WITH THE INCREASING DEMANDS in the microwave and very-small-aperture-terminal (VSAT) radio market, the frequency band of operation has risen while modulation schemes have grown more complex. To design a millimeter-wave radio that can provide the required performance at lower cost than its predecessors, manufacturers have replaced hybrid-mixer technology with solutions based on monolithic-microwave integrated circuits (MMICs). In a seven-page application note titled, "Subharmonic Vs. Fundamental Mixers for High-Capacity Millimeterwave Radios," Hittite Microwave Corp. compares the performance of a double-balanced mixer and a subharmonic mixer in the 27-GHz millimeter-wave radio band.
A subharmonic mixer operates with a localoscillator (LO) frequency at half the RF frequency. In doing so, it eliminates the need for a more complex and costly high-frequency LO. In addition, the subharmonic mixer rejects even-order spurious emissions. A carefully designed mixer can achieve 2LO-RF isolation to 35 dB.
In a subharmonic mixer, it is essential that the diodes are anti-parallel. When an LO signal with sufficient power is incident on the diodes, the diodes will switch on and off in a complementary fashion, thereby creating a waveform that is rich in harmonics. A subharmonic mixer will cancel all harmonics with an even order, such as the RF LO (1 + 1 = 2) while passing the odd-order harmonics, RF 2LO (1 + 2 = 3). Although the fundamental LO will be present at the mixer's RF and intermediate-frequency (IF) ports, it will be significantly lower in frequency.
A double-balanced mixer operates much the same. Yet the currents add differently because of the diodes' ring configuration. Because the sum-and-difference product RF 2LO is naturally suppressed, the double-balanced mixer is unworkable as a subharmonic mixer. Although it rejects more spurious products, the doublebalanced mixer requires a more complex and costly frequency-generation unit. Mixer performance was simulated using a traditional spurious calculator and SPECTRASYS. The subharmonic mixer's spurious response resulted in an input frequency of 3 GHz at 0 dBm and a LO frequency of 12.75 GHz at 4 dBm. For the double-balanced mixer, the input frequency was the same. But the LO was at 25.5 GHz with an input power of +13 dBm. The simulations found that the subharmonic mixer had a spurious product at 3LO2-2IF2. In contrast, the double-balanced mixer had a spurious product at (2IF2 + 2LO2) (LO2 + IF2).
Hittite Microwave Corp., 20 Alpha Rd., Chelmsford, MA 01824; (978) 250-3343, FAX: (978) 250-3373, Internet: www.hittite.com.
