Design And Test Challenges Of Frequency-Hopping Radios

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The dynamic generation of RF waveforms through DSP and the integration of digital and RF circuits—often on the same integrated circuit (IC)—also create issues not seen in traditional RF transceiver designs. These include modulation transients, non-linear effects of amplifiers, and digital-to-RF crosstalk, to name a few.

The performance of SDR transmitters must be verified with measurements that are beyond the traditional RF transmitter conformance tests. Simply passing these tests does not ensure a device will work properly, and system behaviour must be carefully and thoroughly observed since software is continually changing system parameters. Truly addressing these challenges requires SDR designers to fully analyse and characterise their systems.

Discovery of true system behaviour is important to identify potential RF spectrum anomalies. As system parameters change over time, performing frequency-selective triggering is necessary to pinpoint the instant a transient event occurs. Performing time-correlated analysis in multiple domains is required to determine the specific cause of each problem. Capturing the entire event seamlessly into memory is valuable for subsequent analysis, as it can be difficult to recreate the conditions under which the transient occurred. These advanced troubleshooting methods of verifying signal performance over time, combined with traditional conformance tests performed under steady-state conditions, are necessary for comprehensive SDR testing.

Verifying performance and troubleshooting at the system level
Developing a verified system architecture design is vital to the success of a modern communication system. The more the access points that are tested and verified, the less likely the issues that will manifest during the last system integration phase. Also, the later the issues are addressed in the development and integration cycle, the more expensive they will be to resolve. Some of the major contributors to system failures are DSP, RF circuitry and the controlling software. Verification debug tools greatly aid system designers in effectively discovering problems.

Once an error has been identified, it must be isolated and understood. To isolate a problem and determine its root cause, it is important to time-correlate the error back through the signal path. Since the signal information changes form in an SDR design—from digital bits to continuously variable analogue voltages—several pieces of test equipment may be needed to diagnose the exact source of problems. Because the problem may occur at any point in the signal path, and memory capacity in oscilloscopes and logic analysers is limited, the ability to simultaneously trigger multiple test instruments and capture the exact moment in time that the event occurs is important. This requires that each instrument be able to trigger in its domain (logic analysers for digital triggers, oscilloscopes for time-domain amplitude triggers and spectrum analysers for frequency-domain triggers).

An integrated, end-to-end test system comprising real-time spectrum analyser (RTSA), arbitrary waveform generator, oscilloscope and logic analyser can be invaluable for testing SDRs. Select instruments from leading test and measurement vendors are able to work in unison—with cross-triggering and time-correlated subsystem views—to verify SDR performance and perform multiple test procedures at the physical and various software layers. These test systems can also be used to understand the complex interactions between SDR subsystems in the frequency and time domains, especially in bursted or frequency-hopping signals.

When filtered and amplified, software anomalies can create temporal RF impulse bursts of energy at the RF output. To isolate software and hardware performance, RTSAs can be used to trigger on transients in the frequency domain, capture the events into memory and drive the other test instruments to probe possible error sources. The acquired signals are presented in a time-correlated fashion, helping designers see how anomalies in the digital and analogue blocks of an SDR will propagate to the RF output as impulse noise.

The unique ability of these RTSAs to find problems from spectral transients can be used to trigger the other instruments and obtain time-correlated views of vastly different hardware and software functional implementations. For example, the RTSA can capture the signal in the RF and IF portions of the signal paths, and a logic analyser can capture the digital baseband signal and compare it to the symbol table produced by the RTSA. Furthermore, some RTSAs offer offline software that can be used to analyse acquired data from the logic analyser and oscilloscope, allowing hardware and software measurement correlation.

Verifying baseband IQ waveform quality
Verifying baseband IQ waveform quality is important to both system engineers and field-programmable gate array (FPGA) designers. It helps engineers test the baseband to ensure it is properly functional at an early stage of development because many of the problems involved in digital circuits are in the FPGA design.

The baseband signals in the actual designs and applications are differential (I+, I-, Q+ and Q-) and may possess a DC offset. In the past, very few spectrum analysers were able to test IQ signals directly, and fewer spectrum analysers could test the baseband IQ signals with DC offset. Engineers have been forced to use oscilloscopes with additional software for post analysis.

Select RTSAs enable baseband IQ testing using differential inputs. Doing so delivers measurement consistency when analysing IQ, IF and RF signals. Testing IQ signals with an RTSA also reduces system complexity and simplifies testing procedures, while offering higher dynamic range and greater memory depth than general-purpose instruments.

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