Testing and Troubleshooting Digital RF Communications Receiver Designs

Introduction 

This Keysight Technologies, Inc. application note presents the fundamental measurement principles involved in testing and troubleshooting digital RF communications receivers—particularly those used in digital RF cellular systems. Measurement setups are explained for the various receiver tests, and troubleshooting tips are given. 

The demand for ubiquitous wireless communications is challenging the physical limitations of current wireless communications systems. Wireless systems must operate in a very limited area of the radio spectrum and not interfere with other systems. The maturing wireless markets are becoming much more competitive, and product cycle times are now measured in months instead of years. Consequently, network equipment manufacturers must produce wireless systems that can be quickly deployed and provide bandwidth-efficient communications. 

Digital modulation has several advantages over analog modulation. These include bandwidth efficiency, superior immunity to noise, low power consumption, privacy, and compatibility with digital data services. These advantages, coupled with advances in digital signal processing and in analog-to-digital conversion, have spawned the current migration to digital RF communications formats.  

Digital RF communications systems use complex techniques to transmit and receive digitally modulated signals through the radio channel. These complexities challenge designers in the isolation of system problems. Signal impairments can be traced back to a component, device, or subsystem of the digital RF communications system. Successful receiver design depends on the ability to locate sources of error.

The digital radio receiver must extract highly variable RF signals in the presence of interference and transform these signals into close facsimiles of the original baseband information. Several tests verify receiver performance in the presence of interfering signals. These performance verification tests are categorized as either in-channel or out-of-channel measurements. 

This application note includes:

• A block diagram of a digital radio communications system.  
• Common receiver designs. 
• In-channel tests, including sensitivity and co-channel immunity. 
• Out-of-channel tests, including spurious and intermodulation immunity and adjacent and alternate channel selectivity. 
• Best practices in the receiver performance tests.  
• Troubleshooting techniques for receiver designs. 
• An appendix that relates Bit Error Rate (BER) to Error Vector Magnitude (EVM).

The setups required to perform the receiver performance tests are included in this application note along with descriptions of potential errors in the measurement process. Troubleshooting techniques applicable to the design of digital radio receivers are also provided.

Digital Radio Communications Systems

The digital radio signal experiences many transformations in its migration from a baseband signal at the transmitter to its replication at the receiver. A rudimentary block diagram of a digital radio communications system reveals the transformation process the signal undergoes from origination to reception.

The system-level diagram in Figure 1 displays the symmetry of the digital radio. To a certain degree, the receiver can be considered a reverse implementation of the transmitter. Consequently, the measurement challenges are similar for both parts of the digital radio system. However, unique problems exist at various locations in the system. For example, the receiver must detect weak signals in the presence of noise and is therefore tested with very low-level signals. The transmitter must not interfere with other radio systems and is consequently tested for the amount of interference it produces in the nearby frequency channels.

Certain parts of the digital radio may be implemented in a Digital Signal Processor (DSP), an Application-Specific Integrated Circuit (ASIC), or a Digital Down Converter (DDC). The DSP, ASIC, or DDC has different levels of involvement in the various digital radio designs. Sometimes it is difficult to distinguish those problems originating in the digital portion of the radio from those originating in the analog portion. This application note describes how to isolate and clarify sources of error in digital radio receiver tests and designs.