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RF design Approach


Platform-based RF design
Platform-based design has become an accepted approach for digital circuits. It is
a logical next step when reuse of sub circuits by itself does not provide a
sufficiently fast time-to-market. A direct translation of the platform-based
design method for digital circuits to RF design is not likely to be successful,
however, because of a number of basic differences between digital and RF
design:
1. Digital design is based on the robustness to noise, interference and nonlinearity
that is inherent to the processing of quantized, typically binary,
signals. Distortion is completely irrelevant digital circuits, the desired
signal level is typically equal to the maximum signal level, and noise
levels and crosstalk can be as high as 20dBs below the maximum signal.
In RF circuits, the noise level is much further below the maximum signal,
in many cases 80dB or more, and the desired signal can be around the
noise level while interferer levels exceed the desired signal by several
orders of magnitude. This puts very high requirements on the linearity and
noise performance of RF circuits, often close to the limits that can be
achieved in the IC technology used.
2. Digital design is based on the robustness to delay inherent to the
processing of time-discrete signals. This is achieved at the expense of a
much large margin between clock frequencies and the unity-gain
bandwidth of the individual devices. Typically, the clock frequency is less
than 1% of the unity-gain bandwidth. RF circuits often operate with
signals around 20% of the unity-gain bandwidth of individual devices,
and therefore have a much smaller margin between desired and achievable
performance.
3. The complexity in terms of circuit elements in RF circuits tends to be
much less than in digital circuits. Whereas a typical RF transceiver circuit
has in the order of 1000 devices, modern microprocessors use around 6 orders of magnitude more devices to implement their highly complex
required functionality





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