Today, I must introduce to you an innovative balun structure that can be easily implemented.

Wideband mixers are widely used in multifunction wireless transceivers, microwave transceivers, microwave backhaul, radar, and test equipment. Wideband mixers make it possible to use a single mixer in a radio architecture with dynamic programmability of various radio parameters.

Advanced silicon technologies such as CMOS and BiCMOS have proven capable of implementing high performance mixers in relatively narrowband applications. Therefore, the most anticipated implementation of wideband mixers is made using lumped elements or other structures compatible with IC manufacturing techniques and geometries. Balanced mixers are the preferred topology because they have better overall performance in terms of linearity, noise figure, and port-to-port isolation compared to unbalanced mixers. The balun is a key component used in single-balanced and double-balanced mixers to convert RF, LO, and IF signals between balanced and unbalanced configurations. It is critical to be able to integrate the balun in standard IC foundry processes to produce wideband integrated mixers.

This article introduces an innovative balun structure that can be easily implemented in silicon, GaAs or any other integration process. This balun topology has a wider bandwidth than the traditional balun structure. A 3GHz to 20GHz high-performance mixer is designed using a broadband balun in a 0.18µm SiGe BiCMOS process.

The most important performance parameters of a mixer include conversion gain, linearity, noise figure, and operating bandwidth. The balun used in the integrated mixer has a significant impact on the performance of all of these mixers. Key performance of the integrated balun includes operating frequency range, insertion loss, amplitude/phase balance, common mode rejection ratio (CMRR), and physical size.

Two common balun structures in integrated circuit applications are the traditional planar transformer balun and the Marchand balun. Both baluns have good performance in narrowband applications. The planar transformer balun consists of two closely coupled transformers. The self-inductance and resonant frequency of the inductors are the two main limiting factors for bandwidth. Self-inductance limits the bandwidth at the low frequency end, and parasitic capacitance and asymmetric terminations of unbalanced and balanced terminations limit the bandwidth at the high frequency end. The Marchand balun consists of four quarter-wavelength transmission lines and usually requires a lot of space on the chip. Miniature Marchand baluns have been demonstrated in integrated circuits using interleaved transformer layouts. The electrical length requirement of each line segment limits the bandwidth of the Marchand balun. When the electrical length deviates from the required quarter wavelength, the amplitude and phase balance degrade. Generally, a well-designed transformer balun or Marchand balun can cover a frequency range of 3× to 4× the maximum-to-minimum frequency ratio with reasonable performance.

It is well known that the Ruthroff balun has a very wide bandwidth, and many discrete component products are developed based on the Ruthroff structure. However, it has not been found that a similar structure is applied to microwave integrated circuits.

Figure 1a shows a schematic of a Ruthroff-type broadband balun that can be easily constructed in a planar semiconductor process using three inductors. A layout example is shown in Figure 1b. In this layout, only two metal layers are required, one thick metal layer for the three low-loss inductors and one underground via metal layer for the connections. When an additional thick metal layer is available, L1 and L3 can be coupled vertically, which results in a smaller size and potentially better magnetic coupling between them.

(a). Schematic

(b). Layout

Figure 1. Ruthroff-type broadband balun.

Figure 1a shows a schematic of a Ruthroff-type broadband balun that can be easily constructed in a planar semiconductor process using three inductors. A layout example is shown in Figure 1b. In this layout, only two metal layers are required, one thick metal layer for the three low-loss inductors and one underground via metal layer for the connections. When an additional thick metal layer is available, L1 and L3 can be coupled vertically, which results in a smaller size and potentially better magnetic coupling between them.

The broadband characteristics are due to the simple structure, which results in less parasitic capacitance. The single-ended signal is divided by L1 and L2. Therefore, the positive port of the balun is exactly half the voltage of the single-ended signal with the same phase. Due to the negative coupling between L1 and L3, the negative port of the balun is half the voltage of the single-ended signal with a 180° phase shift.

Excellent amplitude and phase balance can be achieved over a very wide bandwidth. Figure 2 shows the simulated performance of the broadband balun configuration. Amplitude imbalance is the difference between S21 and S31, and phase error is the phase difference between S21 and S31 and the desired 180°. The proposed balun has very good amplitude balance, and a phase difference close to 180° between 3GHz and 20Ghz. Common mode rejection is very important when using baluns in many applications such as balanced mixers and push-pull amplifiers. The simulation results shown in Figure 5b show that the CMRR of the 3-inductor balun is better than 20dB from 3GHz to 20GHz.

(a). Amplitude Imbalance and Phase Error

(b). Insertion Loss and CMRR

Figure 2. Simulated performance of a wideband balun.

Like the transformer balun topology, the bandwidth of the 3-inductor balun is limited by the inductance at the low frequency end and the parasitic capacitance at the high frequency end. When the inductance is low, the load impedance has a greater impact on the voltage division between L1 and L2 at port 3 and the converted voltage at port 2. Although the amplitude balance and phase difference are still acceptable in the low frequency range, the insertion loss increases. Therefore, lower terminal impedance or higher inductance will benefit the low frequency performance. At the high frequency end, the parasitic capacitance between L1 and L2 will degrade the performance of the transformer and cause large phase errors. Careful layout and consideration of reducing parasitic capacitance can extend the high frequency operating range of the balun.

The physical size of the integrated balun limits the low-end bandwidth. To explore the feasibility of the proposed balun structure in low-frequency applications, a 0.5 GHz to 6 GHz balun was designed and compared with a traditional transformer-based balun. The performance is shown in Figure 3.

(a). Phase Performance

(b). Amplitude Balance

Figure 3. Comparison of simulated performance of traditional balun and new balun.

The broadband double-balanced passive mixer design uses Jazz's SiGe 0.18µm process and a 3-inductor balun configuration. The mixer's RF, IF, and LO ports are 50Ω single-ended ports with integrated baluns at the RF and IF ports. The integrated RF balun is optimized to cover the 3GHz to 20GHz RF frequency range. The integrated IF balun is optimized to cover an extremely wide frequency range of 500MHz to 9GHz. The single-ended LO signal is internally converted to a differential signal by an active amplifier circuit to reduce chip size. A two-stage broadband amplifier using a high-speed NPN provides sufficient signal voltage swing to the MOSFET gate of the passive mixer with only 0dBm input power in the 1GHz to 20GHz frequency range.

Figure 4. Wideband double-balanced passive mixer.

The mixer is packaged in a small 2mm × 3mm QFN package and uses copper pillar flip-chip interconnects. The additional parasitic capacitance of the copper pillar connections is low, maintaining the broadband performance of silicon. The mixer consumes 132mA at room temperature with a 3.3V bias supply. The measured conversion loss and IIP3 performance are shown in Figure 5. The RF, LO, and IF ports of the mixer are well matched over its wide operating frequency range. Figure 6 shows the return loss of these ports. It should be noted that the RF return loss depends on the IF port impedance, and the results in Figure 6a are measured using an IF frequency of 0.9GHz.

(a). Conversion Loss and IIP3 vs. RF

(b). Conversion Loss and IIP3 vs. IF

Figure 5. Measured performance of a broadband double-balanced passive mixer.

(a). RF and LO Port Return Loss

(b). IF Port Return Loss

Figure 6. Measured return loss of a broadband double-balanced passive mixer.

Compared to wideband mixers on the market (as shown in Table 1), the mixer designed using the 3-inductor balun achieves the widest bandwidth for both RF and IF ranges. It has the lowest LO power consumption and the highest level of integration. The overall performance is better than any reported product or released wideband mixer product.