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MC10E197 Scheda tecnica(PDF) 8 Page - ON Semiconductor |
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MC10E197 Scheda tecnica(HTML) 8 Page - ON Semiconductor |
8 / 16 page MC10E197 http://onsemi.com 8 Filter Input The primary function of the filter input subsection is to convert the output of the phase detector into a single ended signal for subsequent processing by the integrator circuitry. This subsection consists of the 10E197 charge pump current sinks, two shunt capacitors, and a differential summing amplifier (Figure 5). Hence, this portion of the filter circuit contributes a real pole and two complex poles to the overall loop transfer function F(s). Before these pole locations are selected, appropriate values for the current setting resistors (RSETUP and RSETDN) must be ascertained. The goal in choosing these resistor values is to maximize the gain of the filter input subsection while ensuring the charge pump output transistors operate in the active mode. The filter input gain is maximized for a charge pump current of 1.1 mA; a value of 464 Ω for both RSETUP and RSETDN yields a nominal charge pump current of 1.1 mA. It should be noted that a dual bandwidth implementation of the phase lock loop may be achieved by modifying the current setting resistors such that an electronic switch enables one of two resistor configurations. Figure 6 shows a circuit configuration capable of providing this dual bandwidth function. Analysis of the filter input circuitry yields the transfer function: F1(s) = K1 * 1 (s + p1) * 1 where: The gain constant is defined as: K1 = A1 * 1 CIN eqt. 3 A1= op-amp gain constant for the selected pole positions. CIN = phase detector shunt capacitor. [s2 + (2ζω ) s + ω2 ] o1 o1 The real pole is a function of the input resistance to the op-amp and the shunt capacitors connected to the phase detector output. For stability the real pole must be placed beyond the unity gain frequency; hence, this pole is typically placed midway between the unity crossover and phase detector sampling frequency, which should be about ten times greater. ELECTRONIC SWITCH VEEVCO VEEVCO RSETUP RSETDN 464Ω 464Ω 464Ω 464Ω Figure 6. Dual Bandwidth Current Source Implementation The second order pole set arises from the two pole model for an op-amp. The open loop gain and the first open loop pole for the op-amp are obtained from the data sheets. Typically, op-amp manufacturers do not provide information on the location of the second open loop pole; however, it can be approximated by measuring the roll off of the op-amp in the open loop configuration. The second pole is located where the gain begins to decrease at a rate of 40 dB per decade. The inclusion of both poles in the differential summing amplifier transfer function becomes important when closing the feedback path around the op-amp because the poles migrate; and this migration must be accounted for to accurately determine the phase lock loop transient performance. Typically the op-amp poles can be approximated by a pole pair occurring as a complex conjugate pair making an angle of 45° to the real axis of the complex frequency plane. Two constraints on the selection of the op-amp pole pair are that the poles lie beyond the crossover frequency and they are positioned for near unity gain operation. Performing a root locus analysis on the op-amp open loop configuration and adhering to the two constraints yields the pole positions contributed by the op-amp. Determination of Element Values Since the difference amplifier is configured to operate as a differential summer the resistor values associated with the amplifier are of equal value. Further, the typical input resistance to the summing amplifier is 1 kΩ; thus, the op-amp resistors are set at 1 kΩ. Having set the input resistance to the op-amp and selected the position of the real pole, the value of the shunt capacitors is determined using the following relationship: ⎥ p1⎥ = 1 2πR1CIN eqt. 4 Augmenting Integrator The augmenting integrator consists of an active filter with a lag-lead network in the feedback path (Figure 7). VIN VCCVCO MC34182 RIA RA CA VO2 RIA Figure 7. Integrator Subsection |
Codice articolo simile - MC10E197_06 |
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Descrizione simile - MC10E197_06 |
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