Bob is asked to develop a new radio system design, with some tough goals on RF performance and power consumption. Like many RF engineers, Bob has never really been comfortable with mixers. There is something mysterious about how they convert one frequency to another, and his formal education only covered them briefly; you know, the Taylor series expansion of the diode curve with two sine waves. His design experience and wisdom of his colleagues leads him to a simple rule of thumb: If you need good linearity and IMD performance, use a high LO mixer. But a high LO mixer requires a big LO booster amplifier, and that means more power consumption. Bob’s problem is how to distribute the gain and power in his radio design to get the needed linearity and still use the lowest power.
Bob’s design (see Figure 1) has goals that include frequency response and IMD performance; his key goal is +22 dBm IP3 at a frequency of 2.5 GHz. He starts with a Web search for mixers, and finds a lot of results, so he picks a few sites and starts looking over spec sheets. He is determined that he can only afford around +7 dBm from his LO drive, but needs good IP3 for his application. He starts to search through data sheets and nothing really meets his needs; all the level 7 mixers have low IP3. What he finds is that he needs high level mixers, +13 dBm or higher, to achieve his IP3 specs, but that is going to require an additional amplifier and he cannot afford the power.
Figure 1 System block diagram of a receiver.
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