Re: PLL Lock to an Offset Frequency

On Fri, 14 Oct 2005 13:59:36 -0400, Phil Hobbs
<pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>Jim Thompson wrote:
>> On Fri, 14 Oct 2005 11:49:51 -0400, Phil Hobbs
>> <pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>
>>
>>>Jim Thompson wrote:
>>>
>>>>On Fri, 14 Oct 2005 06:15:08 -0500, "hashemi7102"
>>>><novinmadar@xxxxxxxxxxx> wrote:
>>>>
>>>>
>>>>
>>>>>Hi
>>>>>I want to design a PLL which must lock to a
>>>>>frequncy which is a bit different from input frequency.
>>>>>For example input Frequncy is 8MHz and PLL must synthisize
>>>>>a 8.125.000Hz or 7.875.000Hz (8MHz+- 125KHz) output signal.
>>>>>
>>>>>
>>>>>How can i do this?
>>>>>
>>>>>Hanse
>>>>>Hashemi
>>>>>
>>>>>
>>>>
>>>>
>>>>Divide 8MHz by 64 for reference input (125KHz) then use div63 or div65
>>>>in the feedback.
>>>>
>>>>OR:
>>>>
>>>>Use a single-sideband mixer to mix 125KHz with 8MHz.
>>>>
>>>> ...Jim Thompson
>>>
>>>If you use a frequency-phase detector or other very asymmetric PD,
>>
>>
>> Huh? A PFD isn't asymmetric.
>>
>> Reread my first example: 65/64*8MHz = 8.125MHz, 63/64*8MHz = 7.875MHz
>>
>>
>>
>>
>>>you
>>>don't even have to use an SSB mixer, because the unwanted null is
>>>unstable. On each sideband, one null is unstable because of the sign of
>>>the loop gain. On both sidebands, the null at +- pi is unstable because
>>>of huge loop gain. Only one of the four nulls is stable.
>>>
>>>Cheers,
>>>
>>>Phil Hobbs
>>
>>
>> My second example, elaborating: 8MHz/64 = 125KHz, then MIX in an SSB
>> mixer to get the +/- desired. No PLL needed, just some phase shifters
>> ;-)
>>
>> ...Jim Thompson
>Of course it's asymmetric. The voltage vs phase plot is a sawtooth.
>
>Cheers,
>
>Phil Hobbs

Not on either side of zero phase it isn't... it's linear thru zero.
With a PFD you could care less about ±pi.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

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