Multi-order Cancellation Technology

Multi-order Cancellation Technology

This article describes a multi-order cancellation technique that, when applied to microwave mixers, can improve their port-to-port isolation significantly. Two different cancellation methods are discussed and test data are presented.

Frank Xiaohui Li and Greg Corsetto

Watkins-Johnson Co.

Palo Alto, CA

As wireless communication markets continue to grow, new applications are emerging. Examples include local multipoint distribution systems, wireless local area networks (LAN), microwave point-to-point radios, wireless inventory tracking systems, RF identification systems and short-range personal communication systems. Most of these applications utilize a microwave/RF mixer as the key component to achieve frequency conversion. Mixers require an LO signal that is generally at a much higher power level than the other RF signals in the system. Consequently, LO signal leakage through the mixer can be relatively high and often needs to be suppressed to avoid impacting system performance.

Good mixer designs, whether single, double or triple balanced, must provide adequate port-to-port isolation for the particular application. For example, single-balanced mixers exhibit 10 to 25 dB LO-to-RF isolation while double- and triple-balanced mixers achieve 25 to 40 dB LO-to-RF isolation.1 However, in many cases the port-to-port isolation from the mixer itself is not sufficient for proper system operation. This article discusses examples that illustrate the impact of insufficient mixer isolation on system performance.

The first example involves a transmitter for which additional suppression of the mixer leakage signal is required to meet system emission standards. A typical transmitter block diagram is shown in Figure 1 . The transmitter LO leakage is analyzed assuming a +17 dBm LO drive, 25 dB mixer LO-to-RF isolation, 25 dB filter rejection and 40 dB gain from the power amplifier. Based on these assumptions, the resultant LO leakage at the antenna is +7 dBm. This high LO leakage may pose a system problem since it would likely exceed allowable radiated emissions. The second example involves a receiver application in which a high LO leakage exists through the RF mixer into the IF. In this case, high LO leakage can reduce receiver dynamic range, create high order spurious or possibly even saturate IF amplifier stages. These examples show that when isolation from the mixer itself is not sufficient, other methods for reducing LO leakage must be considered. The most common solution is to add highly selective filters around the mixer. However, these types of filters can be expensive and can interact with the mixer, thereby degrading other system-level parameters. In addition, filters generally are not a feasible solution when RF and LO frequencies are close or overlapping. This article discusses a multi-order cancellation technique (MOCT) that can be used to solve these system-level problems by improving the mixer's port-to-port isolation dramatically and thus eliminating the need for highly selective filters. This technique can be applied to any mixer design regardless of its circuit structure. Using MOCT, improved isolation can be achieved on a wide variety of mixer topologies, including double- or triple-balanced mixers, FET mixers, Gilbert Cell mixers2 or even image-rejection mixers (IRM). In addition, this technique can be combined with simple filters to obtain superior mixer isolation performance at low cost.

MOCT Description

Mixer isolation is a measure of the circuit balance and is infinite for a properly balanced mixer. However, in reality, it is impossible to make the mixer circuit perfectly balanced and some amount of leakage or feedthrough exists between the mixer ports. The basic concept of MOCT involves combining two mixers with the proper surrounding circuits so that the leakage signal is canceled while the desired signal is enhanced at the output. By splitting the LO signals to the two mixers equally and recombining them with a 180° phase shift, LO cancellation can be achieved. Of course, the desired converted signal must experience a net 0° phase shift. Figure 2 shows a method to produce this phase shift that uses an in-phase power divider and a 180° hybrid combiner with two identical networks in between. The signal incident at port 1 is divided into two paths: A and B. The signals in paths A and B have the same amplitude after the hybrid, but are 180° out of phase and cancel at port 2. Figure 3 shows another implementation that utilizes two 90° hybrids with two identical networks between them.3 The input signal at port 1 is also divided into paths A and B. Path A experiences 90° plus another 90° phase shift compared to path B. Since both paths have the same amplitude, they cancel at port 2. Both two-port networks represent the leakage path of mixers that, for example, could be LO-to-RF leakage, LO-to-IF leakage or any other port-to-port combination. MOCT can be applied to any mixer products to realize multi-order cancellation of LO leakage. Figure 4 shows an example in which LO signals from the in-phase splitter leak through path A and B mixers, which then are canceled after passing through a 180° balun at the RF port. The LO leakage is canceled similarly at the IF port. The RF signal from the 180° RF balun mixes with two in-phase LO signals. The resultant IF signals experience another 180° relative phase shift through the IF balun and add constructively at the IF port. Thus, both LO-to-RF and LO-to-IF isolation are improved significantly. Another example that demonstrates a power divider configuration is shown in Figure 5 . Here, the LO signal is applied to a 90° hybrid, split into paths A and B and recombined at a second 90° hybrid at the RF port where the LO leakage is canceled. The RF signal is split with a 90° hybrid and mixed with two LO signals with phases that are 90° apart. The two IF signals through paths C and D recombine at the IF port in phase. Although this technique produces improved LO-to-RF isolation as in the previous example, it does not improve the LO-to-IF isolation because the LO-to-IF leakage signals are not 180° out of phase at the IF port. However, LO and RF SWR are improved due to the use of 90° hybrids. Both of the circuits shown can be utilized in downconverter and upconverter applications although, in the latter circuit, the in-phase IF divider should be changed to a 180° hybrid for an upconverter. The working bandwidth of these cancellation methods is limited generally by the bandwidths of in-phase splitters, baluns and 90° hybrids. These types of cancellation circuits can be used repeatedly by combining 2(n - 1) mixers to create an Nth-order cancellation circuit. Assuming that the port-to-port isolation from the mixer itself is referred to as first-order cancellation (that is, the mixing elements and their baluns provide first-order isolation or cancellation), any additional isolation from the circuit other than the mixer itself is referred to as higher order cancellation. Mixers with second-order cancellation can be treated as whole mixers and used to create a third-order cancellation mixer, and so on. This article presents examples with test data up to third-order cancellation. This technique's practical effectiveness is dependent on the electrical repeatability of the two mixers within the balancing structure. In other words, the more identical the leakage amplitude and phase are between the two mixers, the greater the suppression should be through the cancellation circuit. In some of the circuit examples discussed, a small tuning capacitor Ct is included on one of the two paths to optimize this amplitude/phase balance.

MOCT Applications

To further demonstrate the MOCT concept, several example applications have been developed that utilize telecommunication mixer products. Figure 6 shows a 0° + 180° second-order cancellation circuit and Figure 7 shows a similar circuit using a 90° + 90° configuration. The first example utilizes high dynamic range FET mixers to achieve an ultra-high dynamic range mixer with exceptional isolation.

Table 1 lists the 0° + 180° circuit's test data as well as the mixers by themselves. The in-phase power divider/combiner and 180° hybrid are configured for improved LO-to-RF and LO-to-IF isolation. If desired, a configuration for improving RF-to-IF isolation and LO-to-RF isolation can be implemented by exchanging the LO port in-phase power divider/combiner and RF port 180° hybrid. The circuit was tested as an upconverter from the Advanced Mobile Phone Service (AMPS) band to the personal communications service (PCS) band. From the data it can be derived that the LO-to-RF isolation is improved significantly. Corresponding improvements in third-order intercept point (IP3) and 1 dB compression point (P1dB) are close to predicted values. The slight increase in insertion loss is due to losses in the passive circuits. An increase in LO power of 3 dB for the second-order circuit ensures sufficient LO power at each mixer. The next two upconverter examples use 0° + 180° and 90° + 90° cancellation methods, respectively. Both of these applications utilize model WJ-HMJ2 mixers to form second-order cancellation circuits. The WJ-HMJ2 mixer is a high dynamic range FET model designed specifically to convert frequencies between the PCS and IF bands.

 Table 2 lists the test data for the 0° + 180° circuit as well as for a single mixer. It can be seen that both LO-to-RF and LO-to-IF isolations are improved by 15 dB after incorporating second-order cancellation.

Table 3 lists the test data for the 90° + 90° circuit where only LO-to-RF isolation improves because of the particular combiner/splitter arrangement. However, this cancellation scheme provides outstanding SWR to the RF and LO ports using 90° hybrids. SWRs of approximately 1.5 can be achieved using this configuration. If the model WJ-HMJ2 mixers are replaced with model WJ-HMJ1 FET mixers and the experiment is conducted for an AMPS band upconverter, the performance listed in Tables 4 and 5 can be achieved.

Another interesting application using MOCT involves IRMs. Figure 8 shows a second-order cancellation circuit using two IRMs.

Table 6 lists the test data. The carrier (or LO) rejection is improved by 22 dB after applying second-order cancellation. The final example involves a third-order cancellation circuit that demonstrates the performance improvements possible with higher order cancellation. A standard double-balanced mixer, shown in Figure 9 , is utilized in this application as well as a 0° + 180° divider/combiner. The test data are listed in Table 7 .

The LO-to-RF isolation improves from 29 dB at a single mixer level to 56 dB using second-order cancellation, which then increases to 75 dB using third-order cancellation.

Conclusion

Multi-order cancellation techniques have been developed for RF and microwave systems and verified through several experiments using various mixer products. This technique is most applicable in wireless communications and can be used with a wide variety of mixer products. The test data collected from several applications demonstrate a significant improvement in port-to-port isolation, two-tone IP3 and P1dB. However, several possible disadvantages need to be evaluated before using this technique, including extra circuit complexity and additional PCB space required for splitters and combiners. In addition, the overall parts count (as well as LO power) generally will double each time the order of cancellation increases. However, when applied correctly in a system design, this technique can eliminate the need for costly filters and provide ultra-high dynamic range mixer performance.

Acknowledgment

The authors would like to thank Mike O'Neal for many helpful suggestions, Charleen Tipps for preparing the manuscript and Rollin Cabose for dedicated testing work performed during the experiments. n

References

1. RF and Microwave Designer's Handbook, Watkins-Johnson Co., 1997-1998. 2. James N. Wholey and Issy Kipnis, "Silicon Bipolar Active Mixers," Applied Microwave, 1990. 3. Stephen A. Maas, Microwave Mixers, Artech House Inc., Norwood, MA, 1993.

Frank Xiaohui Li received his MSEE degree from Tufts University, Medford, MA, in 1990. Currently, Li is a staff scientist in Watkins-Johnson Co.'s Microwave Products Group where he has been involved in the design and development of new RF/microwave components such as mixers and amplifiers. He has received several patents relative to mixers and multilayer embedded PCB technology. Li's work has also included RF/microwave transceiver designs from detailed circuitry to higher level integration for commercial and military applications.

Greg Corsetto received his MSEE degree from the University of California at Davis in 1978. From 1979 to 1997, he was a design team leader and section head at Watkins-Johnson Co. where he was involved in a variety of RF and microwave product developments for both defense and commercial applications. Corsetto has also been involved as an engineering manager responsible for developing RF products for wireless infrastructure applications. Currently, he is with Clarity Wireless, Belmont, CA