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Since a Gilbert-cell mixer is a balanced mixer design, even-order mixer harmonics should be quite low. Due to the balanced nature of this structure, both the LO and RF ports should ideally be driven differentially. While modern wireless receivers rely more and more on differential components on the intermediate-frequency (IF) and baseband portions of the downconverter, most RF designers still prefer single-ended signal chains for driving the RF and LO ports.

This design preference is reinforced by available components. Most low-noise amplifiers on the market which drive I/Q demodulators and receiver mixers are single-ended circuits. Likewise, most VCOs which drive mixer LO ports are also single-ended devices, such as the model ADL5801 double-balanced mixer (10 MHz to 6 GHz) and model ADF4351 fractional-N frequency synthesizer with integrated VCO, both from Analog Devices.

Differential Drive Optimizes Active Mixers, Fig. 2For using single-ended drive with the RF and LO ports of the ADL5801 double-balanced mixer and ADL5380 I/Q demodulator, straightforward circuit configurations can be applied (Fig. 2). The RF input signal is connected to one of the balanced input ports while the other port is terminated with a capacitor connected to ground; the choice of either positive or negative port is of no consequence. The RF and LO ports are self-biased so a DC blocking capacitor is needed to prevent disrupting the common mode levels.

Driving differential LO and RF ports with single-ended signals comes at the expense of performance degradations, such as increased even-order distortion, noise coupling, and decreased power gain. With a single-ended drive, the balance within the mixer core is no longer maintained since the single-ended drive creates propagation delays or phase shifts of the two signal phases of the LO and RF ports.2 The result is degraded linearity performance—specifically, increased even-order distortion. Figures 3 and 4 compare input third-order-intercept-point (IIP3) and input second-order-intercept-point (IIP2) levels when driving mixer ports with single-ended signals versus differential signals.

Differential Drive Optimizes Active Mixers, Fig. 3

In addition, with the loss of differential balance within the mixer, any noise that is coupled to the mixer input will directly propagate to its output. A single-ended signal is unbalanced by definition and is measured by the difference between the input signal and a constant reference point, which is often ground. When noise or an interference signal couples into the system, the constant reference point will have a signature different than that of the desired input signal. Therefore, when the signals are summed, the unwanted signal does not cancel and directly propagates to the output.

Differential Drive Optimizes Active Mixers, Fig. 4

Differential signals, on the other hand, have equal amplitudes and opposite phases, 180 deg. out of phase with each other. The differential nature of the signals allow any noise or unwanted coupling to directly cancel when summed, since the unwanted signal will affect both signals equally but with opposite phases. This advantage of differential signals proves very beneficial in achieving a quiet and well-controlled high-frequency printed-circuit-board (PCB) design.

Another performance degradation observable in single-ended drive of the RF port is decreased power gain where only one-half the power is available compared to differential drive. The 6-dB power loss is a result of the change in input impedance from a 50-Ω differential impedance to a 25-Ω single-ended impedance. The effect of this becomes more critical for the noise figure of the device, which follows a linear relationship with power gain. Across the component’s full frequency range, the noise figure will degrade by 6 dB for single-ended drive versus differential signals.

As noted, differential drive for a mixer’s RF and LO ports offers performance advantages in linearity and noise figure and helps ease noise coupling compared to single-ended components. However, if the previous stage to the mixer or demodulator is single ended, which is often the case, a balun will be required for the single-ended-to-differential conversion. As an example, the model TCM1-63AX+ from Mini-Circuits is a 50-Ω 1:1-impedance-ratio balun that maintains low insertion loss from 1.1 to 1.8 dB across a broad frequency range of 10 MHz to 6 GHz. The frequency range of this balun is an excellent match for the frequency ranges of the single-ended ADL5801 mixer and ADL5380 I/Q demodulator.

Differential Drive Optimizes Active Mixers, Fig. 5Figure 5 shows how the LO and RF ports of the ADL5801 and ADL5380 components interface to the TCM1-63AX+ balun. Improvements in IIP2 and IIP3 performance from using the mixer and I/Q demodulator with these baluns and differential drive versus a single-ended approach are apparent from the plots in Figs. 3 and 4.

Broadband radio receivers and reconfigurable receivers require broadband components. The ADL5380 I/Q demodulator and ADL5801 and ADL5802 mixers together with the TCM1-63AX+ broadband 1:1 balun from Mini-Circuits provide broadband operation with optimum performance. The TCM1-63AX+ broadband 1:1 balun is an excellent support component for these frequency downconverters and helps to enable the design of broadband and reconfigurable radios.

Qui Luu, Application Engineer

Analog Devices, Inc., 804 Woburn St., Wilmington, MA 01887; (978) 658-8930

Benjamin Sam, Design Engineer

Analog Devices, Inc., Northwest Labs Design Center, 1100 Northwest Compton Way, Ste. 100, Beaverton, OR 97006; (503) 690-1333

References

1. Whites lecture notes, Lecture 27: Mixers. Gilbert Cell.

2. Behzad Razavi, RF Microelectronics, Prentice-Hall, 2nd ed., Englewood Cliffs, NJ, 1998.

3. Carlos Calvo, “The differential-signal advantage for communications system design,” EE Times, February 1, 2010.

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