Introduction
Designing and implementing high-performance RF systems is challenging, especially for GPS/GNSS applications. Software-defined radio (SDR) helps address many limitations of traditional analog radio, including flexibility and size, but nonidealities in signal processing can still disrupt GPS/GNSS performance. One fundamental SDR component is the mixer. For RF simulation environments used in GPS/GNSS testing, it is important to address mixer-induced nonidealities. A key step in developing such systems is analyzing the differences between ideal theoretical signals and real-world signals, and understanding the impact of those differences in practice.
Scope and Structure
This article discusses major differences between real-world signals and ideal theoretical signals. Specifically, it introduces artifacts, spurs, IQ imbalance, compression, and intermodulation. It also reviews mixer concepts and types, and explains how mixer behavior relates to the nonidealities above. Finally, the article examines the implications of each nonideality for the design and implementation of GPS/GNSS test simulators. The discussion assumes a basic familiarity with GPS/GNSS simulators and software-defined radio.
Mixer Basics
To understand how real RF environments affect received signals and how to simulate them, start with mixer characteristics and identify the most important nonidealities encountered in use. Mixers perform upconversion and downconversion, shifting signal frequency content up or down the spectrum. This is a fundamental operation in radio transceivers to handle high-frequency communications at the board level. In SDR architectures, three mixer types are commonly implemented: a true RF mixer, a complex IQ mixer, and a CORDIC mixer. A mixer multiplies the input signal (RF or IF, depending on the conversion) with a local oscillator (LO) signal to shift frequency components up or down the spectrum (see Figure 1). In downconversion, RF input combines with the LO to produce an IF output below the original RF. Upconversion uses the LO to move the RF output beyond the original RF input. Sidebands, often called image sidebands, can be created in the process. Since typically only one sideband is desired, the unwanted sideband must be removed through filtering or phase manipulation. For upconversion, filters or single-sideband (SSB) mixers with Hartley modulators may be used; the Hartley modulator uses phase manipulation to cancel one sideband and thus avoids narrowband filtering. For downconversion, image-reject (IR) mixers or standard mixers with appropriate filtering can eliminate the unwanted image.
Figure 1: Upconversion and downconversion.
IQ mixers go further than single mixers by operating simultaneously on in-phase (I) and quadrature (Q) components, allowing independent processing of both components and providing natural sideband suppression, which reduces the need for complex narrowband filters. An IQ mixer consists of two standard mixers and an LO quadrature splitter that produces two LO outputs with 90 degrees phase shift (see Figure 2). During upconversion, each component mixes with either the in-phase LO or the quadrature LO, and the outputs combine at the RF port. For downconversion, both signals are easily retrieved. A CORDIC mixer (Coordinate Rotation Digital Computer) is essentially a digital implementation of an IQ mixer, often used in the SDR FPGA backend where vector-rotation algorithms are implemented in logic. In that case, the LO is implemented as a numerically controlled oscillator (NCO) inside the FPGA for upconversion and downconversion. For upconversion, interpolation raises the effective sampling frequency, which may introduce image components. For downconversion, decimation after mixing reduces the sampling rate to a level more manageable by the host.
Figure 2: IQ mixer architecture.
In high-end SDR design, engineers use advanced circuit techniques to minimize nonidealities. For example, balanced mixing junctions are used to ensure mixer balance, establishing isolation between ports and suppressing intermodulation products. However, mixer artifacts remain. Common artifacts include LO feedthrough, IF feedthrough, and sideband formation. Feedthrough from LO, RF, or IF ports results from limited isolation between ports due to parasitic capacitance, inductance, and power-supply coupling. These nonidealities introduce harmonics at LO, RF, or IF frequencies. While RF or IF leakage can often be filtered out due to spectral separation from desired harmonics, LO feedthrough can significantly impact performance and is more difficult to remove. Image bands or sidebands are inherent to mixing and must be removed with filtering or phase cancellation architectures. Practically, single-sideband upconversion or image-reject downconversion using IQ architectures with phase-canceling schemes help eliminate unwanted bands. In addition to desired frequency products, RF mixers produce spurious products that can sometimes be suppressed by balanced techniques but may appear under certain tuning and LO conditions.
IQ Balance and Its Effects
Balance is critical when dealing with IQ filters. Any defect in LO or I/Q channel phase balance can reduce sideband suppression and channel isolation, and increase LO feedthrough, RF/IF feedthrough, and spurious products. For example, DC offsets between the I and Q waveforms, caused by imbalance in conversion loss between cores, may allow LO leakage into the output spectrum, reducing LO-to-RF isolation and limiting IQ modulation/demodulation performance. Figure 3 shows the impact of a DC offset on a 16-QAM constellation. Phase and amplitude mismatch between IQ channels can cause significant distortions in the mixer output, deforming constellation diagrams and degrading overall performance (see Figure 4). Both mismatches arise from balance issues in the LO splitter and differences in differential connection lengths and losses. These problems are especially pronounced at higher IF frequencies, underlining the need for identical differential paths to achieve proper balance.
Figure 3: Effect of DC offset on a 16-QAM constellation.
Figure 4: Effect of amplitude and phase imbalance.
Compression and Linearity
Ideally, a mixer behaves as a perfect frequency converter, transferring power linearly between input and output with a constant conversion loss. In reality, conversion loss varies with input power, creating a nonlinear input/output relationship called compression. Figure 5 illustrates this: the IF/RF curve starts linear with constant conversion loss and constant "gain", but as input power increases, the slope drops and the curve becomes nonlinear. The most important compression metric is the 1 dB compression point, defined as the input power required for the output to deviate 1 dB from the ideal linear response. The 1 dB compression point is the recommended maximum input level for RF mixers. One way to increase a mixer’s 1 dB compression point is to raise the conduction potential of diodes in the circuit.
Figure 5: 1 dB compression point.
Intermodulation Distortion
The final nonideality discussed here is intermodulation distortion (IMD). IMD arises from mixer nonlinearity when two or more signals simultaneously enter the input and generate frequency combinations between the fundamentals and their harmonics, as shown in Figure 6. These unwanted products are collectively referred to as IMD. While most harmonics decrease in amplitude with frequency and can be ignored at some distance, third-order IMD is particularly important because it falls near the fundamentals. The third-order harmonic’s conversion slope is three times that of the fundamental, exacerbating the effect. Due to compression, third-order products never exceed the fundamentals, but they approach the 1 dB compression point more closely. Extrapolating the linear slopes of the fundamental and third-order curves gives the third-order intercept point (IP3), an important IMD metric.
Figure 6: Intermodulation distortion.
Impact on Simulators
RF simulators play a key role in GPS/GNSS testing by reproducing realistic RF environments. GPS/GNSS testing is critical to verifying system accuracy and reliability. Therefore, simulators must reproduce RF nonidealities introduced by SDR mixers, including IMD, compression, IQ imbalance, and other mixer artifacts. In GNSS scenarios, simulators must also reproduce receiver operating conditions on moving platforms, including vehicle and satellite motion, signal properties, and atmospheric effects.
Mixer artifacts are common issues that can affect GPS/GNSS receivers. When a received signal mixes with other signals, unwanted components are generated, degrading signal quality and system performance. To simulate this effect, test instruments must be able to adaptively change mixer parameters and operating conditions. CORDIC mixers are useful in such scenarios because the digital implementation allows full reconfiguration of mixer behavior and performance.
IQ imbalance affects RF transceivers used in GPS/GNSS systems when I and Q components are not perfectly balanced, causing imperfect representation of received information. This can produce demodulation errors that lead to inaccurate position and timing results. To simulate this effect, SDR-based test simulators must be able to generate imbalanced signals in phase and amplitude and measure the impact on receiver performance. In traditional analog radio systems, changing phase delay and amplitude of each component for realistic simulation is difficult, especially across L1, L2, and L5 frequency bands. Digital systems like SDRs are therefore preferable for flexible and repeatable simulation.
Intermodulation effects are particularly important in GNSS because received signal power levels are very low and high sensitivity and precision are required for position calculations. IMD can cause a variety of errors in position fixes, including incorrect satellite identification, increased noise and interference, and degraded signal quality. To simulate IMD and third-order intercept (TOI) effects in GNSS systems, RF transceivers must reproduce the nonlinear behavior of practical IQ mixers, including compression effects. Simulating compression is important because GNSS signal amplitudes can vary significantly.
Conclusion
Differences between real signals and ideal signals in SDR-based simulations can significantly affect GPS/GNSS tests. While SDR overcomes many limitations of traditional analog radio, mixers remain a source of nonidealities that can interfere with system operation. This article reviewed mixer characteristics and nonidealities including spurs, IQ imbalance, compression, and intermodulation, and described three common mixer implementations in SDRs: RF mixers, complex IQ mixers, and CORDIC mixers. Mixer artifacts such as LO feedthrough, IF feedthrough, and sideband formation were highlighted. To build reliable RF simulation environments for GPS/GNSS testing, these nonideal factors must be considered and accounted for in system design. By analyzing differences between ideal and real signals, engineers can optimize simulation environments to achieve higher accuracy and reliability in testing.
