Wireless applications for many conjure up images of cellular networks in bands around 800 to 900 MHz and 1800 to 2000 MHz. But a growing number of wireless applications are making use of unlicensed spectrum in the Industrial-Scientific-Medical (ISM) bands around 2400 to 2500 MHz and 5725 to 5875 MHz. To facilitate designs at these higher ISM frequencies, Mini-Circuits (www.minicircuits.com) has developed a highperformance passive mixer that allows original-equipment manufacturers (OEMs) currently working in the 2450-MHz band to upconvert those frequencies to the 5.7-to-5.8-GHz band. The company's model SIM-U63+ mixer is based on a combination of low-temperature-cofired-ceramic (LTCC) technology, semiconductor technology, and a highly manufacturable circuit layout. The patented combination1 results in small size, high insensitivity to electrostatic discharge (ESD), and excellent stability with temperature.

ISM-band frequencies are often referred to as "unregulated frequencies" since they are available for public of private use without any government franchises or regulation. Although unlicensed, products manufactured for ISMband use must meet type-acceptance for power limits and frequency tolerances according to the applicable regulating body, such as the Federal Communications Commission (FCC) in the United States. For that reason, frequencydetermining components, such as mixers and oscillators, must provide performance that is stable over time and under different environmental conditions, including temperature.

The SIM-U63+ mixer (Fig. 1) is built on LTCC substrates, ideally suited for designs with multilayer circuits. In contrast to conventional planar circuit designs, in which all circuit elements are placed on one side of a single-layer printed circuit board, LTCC circuits can be designed and fabricated in three dimensions, even with embedded components between layers to save space. The approach results in a mixer that measures just 0.2 x 0.18 x 0.08 in. (5.1 x 4.6 x 2.1 mm), which is smaller than some commercial semiconductorbased mixers. And although the SIM-U63+ mixer incorporates semiconductors to accomplish its nonlinear frequency-translation function, it is a passive design that operates without DC bias (compared to a standard semiconductor or integrated-circuit mixer which requires the application of constant DC bias).

The SIM-U63+ is a double-balanced mixer (Fig. 2) built around a reliable diode quad. Except for the diodes, the entire structure is implemented in multiple layers of LTCC which is inherently hermetic. By integrating components in LTCC, the mass of the mixer is minimizing, making it extremely rugged in terms of withstanding shock and vibration. In fact, the entire mixer structure can withstand the environmental extremes usually associated with tough military components, in terms of temperature, humidity, vibration, and mechanical shock.

The mixer is RoHS compliant, constructed without lead-based solder or other hazardous materials. It is also built to withstand severe ESD scenarios under conditions normally hazardous to monolithic semiconductor mixers. The SIMU63+, like other members of the company's SIM mixer line, meets Class 1C ESD requirements, a level of 1000 V when tested under the Human Body Model (HBM) conditions (compared to standard semiconductor mixers which are typically rated for Class 1A of 250 V for HBM testing). The SIM-U63+ mixer also meets Class M2 ESD requirements (testing at 100 V according to the ESD Machine Model).

EVALUATING PERFORMANCE
The SIM-U63+ mixer accepts intermediatefrequency (IF) signals from 2400 to 2500 MHz and local-oscillator (LO) signals from 3100 to 3600 MHz and a nominal level of +7 dBm to produce RF output signals from 5500 to 6000 MHz (well in excess of the ISM-band range of 5725 to 5875 MHz). It performs the frequency upconversion with typical conversion loss of 6.8 dB (see table). The mixer's conversion loss is very well behaved with frequency even at other LO drive levels. For example, the SIM-U63+ was tested with LO drive levels of +4, +7, and +10 dBm and exhibited very consistent conversion-loss profiles across the 5500-to-6000-MHz RF output range (Fig. 3). Performing swept-frequency testing at different LO drive levels simulates the effects of variations in LO power from unit to unit as well as across wide frequency ranges. The variation with LO drive power is typically +0.7/0.3 dB across the 500-MHz measured bandwidth.

The integrity of mixer's circuit design is most evident in the port-to-port isolation. High isolation is instrumental in achieving good performance in dualmixer in-phase (I) and quadrature (Q) modulators. Also, a mixer with high isolation requires less additional external filtering to reduce unwanted signal content, such as LO feedthrough. The LO-to-RF isolation of the SIM-U63+ mixer was also evaluated at the three LO drive levels of the conversion-loss tests, to compare performance at different LO levels but also to understand the effects of variations in LO power on isolation. As Fig. 4 shows, the LO-to-RF isolation is high (typically 40 dB) and very well behaved at all three LO drive levels. Variations in isolation as a function of LO power are typically 1 dB.

Similarly, the LO-to-IF port isolation was also evaluated at the three LO drive levels of the conversion-loss tests. While isolation between these two ports is less critical in an upconverter applications (where a greater concern is feedthrough of LO signals to the RF output port), the SIM-U63+ mixer nonetheless exhibited typically 16 dB across an LO frequency range of 3100 to 3600 MHz (Fig. 5).

Since wide dynamic range is important in many ISM-band applications, the input thirdorder intercept point (IP3) of the SIM-U63+ mixer was also evaluated at the three LO drive levels (+4, +7, and +10 dBm) and an RF output range of 5500 to 6000 MHz. With the exception of some peaking at an RF output of 5950 MHz for the lowest LO drive level, the mixer's input IP3 is consistently about +11 dBm across most of the range of coverage (Fig. 6).

The LTCC double-balanced mixer features typical LO port return loss of 8 dB. The return loss measured at the RF port varies from 12 to 25 dB for RF output frequencies from 5500 to 6000 MHz while the return loss at the IF port was typically within 6 to 8 dB across the full IF band.

For applications in which ISM coverage is being extended from the 2450-MHz range into the 5-GHz band, the SIMU63+ double-balanced mixer provides an extremely compact, high-performance solution based on a stable LTCC process. The mixer supports conventional surface-mount applications, and can be supplied in tape-and-reel formats for use with automated assembly equipment. The RoHS-compliant mixer fills a wide range of global ISM applications and is designed to withstand high levels of ESD mishandling compared to more sensitive, and often larger, semiconductor mixers.

REFERENCES

  1. United States Patent No. 7,027,795 (2006).
  2. Engineering Staff, Mini-Circuits, "LTCC Launches Miniature, Wideband, Low-Cost Mixers," Microwaves & RF, June 2006, pp. 107-110.
  3. Mini-Circuits website, http://www.minicircuits.com/cgi-bin/inquery.cgi?querystring=SIM&searchtype=modelfamily.