[SI-LIST] Fwd: Re: Re: PLL Natural Freq
- From: steve weir <weirsp@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sun, 02 May 2004 14:25:52 -0700
>Date: Sun, 02 May 2004 14:22:44 -0700
>To: a.ingraham@xxxxxxxx
>From: steve weir <weirsp@xxxxxxxxxx>
>Subject: Re: [SI-LIST] Re: PLL Natural Freq
>
>Andy, perhaps I should have been more specific, since his application is a
>clock replicator, where near zero phase error needs to be maintained. In
>my experience we refer to first order loops as FLL's since they lock
>frequency and do not specifically control the phase; the phase depends on
>the frequency operating point.
>
>Regards,
>
>
>Steve.
>
>At 10:24 AM 5/2/2004 -0400, you wrote:
>>Steve, I disagree that a PLL with just a phase comparator and VCO is not
>>particularly useful as "something has to be done to establish the DC
>>operating point." A plain vanilla PLL will find its DC operating point just
>>fine, as long as the necessary VCO input voltage is within the range of the
>>phase detector's average output voltage. But, without the addition of the
>>second integrator, it has zero frequency error but non-zero phase error. In
>>many cases this is just fine.
>>
>>The other problem with the bare-bones PLL is that it has no filtering to
>>reduce sidebands (VCO jitter) on account of the phase detector's output,
>>which is usually an AC waveform of some variety.
>>
>>The addition of either the second integrator (making it a type II control
>>loop), and/or low-pass filtering inside the loop, are what could cause
>>closed-loop instability if not designed well, as well as the second- or
>>higher-order closed-loop response which can be somewhat oscillatory
>>depending on damping factor. We usually analyze them as if they are
>>second-order loops even if the transfer function is higher order.
>>
>>Regards,
>>Andy
>>
>>
>> > Parthal, it is the natural frequency of the typical second-order transfer
>> > function of the closed loop. Phase comparator gain is in volts / radians,
>> > while VCO gain is in radians / second / volt. Given only DC gain, the
>> > system is first order and unconditionally stable yielding
>> > V/S/V. Unfortunately, it is not particularly useful as something has to
>>be
>> > done to establish the DC operating point. In most implementations an
>> > integrator serves this purpose, adding a second order to the transfer
>>function.
>>
>>
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