Accurate Phase Noise Prediction in PLL Synthesizers
نویسنده
چکیده
Op-amp in loop filter While the cases using the passive loop filter (no op-amp) are simply a matter of circuit analysis, the case using the active filter requires some explanation. This case will only be described here; the accompanying analysis can be found in the supporting MathCad documents. With the op-amp in the loop, and the filter configuration shown in Figure 1, four different noise sources and important factors exist within the loop itself: R2, the op amp itself, the gain of the op-amp, and R3. The noise within R2 is the same as the cases previously mentioned. However once this noise is determined, the gain of the amplifier needs to be applied to it (amp_gain in Figure 1). The output of the op-amp is again filtered by R3 and C3. A schematic of this is pictured in Figure 2a. The op-amp itself contributes noise, and this is one reason to place the op-amp after the second order filter section but before the third pole. The third pole can then provide some attenuation of the broadband noise. Manufacturer’s data sheets will usually specify the input noise of the op-amp in nV/ . This noise voltage is simply multiplied by the amplifier’s gain (amp_gain), and then passed through the filter formed by R3 and C3. Op-amps are usually regarded as very lowoutput-impedance devices. For this reason, the analysis of the noise due to R3 can be greatly simplified if an op-amp is in the loop as shown in Figure 1. If it is assumed that the op-amp output impedance is virtually a short (which would be accurate, even if the op-amp output were a few hundred ohms), then the noise voltage generated in R3 is simply connected to ground, then filtered through R3 and C3.
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