LTCC Arms Mixer For 7.3-To-20.0-GHz Systems
Frequency conversion is one of the more critical functions within the RF/microwave portion of a highfrequency system. In spite of continuing advances in the speed and bit resolution of analog-to-digital converters (ADCs), they rely on a frequency mixer to translate high-frequency signals within their bandwidth range. The model SIM-24MH+ frequency mixer from Mini-Circuits (Brooklyn, NY) is an example of such a mixer for extremely broadband commercial and military applications (Fig. 1). It combines the performance advantages of low-temperature-cofired-ceramic (LTCC) technology and advanced semiconductor technology with a highly manufacturable circuit layout for outstanding electrical performance as a frequency upconverter or downconverter from 7.3 to 20.0 GHz.
The patented combination of technologies1,2 results in small size, high insensitivity to electrostatic discharge (ESD), excellent stability with temperature, and wide bandwidth. The versatile mixer is well suited as the first downconversion stage in extremely broadband receiver systems, but can also be used as an upconversion mixer.
The SIM-24MH+ mixer (Fig. 2) features RF and local-oscillator (LO) coverage of 7.3 to 20.0 GHz. It is designed for LO signals at a nominal level of +13 dBm (see table) and yields intermediate- frequency (IF) signals from DC to 7.5 GHz. The SIM-24MH mixer's bandwidth is so wide that it can effectively replace two or three commercial mixers in broadband applications (Fig. 3). It performs the downconversion function with typical conversion loss of 6.5 dB from 7.3 to 16 GHz, and typically 8.0 dB from 16 to 20 GHz (Fig 4).
The mixer's conversion loss is very well behaved with frequency even at other LO drive levels. For example, the SIM-24MH+ was tested with LO drive levels of +10, +13, and +16 dBm and it exhibited consistent conversion-loss profiles across the full RF range of 7.3 to 20 GHz (Fig 4). Performing swept-frequency testing at different LO drive levels simulates the effects of variation in available LO drive power from unit to unit as well as across the wide frequency range. The variation of conversion loss with LO drive power is typically +1.8/-0.2 dB across the full 12.7-GHz-wide frequency band.
The SIM-24MH+ mixer (Fig. 1) is built on LTCC substrate material, resulting in a compact component with outstanding temperature stability. In contrast to conventional planar circuit designs, in which all circuit elements are placed on one side of a single-layer printed circuit board (PCB), LTCC circuits can be designed and fabricated in three dimensions, even with embedded components between layers, to save space. With LTCC technology, the SIM-24MH+ measures just 0.2 x 0.18 x 0.087 in. (5.1 x 4.6 x 2.2 mm), smaller than some commercial semiconductor-based mixers. And, although the SIM-24MH+ mixer incorporates semiconductors (diodes) to accomplish its nonlinear frequencytranslation function, it is a passive design that does not require DC bias (compared to a standard semiconductor or integrated-circuit mixer that requires the application of constant DC bias).
Mini-Circuits has achieved high performance and good manufacturing yields with its LTCC products through the development of three-dimensional computer-aided-engineering (CAE) models using advanced electromagnetic (EM) simulation tools. LTCC processing involves the fabrication of circuits on ceramic tapes, which are then laminated together and co-fired in a kiln to form a compact, three-dimensional structure. The LTCC technology also allows passive components to be embedded within the layers for small size. The LTCC technology makes possible multilayer, three-dimensional designs that are much more compact than components based on planar circuit approaches.
The SIM-24MH+ is built around a reliable diode quad custom developed for the mixer. Except for the diodes, the entire structure is implemented in multiple layers of LTCC, which is inherently hermetic. The high level of integration made possible by LTCC minimizes the mixer's mass, making it extremely rugged. The mixer can withstand environmental extremes usually associated with military-grade components in regards to temperature, humidity, vibration, and mechanical shock.
The SIM-24MH+ mixer is RoHS compliant, constructed without leadbased solder or other hazardous materials. It is also built to withstand severe ESD scenarios under conditions normally hazardous to monolithic semiconductor mixers. Like other member's of the company's SIM mixer line, the SIM-24MH+ meets Class 1C ESD requirements, a level of 1000 V when tested under the Human Body Model (HBM) conditions. (This can be compared to standard semiconductor mixers that are typically rated as Class 1A, 250 V for HBM testing). The SIM-24MH+ mixer also meets Class M2 ESD requirements (testing at 100 V according to the ESD Machine Model).
The integrity of the mixer's circuit design is most evident in its high port-to-port isolation. High isolation is instrumental in achieving good performance in dual-mixer in-phase/ quadrature (I/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-24MH+ mixer was evaluated at the three LO drive levels used for the conversion-loss tests, to understand the effect of variations in LO power on isolation. As Fig. 5 shows, the LO-to-RF isolation is high (typically 36 dB from 7.3 to 15.0 GHz) and very well behaved at all three LO drive levels. Similarly, the LO-to-IF port isolation was evaluated at the three LO drive levels. The SIM-24MH+ mixer exhibited typically 20 dB isolation across the LO frequency range of 7.3 to 20 GHz (Fig. 6).
Since wide dynamic range is important in many applications, the input third-order intercept point (IP3) of the SIM- 24MH+ mixer was also evaluated at the three LO drive levels (+10, +13, and +16 dBm) from 7.3 to 20 GHz. The mixer's input IP3 is consistently above +14 dBm across most of the range (Fig. 7) and is +25 dBm from 17 to 20 GHz. The LTCC doublebalanced mixer features typical LO-port VSWR of 2.5:1. The VSWR (return loss) measured at the RF port is typically 3.0:1 while the VSWR measured at the IF port is typically 2.0:1.
For millimeter-wave applications, the SIM-24MH+ mixer provides an extremely compact, high-performance solution based on a stable LTCC process. It supports conventional surface-mount applications, and can be supplied in tapeand- reel format for use with automated assembly equipment. This RoHS-compliant mixer fills a wide range of applications and is designed to withstand high levels of ESD mishandling compared to more sensitive, and often larger, semiconductor mixers. With the addition of SIM-24MH+, the SIM family of mixers3 cover 0.75 to 20 GHz (Fig. 8) frequency range. All have same size and foot print making it easy for customers to change frequency without changing the PCB layout.
Even though the SIM-24MH+ mixer is specified as a downconverter, it can also be used in upconversion applications for added versatility. In addition, for coaxial system applications, a connectorized version will also be available (consult factory for availability).4 The connectorized version simplifies quick prototype and proof-of-concept projects as well as evaluation in test laboratories.
Mini-Circuits, P. O. Box 350166, Brooklyn, NY 11235-0003; (718) 934-4500, FAX: (718) 332-4661, Internet: www.minicircuits.com.
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, web site, http://www.minicircuits.com/cgi-bin/inquery.cgi?querystring=SIM&searchtype=modelfamily.
4. Mini-Circuits, web site, http://www.minicircuits.com/.