Key Mixer Performance Parameters
RF Component & Device Test Series: RF Mixer Testing
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RF Mixers are 3-port frequency translation circuits that “mix” the signal inputs from two of its ports and outputs a third signal, along with other products, that is a function of the two inputs. We offer a variety of RF mixer types including 2.92mm mixers, SMA mixers, field replaceable 2.92mm mixers, and field replaceable SMA mixers. These mixer lines include double balanced mixers as well as triple balanced mixers.
Depending on the design and the way a mixer is operated, the resulting output could be a higher frequency or lower frequency than the two inputs. Some mixers are also active devices and require some form of biasing to operate. Moreover, mixers are intrinsically nonlinear devices, and this must be considered when attempting to analyze the performance of a mixer or when characterizing one.
With a passive mixer, a conversion loss is present where some of the signal energy is lost to internal resistances and other losses. With an active mixer, a conversion gain is possible, though this type will likely also have a higher added noise figure.
Key Mixer Performance Parameters
- RF, IF, and LO Port Frequency Operation
- Conversion Gain/Loss
- Isolation
- 1-dB Compression Point
- Output and Input Two-tone 3rd Order Intercept Point Indicating Intermodulation Distortion (OIP3 and IIP3)
- Harmonic Suppression
- Local Oscillator Power Requirements
- Dynamic Range
- Noise Figure
- Input and Output VSWR and Return Loss
Given that mixers are 3-port devices that require two signal inputs to operate, multiple test instruments are needed to evaluate a mixer’s performance. The frequency measurements, such as the operation frequency and conversion gain/loss can generally be measured with a vector network analyzer (VNA) and a calibrated signal generator. For these measurements, the signal generator is acting as the LO and the VNA is providing the RF/IF inputs and measuring the output. As a mixer may produce substantial harmonics below the frequency of interest, a low-pass filter or high-pass filter may be needed at the measurement port to more accurately capture the desired function of the mixer.
In practical use, mixers are often followed by RF filters to attenuate the undesirable low and high frequency products they produce. Attenuators may also be needed at the VNA ports, as active mixers may have high conversion gain that amplifies the signal beyond safe limits for the VNA port, or the LO drive level may need to be high enough to result in excessive powered signals at VNA ports. A VNA may also require a scalar mixer calibration as the two VNA ports connected to the RF and IF of the mixer will be operating at different frequencies. A vector mixer calibration can also be performed, though more complicated than a scalar mixer calibration, allows for measurements of a mixer’s phase and group delay performance.
Isolation measurements generally require a VNA measuring two ports of a mixer while the third port is terminated in a load (with the LO remaining unterminated). In this way each of the 6 isolation measurements can be made, though the reciprocal nature of these measurements mean that usually only LO-RF, LO-IF< and RF-IF isolation measurements are made.
In order to measure the nonlinear aspects of a mixer’s behavior, such as OIP3, IIP3, and harmonic suppression (spurious measurements), a signal generator and spectrum analyzer may be used. With this test, a signal generator with multiple ports, or multiple signal generators outputting the LO and either RF/IF are connected to the mixer inputs and the spectrum analyzer is connected to the mixer’s output (either RF or IF depending on if the mixer is designed for upconversion or downconversion). The spectrum analyzer is used to sweep and measure the power and frequency content of the harmonics, spurs, and mixer products. A spectrum analyzer, signal generator, and calibrated noise generator may also be used to analyze the noise figure of a mixer in a similar way, as injected noise power can be captured and measured with a spectrum analyzer.
Given the power levels involved, signal generators and a power meter may be needed to measure the 1dB compression point. A directional coupler may also be needed to capture the return loss and VSWR, especially when high power levels are present that would otherwise damage sensitive test equipment.
Learn more about Fairview’s line of test accessories and cables:
- Fairview VNA Calibration Kits & Components
- Fairview Phase Stable (VNA) Coaxial Cables
- Fairview Precision Coaxial Adapters
- Fairview RF Couplers
- Fairview RF Terminations
- Fairview RF Attenuators