Upconverters for ISM applications leverage the advantages of LTCC technology

Most wireless applications conjure up the 800 to 900 MHz and 1800 to 2000 MHz cellular bands, but an increasing number of wireless applications are beginning to use the unlicensed Industrial-Scientific-Medical (ISM) bands, which range from 2400 to 2500 MHz and 5725 to 5875 MHz. To advance design technology in these higher ISM bands, Mini-Circuits has developed a high-performance active mixer that allows original equipment manufacturers (OEMs) to up-convert current 2450 MHz operating frequencies to 5.7 to 5.8 GHz. The company's SIM-U63+ mixer is based on a combination of low-temperature co-fired ceramic ( LTCC ) technology, semiconductor technology and a highly manufacturable circuit design technique. This patented combination of technologies results in a small device size, high insensitivity to electrostatic discharge (ESD) and excellent temperature stability.

ISM band frequencies are often referred to as "unregulated frequencies" because they are available to the public for independent use without the need for government authorization or regulations. Although unauthorized, products manufactured for ISM band use must meet power limits and frequency tolerances as set by applicable rulemaking agencies, such as the Federal Communications Commission (FCC). Therefore, frequency-critical components, such as mixers and oscillators, must have performance stability over time and under different environmental conditions, including temperature.

The SIM-U63+ mixer (Figure 1) uses LTCC as a substrate, which is well suited for multilayer circuit design. Compared with traditional planar circuit designs, LTCC circuits can be designed and manufactured in three dimensions, and even components can be embedded between layers to save space, while all components are placed on one side of a single-layer printed circuit board. The above approach enables the mixer to measure only 0.2×0.18×0.08 inches (5.1×4.6×2.1 mm), which is smaller than some mixers based on commercial semiconductor technology. In addition, although the SIM-U63+ mixer incorporates semiconductor technology to obtain its nonlinear frequency conversion function, the active design allows it to operate without DC bias (compared to standard semiconductor or integrated circuit mixers that require fixed DC bias for application).

The SIM-U63+ is a double-balanced mixer (Figure 2), and except for the diodes, the entire structure is realized on a naturally sealed LTCC multilayer board. By integrating the components on the LTCC, the mixer volume is minimized, which makes it extremely robust in terms of shock and vibration. In fact, often combined with military-grade components with excellent environmental performance, the entire mixer structure can withstand extreme environmental conditions in terms of temperature, humidity, vibration and mechanical shock.

The mixer is RoHS-compliant and manufactured without the use of graphite flux or other hazardous materials. It also withstands the severe ESD environments that are typically harmful to monolithic integrated circuit semiconductor mixers. Similar to the company's other SIM mixer product lines, the SIMU63+ meets Class 1C ESD requirements, tested to 1000V levels under human body model conditions (by comparison, standard semiconductor mixers typically only meet Class 1A 250V HBM testing). The SIM-U63+ mixer also meets Class M2 ESD requirements (tested to 100V per the ESD design model)

Performance Evaluation

The SIM-U63+ mixer receives an intermediate frequency (IF) signal between 2400 and 2500 MHz and a local oscillator (LO) signal frequency between 3100 and 3600 MHz with a nominal amplitude of +7 dBm, and generates an RF output signal between 5500 and 6000 MHz (enough to extend beyond the ISM band from 5725 to 5875 MHz) with a typical conversion loss of 6.8 dB for upconversion. The mixer conversion loss is consistent even at other LO drive levels. For example, the SIM-U63+ exhibits very consistent conversion loss curves across the entire 5500 to 6000-MHz RF output range when tested at LO drive levels of +4, +7, and +10 dBm (Figure 3). Frequency sweeps at different LO drive levels simulate the effects of LO power variations and crossing a wide frequency range. Typical LO power variations across the 500-MHz measurement bandwidth are +0.7/-0.3 dB.