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Knowing What's Missing The cost of missing functionality is the cost of inadequately supporting the desired level of features and performance. At the system-level, this cost manifests itself in the form of lost sales, increased pressure on selling price and therefore profit margin, and customer dissatisfaction. It is therefore essential to understand what the required level of features and performance is now and in the likely future for the set of systems being built.

CA3101FA36-10PF80F85F0

Knowing What's Missing The cost of missing functionality is the cost of inadequately supporting the desired level of features and performance. At the system-level, this cost manifests itself in the form of lost sales, increased pressure on selling price and therefore profit margin, and customer dissatisfaction. It is therefore essential to understand what the required level of features and performance is now and in the likely future for the set of systems being built.

Diode mixers usually have 5-8 dB conversion loss, while active mixers usually can achieve at least a few dB of gain. Although properly designed active mixers can achieve somewhat lower noise figures than diode mixers, most systems can tolerate a relatively noisy mixer, so the diode mixer's loss and noise are rarely a significant disadvantage. Broadband diode mixers usually do not require more local-oscillator (LO) power than active mixers, but narrowband active mixers may have an LO-power advantage. Finally, balanced active mixers always require an IF hybrid or balun; diode mixers generally do not. When the IF frequency is low, the resulting large size of the IF balun may be troublesome, especially in monolithic circuits. Finally, even balanced active mixers require matching and filtering circuits, while balanced diode mixers largely do not.

Active mixers have a few important advantages over diode mixers besides their superior gain and noise figure. High-quality diodes are often difficult to produce in FET monolithic circuit technologies, so active FET mixers often are easier to integrate. Diodes in such technologies usually consist of a FET gate-to-channel junction, which usually is a very poor diode. Dual-gate FET mixers offer inherent LO-RF isolation, even in single-device circuits, although noise figure and gain usually are slightly worse than in single-gate FET mixers.

CA3101FA36-10PF80F85F0

Mixer Design

The design of balanced mixers-passive or active-involves two fundamental tasks: (1) design of the baluns and passive matching circuits, and (2) design and analysis of the complete mixer.

The design of baluns and passive circuits for discrete-component mixers is very mature. Figure 1 shows a common structure. In this mixer, the baluns consist of simple, parallel-coupled strips mounted on a suspended substrate. Often, the lower strip (which is connected to the ground surface of the housing) is tapered to improve the balun's performance.

CA3101FA36-10PF80F85F0

But such baluns – widely used in hybrids – are impractical for new-generation RF ICs, and attempts to translate” suspended-substrate baluns into planar monolithic form have been largely unsuccessful. The fundamental problem is in the extra capacitance between the monolithic circuit's microstrips and ground. Because the substrate is thin (usually 100 microns) and has a high dielectric constant (12.9), this capacitance is unavoidably large. It allows an even mode impedance to exist on the balun. The even mode unbalances the mixer and allows input-to-output coupling, which reduces port-to-port isolation. Unless special efforts are made to reduce it, the imbalance is severe.

Practical approaches to the design of on-chip baluns for braodband circuit are still scarce. The Marchand balun offers some hope as a building block for broadband, planar monolithic mixers. Although its even-mode characteristic impedance is no higher than that of other structures, its performance tolerates low even-mode impedance much better.

CA3101FA36-10PF80F85F0

Figure 2 shows a planar Marchand balun, and Figure 3 shows its calculated performance. Clearly, the Marchand balun is intrinsically capable of good performance over a multioctave band. In less idealized cases, we find that an octave bandwidth, or slightly greater, is practically achievable.

Commercially available hardware acceleration solutions also had significant shortcomings. While those products could accelerate the RTL code, there was little or no support for testbenches. Regression would be very difficult and could not support real-life traffic. In most cases, by the time hardware acceleration was accomplished, the chip could already be back from fabrication. We did not have the luxury of unlimited time and money and, at the same time, the chip had to function properly at first silicon.

After weighing the options, we rejected all commercially available hardware acceleration and emulation solutions in favor of building our own emulation system.

To understand the size and complexity of the chip, it is useful to walk through the block diagram shown in Fig. 1. Packets or cells are received from either the line or the switch fabric at the incoming interfaces. The data proceeds to the lookup engine, where one of several lookups is performed, including an ATM lookup, a multiprotocol label switching (MPLS) packet lookup and an MPLS-like tag from an network processing unit or classifier to identify the flow.

Next, the segmentation and reassembly engine, with the help of external data memory, segments packets into cells, reassembles cells into packets or propagates packets or cells. External data memory is used to enqueue the packet or cell payload until it is scheduled for dequeue onto an outgoing interface. The header append block adds the outgoing header onto packets or cells, which could be either ATM cells or cells specifically configured for the switch fabric. The header append block also does MPLS processing such as tag translation and push/pop. Finally, the outgoing interfaces send the packets or cells out of the device to the line or switch fabric.

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