An X-band, Low Cost, LTCC Double-balanced Mixer

Double-balanced mixers are used extensively in transmitters and receivers. For frequencies above 5 GHz the designs are normally realized using strip line and microstrip techniques. This is because distributed element baluns become smaller in size and more attractive for design as the frequency increases.

Semiconductor-based mixers are the most compact. However, changing the design is very time consuming and

Blue Cell™ low temperature co-fired ceramic (LTCC)-based mixers¹ are small in size and lower in cost, and can be customized to meet various needs. They are easier to customize than semiconductor mixers and are ideal for moderate to high volume applications. A series of Blue Cell LTCC-based mixers has been designed using patent pending techniques. These mixers are offered as standard catalog products.

Blue Cell LTCC

Blue Cell LTCC technology provides the means to produce multi-layer circuits with ceramic substrate tapes (sheets). Conductive, dielectric and resistive pastes are applied on each ceramic sheet; numerous sheets are then laminated together and fired (often in a precisely controlled oven or kiln) in one step. The resulting design is a hermetic, monolithic structure. A typical Blue Cell LTCC structure is illustrated in Figure 1 . It consists of multiple dielectric layers, screen-printed or photo-imaged low loss conductors, embedded resistors and capacitors, and via holes for interconnecting the multiple layers.

Each layer can be inspected before firing to replace damaged circuits and improve yield. Because firing takes place at low temperatures, typically about +850°C, low resistivity conductor materials such as silver and gold can be used on Blue Cell LTCC, instead of the molybdenum and tungsten materials typically used with high temperature co-fired ceramic (HTCC) processes. Since Blue Cell LTCC is a repeatable process, it can reliably produce large quantities of RF and microwave components measuring a fraction of the size of components fabricated with conventional substrate materials. To apply Blue Cell technology practicably and cost-effectively, a series of sophisticated three-dimensional computer models has been developed using the latest electromagnetic (EM) simulation tools.

These proven computer models allow new and custom LTCC components to be developed quickly, economically and reliably, with a high degree of first-pass design success. Blue Cell LTCC technology yields a highly integrated surface-mount module that can in turn be used as a platform to mount active components, such as transistors and monolithic amplifiers, to form a miniature multifunction assembly.

The resulting Blue Cell LTCC unit is a multi-layer, three-dimensional design that is considerably more compact than a traditional, planar microwave component. High level integration allows Blue Cell LTCC components to be combined within a miniature, surface-mount package, eliminating the need for many external components. Moreover, Blue Cell LTCC designs can employ circuit elements in die form for further reduction in size. Since the ceramic materials used in these designs are inherently very temperature stable, the need to compensate for variations in temperature is greatly reduced.

Construction

Figure 2 shows a schematic of a wide bandwidth double-balanced mixer¹ whose intermediate frequency response goes to DC. Transformers T1 and T2 are implemented using transmission line baluns distributed over multiple layers. Proven reliability Schottky barrier diodes are used as mixing elements. Solder plated leads are welded using a proprietary technique to the bottom of the LTCC board to provide interface to the customer circuit.

These new mixers, with a bandwidth of 3800 to 12000 MHz, have exactly the same foot print and pin configuration as the lower frequency members of the MCA1 series that cover multi-octave bands from 300 to 8500 MHz.

Electrical Performance

The newly developed series of mixers operate to 12 GHz, traditionally referred to as X-band. Model MCA1-12G is designed for 7 dBm of LO power, and MCA1-12GL for 4 dBm LO. Appendix A summarizes the performance of these LTCC mixers. MCA1 models for lower frequency bands include versions that require 4, 7, 10 and 13 dBm LO power.

The graphs in this article are measured performance of model MCA1-12G. Conversion loss as a function of frequency at three temperatures, at a fixed 30 MHz IF, is shown in Figure 3 . Losses in LTCC substrate are very moderate, resulting in a conversion loss in the range 5 to 7 dB. Low temperature-sensitivity of material used in LTCC generally results in negligible variation of electrical performance. Variation of conversion loss with temperature is within ±0.3 dB typ. These mixers are specified to operate over -55° to 100°C and satisfy most commercial and military applications.

Variation of conversion loss with LO power indicates how good an oscillator is required to drive the mixer. For the MCA1-12G it is within ±0.3 dB of nominal with a wide LO power variation, 7 ±3 dBm. Variation of L-R isolation with LO power is also low, typically less than ±3 dB for a 7 ±3 dBm LO. Figure 4 shows that L-R isolation is typically in excess of 30 dB from 6.4 to 9.5 GHz and more than 20 dB over the remainder of the range. This characteristic is very useful when the mixers are used in image reject, single side band and I/Q applications. Note again that the variation with temperature is negligible.Figure 5 shows how L-I isolation of the same mixer varies with frequency and temperature. The L-I isolation is greater than 20 dB over most of the range and temperature has very little effect.

Figure 6 shows that the IP3 of the mixer is around 10 dBm. Figure 7 shows the VSWR at each of the three ports, including the effect of varying the LO power.

Conclusion

A new series of double-balanced mixers has been developed using LTCC to cover the frequency range of 300 to 12000 MHz. These mixers have low height profile 0.065", small size 0.25" x 0.30", and are temperature-insensitive and low in cost. Additional information may be obtained from the company's Web site at www.minicircuits.com.