Anritsu HFE0104 Campbell

26 High Frequency Electronics

High Frequency Design

INTEGRATED I-Q MIXER

An Integrated I-Q Mixer for Software-Defined Radio Applications

By Rick Campbell TriQuint Semiconductor

he software defined radio takes advan-

tage of digital signal processing to move channel filtering, interference suppression, automatic gain control, modulation and demodulation from hardware into soft-

ware. As shown in Figure 1, the block diagram for a software defined radio includes RF functions and analog and digital baseband signal processing. For simple, high volume, low performance applications, all of these functions might be implemented on a single chip. For higher performance or lower volume applications, partitioning the radio into a general purpose baseband DSP, analog circuitry using standard products, and a few RFICs is a good approach. This paper describes an I-Q frequency converter with integrated microwave splitters and phase shifters, so that the application circuit does not require any critical microwave layout techniques or off-chip components. The microwave ports are 50 ohms, the baseband ports are 200 ohm balanced, and the only offchip component is a single non-critical bypass capacitor on the DC supply line.

Basic Mixer Cell

A mixer for low and zero-IF applications has several important criteria in addition to noise figure, conversion loss and third-order intercept. These include 2nd order distortion, 1/f noise, and the suppression of LO and LO harmonics at the RF port. The conventional superhet block diagram with intermediate frequency near 10 percent of the RF input band is an old and elegant solution that avoids many

problems through the judicious use of filters [1]. Optimum mixers for direct conversion receivers have been discussed in the amateur literature for several decades [2, 3] and many successful developments have been based on the pioneering work of Barrie Gilbert. The balanced mixer cell shown in Figure 2 is a recent evolution of fundamental work by Steven Maas [4] and Wes Hayward [5,6]. Similar passive MESFET mixers have been used extensively in ASICs for cellular handsets.

Two subtleties that may not be immediately apparent from the schematic are the overlaid inductor tuned LO drive circuit and the fact that the mixer FETs are operated with zero DC potential between source and drain.

This design case history

provides a lesson in prod-

uct development, describ-

ing a new microwave I-Q

mixer that addresses many

of the shortcomings found

in earlier designs.

Figure 1 · Software radio architecture.

From January 2004 High Frequency Electronics

28 High Frequency Electronics

High Frequency Design

INTEGRATED I-Q MIXER

Tight electromagnetic coupling in the LO tuned circuit results in nearly perfect 180 degree balanced LO drive to the mixer FET gates. Balance is inherent in the circuit topology. Operating the mixer FETs as variable channel resistors rather than variable gain elements has a number of implications for mixer operation. First, the mixer has conversion loss rather than gain. Commercial resistive FET mixers such as the CMY210 have conversion loss just under 6 dB, with a 50 ohm RF source and 50 ohm IF load. (Much lower conversion loss numbers, or even conversion gain, may be obtained by using stepup transformers or high impedance loads on the IF port—but such numbers have questionable merit). Not only is the conversion loss acceptably low, it is exceptionally constant. Typical production spreads of conversion loss are on the order of 0.1 dB, and pairs of IC mixers on the same die have conversion loss matched to within hundredths of a dB.

Electromagnetic coupling in the LO driver transformer and zero source-drain voltage of the FET have another interesting result: either one will cause most simulations to crash. To paraphrase Wes Hayward lecturing in a GaAs design class, “We know the mixers work—there are hundreds

of millions of them in cell phones— it’s the simulations that are experimental!”

One further refinement is needed for a mixer to be useful for direct conversion receiver applications. GaAs MESFET mixers are typically unsuitable for IFs below about 10 MHz due to high 1/f noise. The 1/f noise problem was attacked in three different ways. First, 1/f noise in GaAs MESFETs is strongly correlated with DC current in the channel. This source of 1/f noise is reduced by operating the device with zero source-drain voltage. Second, the density of semiconductor defects that result in 1/f noise is higher near the semiconductor surface. We operate the FET as a deep-channel device by biasing the gate near pinch-off. With zero drain-source voltage and a deepchannel FET, the remaining dominant source of 1/f noise is the pinchoff bias generator, the 100 micron DFET on the right in Figure 2. Since the bias noise is present on both mixer FETs, it can be cancelled by using a balanced IF connection. Both transformers and active balanced circuitry have been used. These three 1/f noise reduction techniques result a mixer with a noise figure within 1 dB of the conversion loss at 10 MHz IF and only 6 dB higher at 1 kHz IF.

This compares well with other low 1/f noise microwave mixers.

The IF ports are DC coupled and

may be shorted to ground or V

without harming the mixer cell. In normal operation, the IF ports float at V

, the pinch-off voltage of the active devices. This is typically between 0.4 and 0.8 volts in the TQTRx process.

One final comment on the basic balanced mixer cell. This is a “one deep” circuit topology, meaning there are no series connections of active devices between DC ground and V

. Thus the mixer will operate properly as long as the supply voltage is high enough to activate the pinch-off voltage generator. This design was optimized for 2.8 volt supplies, but functions properly with V

between 1.2 volts and 6 volts. Below 2 volts, the drive to the mixer FET gates is lower than optimum, and intercept performance suffers.

The I-Q Mixer Topology

The next step is to extend the balanced mixer to an image reject design. This is where the advantage of the low loss GaAs substrate becomes evident. Signal splitters and phase shift networks may be built using standard topologies from a large catalog of active or passive circuits. Passive circuits have the

Figure 2 · Balanced mixer cell. Figure 3 · I-Q mixer block diagram.

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