Ku- and Ka-Band Subharmonic Mixers Improve System Performance
Since enabling the superheterodyne receiver some 100 years ago, mixers have been a constant in RF/microwave systems, while the circuit topologies and technologies used to build them have been constantly optimized to improve performance, shrink size and reduce cost. The most common topology has been the fundamental mixer, where the mixing generates IF frequencies at the sum and difference of the RF and local oscillator (LO) frequencies, i.e., fLO + fRF and fLO − fRF if fLO > fRF. The LO signal is typically much higher in power than that of the RF signal since the LO is used to turn on the diodes and drive the mixing process. In a practical mixer, unwanted signals such as LO and RF leakage and spurs will appear at the IF port. One of the challenges faced by system designers is minimizing these unwanted signals to achieve the spurious-free dynamic range required by the system.
A fundamental mixer is an efficient solution at lower frequencies, where it is relatively easy to generate a low phase noise LO signal. At higher frequencies - say above 15 GHz - generating such a signal can be difficult and expensive, which makes fundamental mixers a non-optimal choice in many applications. One alternative is to generate the LO at half the required frequency and use a x2 multiplier to double the frequency. A passive multiplier topology is one approach, although it can be quite lossy and require a buffer amplifier at the output to increase the LO power. Alternatively, an active multiplier topology will provide higher output power, although it requires DC bias and may still require an output buffer amplifier. Whether passive or active, a discrete multiplier topology requires additional components between the LO source and the mixer, increasing circuit complexity and consuming additional board area.
Custom MMIC has developed an elegant solution to this problem by releasing three GaAs MMIC subharmonic mixers, the first in a new family, which can be used as down-converters in a receiver or up-converters in a transmit chain. In Custom MMIC’s designs, the LO is input at half the desired LO frequency and an anti-parallel diode pair creates the desired effect of mixing at twice the provided LO frequency (see Figure 1). The integrated buffer amplifier also reduces the required input LO power considerably below that needed to drive a passive fundamental mixer. Another benefit of a subharmonic mixer is high rejection of the even LO harmonics at the RF and IF output ports (i.e., 2fLO, 4fLO, etc.), which reduces the required LO filtering.
Fabricated with GaAs MMIC technology, Custom MMIC’s subharmonic mixers are a new MMIC category for the company. Two of the three mixers are available as die, with RF frequency ranges covering 13 to 21 GHz (CMD303) and 20 to 32 GHz (CMD310); the latter design is also available in a ceramic 3 mm x 3 mm QFN package (CMD310C3). The performance of the three products is summarized in Table 1. All operate from a single supply voltage, nominally +3 to +4 V, and typically require approximately 30 mA.
Figure 2 shows the conversion loss performance of the CMD303 used as an up-converter. Over the specified RF frequency range of 13 to 21 GHz, the conversion loss is approximately 10 dB with LO drive at or above the rated minimum of 0 dBm. Similar performance is achieved with the higher frequency CMD310 used as an up-converter (see Figure 3). Across the specified RF frequency range of 20 to 32 GHz, the conversion loss is approximately 10 dB with the LO drive at or above −4 dBm.
These highly integrated GaAs subharmonic mixers only require external bypass capacitors to achieve the performance shown on the datasheet, making them easy to design in and minimizing circuit area. The die versions are rated for operation from −55°C to +85°C, while the packaged version is rated from −40°C to +85°C. Mixer performance is stable with temperature variation, with conversion loss changing less than 1 dB over the full operating temperature range.
In addition to these subharmonic mixers, Custom MMIC offers products for the entire RF signal chain - from the CMD283C3 low noise amplifier with 0.6 dB noise figure to the CMD304 DC to 67 GHz distributed amplifier. Constantly developing products to meet tough technical requirements, Custom MMIC introduced 39 high performance GaAs and GaN MMICs in 2019, a record for the company.
Read more about Custom MMIC’s history and capabilities in the Fabs and Labs profile published in the June 2019 issue of Microwave Journal.