3
\$\begingroup\$

For a PLL in short,

1) What controls loop bandwidth?

2) What impact does it have on output phase noise/jitter?

3) What impact does loop bandwidth have on PLL lock time?

I am trying to find answers to these questions, could you help?

\$\endgroup\$
  • \$\begingroup\$ I don't want anyone to do the homework. Just some direction is all I need. \$\endgroup\$ – quantum231 Jul 17 '13 at 14:24
6
\$\begingroup\$

Loop bandwidth is mainly controlled by the filter after the phase/frequency detector. Normally this filter is a low-pass filter, and the lower the cutoff frequency of this filter the lower the loop bandwidth is.

Other factors also can affect loop bandwidth, such as the speed that the VCO can change the output frequency in response to its input. Also, any frequency dividers in the PLL can potentially change the loop bandwidth. But the overwhelming factor in loop bandwidth is the filter.

As the bandwidth increases, the lock time decreases. Basically, the higher the bandwidth the faster the PLL can adjust the output frequency. The faster it can adjust the frequency, the faster it can lock.

A side effect of higher bandwidth is that the PLL is harder to control. It might over-adjust, increasing output noise and jitter. In some cases, if the loop bandwidth is low enough then jitter on the reference clock will be removed. But if the loop bandwidth is too high then incoming jitter can be passed to the output or even amplified.

Some PLL's actually adjust the loop bandwidth on the fly. When the PLL has not locked, the bandwidth is high and lock time is improved. But after locking, the bandwidth is reduced to reduce output jitter and noise.

\$\endgroup\$
  • \$\begingroup\$ This is sufficient. \$\endgroup\$ – quantum231 Jul 18 '13 at 10:30
  • \$\begingroup\$ This is wrong, but don't worry, few people get it right. \$\endgroup\$ – Neil_UK Feb 6 '18 at 16:18
  • \$\begingroup\$ Please enlight us how this is wrong. \$\endgroup\$ – user94729 Feb 12 '18 at 10:17
1
\$\begingroup\$

The loop bandwidth is entirely controlled by the gain of the loop.

Of necessity, the loop filter is essentially inactive at the frequency of the loop bandwidth, and so has little influence on the loop bandwidth. It does have a large influence on loop stability, and loop quality.

Above the loop bandwidth, the loop filter provides a low pass filter response to reduce the modulation of the VCO by phase detector noise and reference noise. In order to maintain loop stability, the low pass filter must create no more than 20 or so degrees phase shift at the loop bandwidth, and so has a gain very close to unity.

Below the loop bandwidth, the loop filter often provides a broken integrator response to reduce the error between the VCO and the reference frequency. In order to maintain loop stability, the integrator must create no more than 20 or so degrees phase shift at the loop bandwidth, and so has a gain very close to unity.

Together, the low pass filters are limited to creating 40 or so degrees of phase shift at the loop bandwidth. As the VCO behaviour already creates 90 degrees, anything approaching 90 degrees from the loop filter would make for a very unstable loop.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.