By combining sophisticated circuit techniques with advanced LTCC material technology, the SIM line of extremely compact, extremely broadband mixers is born.
Low temperature co-fired ceramic (LTCC) circuit materials lend themselves to high-performance components in extremely small footprints. They also are the foundation for a new series of broadband, low-cost mixers from a company synonymous with mixers, Mini-Circuits (Brooklyn, NY). The new mixer line employs patented techniques using LTCC and advanced semiconductor technologies to cover bandwidths from 750 MHz to 15 GHz. 1The versatile mixers can be used as upconverters and downconverters; their wide frequency ranges makes them suitable for a host of applications.
LTCC technology is based on the use of ceramic substrates to form multilayer circuits. Since circuitry can be designed in a three-dimensional, rather than planar, configuration, with embedded passive elements such as resistors and capacitors, LTCC components can be produced in a fraction of the size of conventional lumped-element or microstrip components.
The new model SM-153+ LTCC double-balanced mixer (Fig. 1, left) , for example, measures just 0.2 0.18 0.08 in. (5.1 4.6 2.1 mm), considerably smaller than commercial FET-and diode-based mixers. Model SIM-153+, unlike some semiconductor mixes, is passive and requires no bias energy and is insensitive to electrostatic discharge (ESD). It operates with RF signals from 3.4 to 15.0 GHz and has an intermediate-frequency (IF) range of DC to 4.5 GHz. Since it can be used as both and upconverter and a downconverter, it can be used in a wide range of commercial and military applications. It is also RoHS compliant for use in systems requiring lead-free components.
The entire structure of the mixer (Fig. 1, right), except for the diodes, is implemented in multiple layers of LTCC circuitry. With strong adhesion between its multiple layers, LTCC is inherently rugged and hermetic. The mixer structure is built to withstand the demanding conditions required of commercial and some military applications for temperature, humidity, vibration, and mechanical shock.
In contrast to existing mixers, the new SIM mixers occupy very little PCB real estate and are easier to mount and connect. For example, drop-in and opencarrier mixers require cut-outs in the PCB for mounting. The open-carrier mixer package must be secured with screws, a labor-intensive process. The new SIM mixers are considerably smaller than either of these mixer options. In fact, the SIM mixers are about one-half the size of the MCA1 series2 of mixers from Mini-Circuits, which have been successfully used in many systems. With the advent of monolithic (semiconductor) integrated-circuit (IC) mixers, components became available in ceramic and plastic packages, such as the MSOP-8 housing. The SIM mixers are still smaller than ceramic-packaged IC mixers and only slightly larger than their plastic-packaged counterparts.
The performance of the SIM-153+ mixer model is summarized in the table. It is designed for nominal local-oscillator (LO) power of +7 dBm, but is versatile enough to deliver fine performance over a wide range of LO levels. The mixer was tested at three different LO levels in order to simulate the effects of LO variations on mixer performance. The conversion loss at nominal LO power (+7 dBm) and above is less than 10.5 dB to an IF of 4.5 GHz. While some slight variations in conversion-loss performance occur at different LO levels when operating with a fixed LO frequency of 8 GHz (Fig. 2, left) , the conversion-loss performance is remarkably well behaved when tested with an RF input frequency of 15 GHz for LO levels of +4, +7, and +10 dBm (Fig. 2, right) . Even when tested over wide temperature variations (from -55 to +100C), the mixer's conversion-loss performance referenced to room temperature (+25C) remains for the most part within a tight 0.5-dB window (Fig. 3) . It should be noted that the mixer was put to the test over this broad temperature range to reveal any performance flaws; it is actually specified over a "narrower" temperature range of -40 to +85C.
When tested at various LO levels and a fixed IF, the SIM-153+ mixer maintained low conversion loss of typically 6 dB through 9 GHz and typically 8 dB through 15 GHz. Because the IF bandwidth of these mixers is so wide (DC to 4.5 GHz), they are useful in systems requiring multiple frequency conversions (such as a multiple-downconversion receiver or multiple-upconversion transmitter), with a higher IF used for the first conversion stage. The mixer provides enough flexibility that it can handle downconversion of signals as high as 15 GHz to standard IFs, such as 70 MHz, or upconversion of that same IF to microwave frequencies.
Port-to-port isolation is one of the key performance parameters for an RF/microwave mixer, with high isolation showing good circuit design. When the SIM-153+ mixer was evaluated for LO-to-RF port isolation at three different LO levels, it exhibited excellent performance of typically 40 dB from 3.4 to 5.0 GHz and 28 to 43 dB over the rest of the frequency range. For specific application frequencies, such as the classic 3.7-to-4.2-GHz satellitecommunications (satcom) band, the SIM-153+ provides typical isolation of 40 dB. Such high isolation reduces the amount of additional filtering that must be provided with the mixer. It is also invaluable when two mixers are used to realize a single-sideband (SSB) mixer or in-phase (I) and quadrature (Q) modulator/demodulator. In applications like these requiring closely matched mixers, the high isolation translates into excellent carrier rejection.
In addition to excellent unit-to-unit performance repeatability, the LTCC construction also yields minimal performance-variations as a function of frequency. When the LO-to-RF isolation was evaluated at three temperatures ranging from -55 to 100C, the isolation was found to remain within a 2 dB window across the full frequency range when the LO frequency was swept from 3.5 to 15.0 GHz (Fig. 4).
Isolation from the LO port to the IF port is essential in preventing LO frequencies from leaking or mixing with IF signals. Again, poor isolation can increase the requirements (and the cost) for additional filtering. When evaluated with LO levels of +4, +7, and +10 dBm from 3.5 to 15.0 GHz, the SIM-153+ mixer dropped to a low point at about 8 GHz (about 13 dB) but maintained LO-to-IF isolation of well above 19 dB for all three LO power levels and for frequencies beyond 9 GHz (Fig. 5, left). When the LO-to-IF isolation was tested at three different operating temperatures (Fig. 5, right), the isolation was within 1 dB over the entire temperature range.
The linearity of the SIM-153+ mixer, in terms of third-order intercept (IP3) performance, was evaluated at three different LO levels and found to vary typically from +6 to +18 dBm from 4 to 9 GHz (Fig. 6). To verify the consistency of the LTCC processed used to fabricate the mixers, five different units were measured for IP3 performance across the same frequency range and found to vary by less than 1 dB. Since the peaks and valleys in the IP3 performance are repeatable from unit to unit, the mixer can be counted upon to deliver high IP3 performance of +18 dBm at 4.8 GHz and +15 dBm at 9 GHz. Widerbandwidth measurements of IP3 performance are available upon request.
The SIM-153+ and other members of the SIM mixer family are built to withstand severe electrostatic-discharge (ESD) environments even under conditions that would cause damage to semiconductor IC mixers. SIM mixers are fairly insensitive and can be used with normal ESD precautions. For example, the SIM mixers pass 1000-V ESD for Human Body Model (HBM) or Class 1C testing. In contrast, monolithic semiconductor mixers are typically only Class 1A (250-V capability) for HBM testing. The SIM mixers also pass 100 V ESD for Machine Model (Class M2) testing. Published data are not available for semiconductor mixers regarding ESD Machine Model sensitivity levels.
The SIM mixers are priced at about one-half to one-third the price of semiconductor mixers, even though they provide much wider bandwidths. The wide bandwidth of the SIM-153+ (3.5 to 15.0 GHz) is in sharp contrast to the much more narrow bandwidth of semiconductor mixers which tend to provide an octave or less bandwidth starting at about 4 GHz. The wide bandwidth of the SIM-153+ mixer makes it of great interest for instrumentation and military applications. In addition, a single mixer model such as the SIM-153+ can be used in multiple commercial applications as a way of simplifying inventory and product bill of materials (BOMs). In comparison to most semiconductor mixers that require LO drive of about +13 dBm, the nominal LO drive level of the SIM-153+ mixer is only +7 dBm. For applications not requiring high IP3, this lower LO power level can translate into considerable power savings at the system level.
Mini-Circuits has developed extensive experience in the design of LTCC components (see sidebar). The firm's LTCC mixers have been tested extensively and qualified for environmental conditions such as humidity, shock, and vibration.3
To evaluate the durability and reliability of their solder joints, 60 of the LTCC mixers were soldered onto FR-4 PCB motherboards and thermally cycled over a temperature range of -55 to 125C (MIL-STD-202, Method 107G, test condition B3, except -55C instead of -65C). The DC continuity was measured from the motherboard trace to the top of the LTCC board for 100 cycles, with no failures found. It should be noted that the temperature range used for thermal cycling is much higher than the mixer's operating temperature range and hence represents an accelerated operating-life condition.
The LTCC-based SIM mixers are reliable and broadband, approximately the size of more-expensive semiconductor mixers but with several advantages. For applications requiring low conversion loss and high isolation with trouble-free device handling through 15 GHz, the SIM mixers offer a practical solution. 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. Mini-Circuits Engineering Staff, "Double-Balanced Mixer," Microwave Journal, October 2002, pp. 100-104.
3. "Qualification of LTCC Double Balanced Mixer," Mini-Circuits Qualification Report D4-QR-DZ-2.
