New Mixer from Analog Devices Targets High-Frequency Applications
The HMC553ACHIPS is a passive double-balanced mixer that supports upconversion and downconversion with RF frequencies from 6–14 GHz.
Leakage in RF Circuits
One of the fundamental difficulties of RF design is the tendency for high-frequency signals to break free of typical conductive pathways and end up in places where they don’t belong. This phenomenon is referred to as leakage.
At low frequencies, signals are largely restricted to the metallic connections leading from one component to another. These signals can’t travel through insulating materials, and they don’t generate significant amounts of electromagnetic interference.
High-frequency signals, on the other hand, are transferred not only by metallic connections but also via capacitive, radiated, and inductive coupling. Parasitic capacitance is everywhere, and since capacitance presents less resistance to current flow as frequency increases, the transmission paths created by parasitic capacitance become more problematic in RF systems.
Parasitic capacitance is particularly significant in PCB environments, where conductive features are often separated by relatively thin sections of insulating material.
Local Oscillator Leakage
One specific type of leakage that has caused headaches for RF engineers is called local oscillator leakage. The local oscillator, often abbreviated LO, is one of the signals that is applied as an input to an RF mixer, an active component in RF circuits. One of the challenges of working with mixers is preventing problems associated with the LO’s tendency to find coupling paths.
The following diagram is an example of LO leakage occurring in the context of downconversion. The LO signal is intentionally delivered to the LO port, but it also couples unintentionally into the other input port.
LO leakage occurring in downconversion.
Mixing two waveforms that have the same phase and frequency produces an undesirable DC offset in the downconverted signal.
LO leakage is also a concern when mixers are used for upconversion. If the LO signal leaks into the output signal, the RF spectrum will contain not only the energy from the upconverted signal but also some energy at the LO frequency:
RF spectrum with energy from the upconverted signal and leaked LO.
If the transmitted signal needs to contain both of the output sidebands, the leaked LO energy cannot be removed via filtering, and specialized cancellation techniques must be used in cases where the LO-leakage amplitude is high enough to interfere with system operation or violate emission requirements.
General-Purpose, Wide-Frequency-Range Mixing
The block diagram for the HMC553ACHIPS is pleasantly straightforward.
Diagram of the HMC553ACHIPS. Image used courtesy of Analog Devices
This chip functions as a downconverter or an upconverter. For downconversion, it accepts an RF input from 6–14 GHz and shifts the signal down to produce an IF output that can be anywhere between DC and 5 GHz. For upconversion, it shifts an IF input (again, DC to 5 GHz) to higher frequencies, producing an RF output in the same 6–14 GHz range.
The datasheet actually contains the exact phrase “does not require any external components.” That’s one of the best things that I can find in a part description, especially when I’m working in the intimidating world of RF.
However, I’ll have to make sure that all my RF designs have IF signals that extend to DC, because in systems that don’t require IF operation to DC, one external capacitor in series with the IF port is “recommended.”
Reducing LO Leakage
The HMC553ACHIPS datasheet emphasizes the part’s high leakage suppression, which it ascribes to “optimized balun structures.” The typical LO-to-RF isolation is 37 dB, with a minimum value of 32 dB. The typical LO-to-IF isolation is 33 dB when operating from 6 GHz to 11 GHz and 35 dB when operating from 11 GHz to 14 GHz.
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Have you ever worked on an RF system that had problems related to LO leakage? Feel free to leave a comment if you want to share any experiences or advice.