Reported In shows products that are verified to work for the solution described in this article. This solution might also apply to other similar products or applications. How does a phase-locked loop work? What hardware connections are required for PLL circuits?
Solution Phase-locked loop PLL A phase-locked loop PLL is a feedback circuit designed to allow one circuit board to synchronize the phase of its on board clock with an external timing signal. PLL circuits operate by comparing the phase of an external signal to the phase of a clock signal produced by a voltage controlled crystal oscillator VCXO. Thus, the original reference signal and the new signal are precisely in phase with each other.
There may be a phase difference between the two signals. This only means that they do not reach the same point on the waveform at the same time. If the phase difference is fixed it means that one is lagging behind or leading the other signal by the same amount, i.
A phase locked loop, PLL, is basically of form of servo loop. Although a PLL performs its actions on a radio frequency signal, all the basic criteria for loop stability and other parameters are the same. In this way the same theory can be applied to a phase locked loop as is applied to servo loops. The basic concept of the operation of the PLL is relatively simple, although the mathematical analysis and many elements of its operation are quite complicated. The diagram for a basic phase locked loop shows the three main element of the PLL: phase detector, voltage controlled oscillator and the loop filter.
In the basic PLL, reference signal and the signal from the voltage controlled oscillator are connected to the two input ports of the phase detector. The output from the phase detector is passed to the loop filter and then filtered signal is applied to the voltage controlled oscillator.
Here the phase of the signals from the VCO and the incoming reference signal are compared and a resulting difference or error voltage is produced. This corresponds to the phase difference between the two signals.
The error signal from the phase detector passes through a low pass filter which governs many of the properties of the loop and removes any high frequency elements on the signal. Once through the filter the error signal is applied to the control terminal of the VCO as its tuning voltage.
The sense of any change in this voltage is such that it tries to reduce the phase difference and hence the frequency between the two signals. Initially the loop will be out of lock, and the error voltage will pull the frequency of the VCO towards that of the reference, until it cannot reduce the error any further and the loop is locked. These specifications form part of a wireless communications standard.
The blocking specifications directly influence the performance requirement of the VCO. VCO noise blockers. The next PLL circuit element to be considered in our circuit is the voltage controlled oscillator. With VCOs, a fundamental trade-off between phase noise, frequency coverage, and power consumption is necessary. The higher the quality factor Q of the oscillator, the lower the VCO phase noise is.
However, higher Q circuits have narrower frequency ranges. Increasing the power supply will also lower the phase noise.
This increases PLL circuit complexity, as most PLL charge pumps can only tune to 5 V, so an active filter using operational amplifiers is used to increase the tuning voltage of the PLL circuit on its own.
Another strategy to increase frequency coverage without degrading VCO phase noise is to use a multiband VCO, in which overlapping frequency ranges are used to cover an octave of frequency range, and lower frequencies can be generated by using frequency dividers at the output of the VCO. The internal reference and feedback frequency dividers are used by the device to choose the appropriate VCO band, a process known as VCO band select or autocalibration.
The wide tuning range of the multiband VCOs makes them suitable for use in wideband instrumentation, in which they generate a wide range of frequencies. The 39 bits of fractional-N resolution also makes them ideal candidates for these precise frequency applications. In instruments such as vector network analyzers, ultrafast switching speed is essential. This can be achieved by using a very wide low-pass filter bandwidth, which tunes to final frequency very quickly.
The automatic frequency calibration routine can be bypassed in these applications by using a look-up table with the frequency values directly programmed for each frequency, true single core wideband VCOs like the HMC can also be used with less complexity. For phase-locked loop circuits, the bandwidth of the low-pass filter has a direct influence on the settling time of the system. The low-pass filter is the final element in our circuit.
If settling time is critical, the loop bandwidth should be increased to the maximum bandwidth permissible for achieving stable lock and meeting phase noise and spurious frequency targets. The narrow-band demands in a communications link mean the optimal bandwidth of the low-pass filter for minimum integrated noise between 30 kHz to MHz is about kHz Figure 20 using the HMC By contrast, the wideband HMC covering from 4 GHz to 8 GHz achieves the optimum rms phase noise with a wider bandwidth closer to kHz bandwidth Figure 21 , achieving —44 dBc of integrated noise.
Correct part selection and the surrounding circuit design are all critical for achieving the best outcome for the application. For high speed digital-to-analog converters DACs and high speed analog-to-digital converters ADCs , a clean low jitter sampling clock is an essential building block.
To minimize in band noise a low N value is desired; but to minimize spurious noise, integer N is preferred. Clocking tends to be fixed frequency so the frequencies can be chosen to ensure that the REF IN frequency is an exact integer multiple of the input frequency. This ensures the lowest in-band PLL noise. The VCO whether integrated or not needs to be chosen to ensure that it is sufficiently low noise for the application, paying particular attention to the wideband noise.
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