The correlation between a microwave mixer's conversion loss and input signal frequency is a key metric for measuring its frequency conversion efficiency. Its characterization requires analysis from three perspectives: nonlinear conductance characteristics, parasitic parameter effects, and port matching.
At the nonlinear conductance level, a mixer's conversion loss stems essentially from the way the diode's nonlinear characteristics distribute harmonic energy. As the input signal frequency changes, the diode's junction capacitance and series resistance create a dynamic voltage/current divider effect. At high frequencies, the junction capacitance decreases, causing some signal power to be diverted to parasitic paths rather than participating in mixing, thereby increasing net conversion loss. This loss is positively correlated with the square root of frequency; with each doubling of frequency, the loss due to junction capacitance increases significantly. Furthermore, the modulation effect of local oscillator power on nonlinear conductance is frequency-dependent. At low frequencies, losses can be reduced by increasing local oscillator power. However, at high frequencies, due to the diode's cutoff frequency, the room for increasing local oscillator power is limited, making loss control more challenging.
The relationship between parasitic losses and input frequency is even more complex. The energy distribution of parasitic frequency components, such as image frequencies and sum frequencies, generated during the mixing process is directly affected by the input frequency. The image frequencies are separated from the signal frequency by twice the intermediate frequency. When the input frequency approaches the local oscillator frequency, the image frequencies may fall within the signal passband, dissipating energy through the signal source's internal resistance, resulting in irreversible losses. Furthermore, the junction resistance and capacitance of a diode vary with increasing frequency, causing the loss contribution of parasitic parameters (such as voltage divider due to series resistance and current shunting due to junction capacitance) to increase nonlinearly. For example, when the input frequency approaches the diode's cutoff frequency, the junction capacitance equals the series resistance. At this point, half of the high-frequency signal's power falls on the parasitic path, sharply reducing the frequency conversion efficiency.
The frequency sensitivity of the port matching state to the frequency conversion loss is reflected in impedance matching. The input port of a microwave mixer must achieve conjugate matching with the signal source over a wide bandwidth, but in practical designs, the matching network is typically optimized for a specific frequency band. When the input frequency deviates from the designed center frequency, the reflection coefficient increases, causing the signal power to form standing waves at the port, with some energy reflected back to the source rather than entering the mixer. The frequency dependence of mismatch loss can be described by the standing wave ratio (VSWR): VSWR deteriorates as frequency deviates from the center frequency, increasing by approximately a certain amount per octave, directly driving up conversion loss. For example, in the millimeter-wave band, the shortened wavelength increases the relative error between component size and wavelength, significantly increasing matching difficulties. Mismatch loss becomes the dominant factor in high-frequency bands.
From a system perspective, the correlation between conversion loss and input frequency is also affected by mixer type. Single-ended mixers, due to their simple structure, exhibit monotonically increasing loss with increasing frequency. Double-balanced mixers, by suppressing even-order harmonics through a differential structure, exhibit minimal loss fluctuation across a wide bandwidth, but are still limited by the diode cutoff frequency at high frequencies. Passive mixers, lacking a gain mechanism, have losses determined entirely by the aforementioned nonlinearities and parasitic effects. Active mixers, while partially compensating for losses through amplification links, suffer from frequency-dependent noise figure and linearity degradation.
In practical applications, the frequency dependence of conversion loss requires optimization through calibration and compensation techniques. For example, frequency response testing using a vector network analyzer can identify the frequency corresponding to the loss peak, and loss reduction can be achieved by adjusting the matching network or local oscillator power. In the terahertz frequency band, due to material and process limitations, harmonic recovery technology is also required to reflect the image frequency or sum frequency energy back to the diode for secondary mixing to reduce net frequency conversion losses.
