0
\$\begingroup\$

I've been learning from many publications that DLLs offer various advantages over PLLs, such as low-jitter performance and fast locking. So, recently, DLLs have been used for local clock generation in various applications too (edge combining DLL or multiplying DLL).

I was wondering what the disadvantages of DLLs for clock generation are with respect to PLLs, since the latter are still widely used for this purpose.

\$\endgroup\$

1 Answer 1

1
\$\begingroup\$

There are a number of disadvantages to DLLs. The four I can think of right now:

  1. The delay though each element is often highly temperature dependent, which means the number of delay elements changes as the temperature changes. As each delay is a discrete time step, this can cause significant jitter and phase noise.

This effect can also mean that the DLL is unusable over a wide temperature range as the part can literally run out of delay elements or have insufficient (depending on which extreme of temperature you are at).

  1. The delay through each element is not the same and has a stochastic mismatch and therefore jitter can result.

  2. They can struggle with a varying reference input (so we cannot use spread spectrum sources with them).

  3. They were, the last time I used them, unsuitable for fractional N synthesis.

Excellent source at this paper.

DLLs have got better over the years, but they are usually unsuitable if you need a high purity output signal or want to use spread spectrum for EMI control reasons.

\$\endgroup\$
2
  • \$\begingroup\$ IOW: You think you need the oven just for the oscillator, with DLLs you may need an oven for the DLL chips too :) \$\endgroup\$ Jan 25, 2021 at 16:51
  • \$\begingroup\$ I have not run into issue (1) myself. What kind of temperature ranges are we talking about and is this perhaps specific to certain implementations of delay elements? And in (2), why does an unequal delay lead to jitter instead of a static error compared to PLL's? (It also seems to contradict OP's info of DLL's having low jitter). \$\endgroup\$
    – Sven B
    Jan 25, 2021 at 17:48

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.