degrades system performance. The receiver spurious emissions and intermodulation tests help identify potential problems with your designs and prevent system degradation. When you perform these tests, be aware of port termination, signal isolation, and band rejection to improve measurement accuracy.