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February 2, 1998
Off-the-shelf MMIC suits mixer applications
J Leira Paz, M Pereira Varela, and FP Fontán, University of
Vigo, Spain
 You can use a standard monolithic-microwave IC
(MMIC) to configure an efficient and economical microwave mixer. Some systems require a
mixer with an input level lower than 10 dBm. Many available mixers, however, require
greater than 10-dBm input power. The circuit in Figure 1 exploits
the inherent nonlinearity of a Mini-Circuits (Brooklyn, NY) MAR6 microwave amplifier to
configure a low-level mixer. The idea is to add the input signals and feed the sum to the
MMIC (Figure 1a). The amplifier's saturation characteristic produces the harmonics and
intermodulation products. The MMIC has greater than 10-dB gain at frequencies over 1 GHz,
a desirable characteristic for making mixers for the 0.9, 1.8-, and 2.5-GHz bands.
The MMIC also has a maximum power output of 2 dBm at 1-dB compression, a useful trait
for obtaining intermodulation products with low input power. The gain of the MMIC varies
from 17 dB at 100 MHz to 10 dB at 2.4 GHz. The gain can thus generate intermodulation
products using two input signals whose power is lower than 10 dBm. Finally, the MMIC's
3-dB noise figure is an important factor for low-level input signals. The adder uses two
standard-value 22 ohms
resistors and thus presents a reasonably good impedance match in 50 ohms systems (Figure
1b). For perfect impedance matching, you would need 20.7 ohms (unavailable) resistors. With
the 22 ohms
resistors, the input VSWR is less than 1.4 at 1 GHz. To obtain optimum results, you should
use microstrip techniques, with the dimensions in Figure 1b, in
designing the pc board.
 You can use this mixer in a variety of
applications: downconverters and modulators, for example. Another example is an
amplitude-shift-keying (ASK) modulator for digital applications (Figure
2). The mixer block in Figure 2 uses the circuit in Figure 1a.  The measurements of
Figures 3, 4, and 5
reflect a 1-GHz carrier frequency and a 4-Mbps digital signal. To obtain a good
relationship between the carrier level and the modulated-signal level, use a 5-dBm
input-carrier level, a ±0.5-dBm digital signal, and a 12V supply. The output-signal level
is 0 dBm.
 Figure 3 shows the magnitude of the
intermodulation products as a function of the 100-MHz signal power. The input frequencies
are 1.3 GHz and 100 MHz. The most important intermodulation frequencies are the
sum-and-difference intermodulation products, 1.2 and 1.4 GHz.  Figure 4 shows the input-input and output-input isolation characteristics. Figure 5 gives the variation of the input and output VSWR as a
function of the input frequency.
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