Re: PLL Lock to an Offset Frequency

On Fri, 14 Oct 2005 15:07:11 -0400, Phil Hobbs
<pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>Jim Thompson wrote:
>> 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
>
>Jim, with all due respect, you need to think about what I wrote in my
>original post. The sign of the loop gain is opposite for USB and LSB
>for a given PD null, i.e. if you use an XOR or a diode mixer whose nulls
>are at quadrature, if the loop wants to lock up at +pi/2 on USB, it'll
>lock up at -pi/2 on LSB. With a PFD, one sideband will have the right
>sign of loop gain to lock up at 0, where (as you point out) everything
>is copacetic.

Aha! Now I see our miscommunication. You said, "If you use a
**frequency-phase detector** or other very asymmetric PD". An XOR is
NOT a PFD (phase-frequency detector, AKA edge-matcher), it's a PD
(phase detector, AKA MIXER). PFD's are used in synthesizers. I am
co-inventor of the first PFD, the Motorola MC4044.

>
>The point I was making in my original post is that the other sideband
>will have to try locking up at +- pi, where there's a ruddy great
>cliff--its PD gain there is like Vdd /2*pi*(f_0*t_PD), i.e. something
>like 500 times larger than the other null. Of course it's noisy and
>possibly metastable there, but the point is that the loop gain is
>_huge_, so no lock will occur there. Thus with a tiny bit of
>acquisition aiding, e.g. 2 resistors and a cap in positive FB around the
> loop amplifier, you can make a reliable lock to one sideband and not
>the other, _without_needing_a_SSB_mixer_.
>
>Cheers,
>
>Phil Hobbs

Huh? The OP wanted to **generate** either an upper or lower sideband.
An XOR mixer isn't going to do that.

...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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Relevant Pages

  • Re: PLL Lock to an Offset Frequency
    ... This is an alternative to the 64x PLL with the 125-kHz reference frequency, that avoids the phase noise penalty of frequency ... The nice feature of a correctly-designed offset PLL, using a PFD, is that you can get rid of the other sideband to any accuracy you like with a lowpass filter on the loop control voltage--using all jellybean parts zero inductors, and zero tweaks. ... You can choose USB or LSB by switching the sign of the loop gain, e.g. by swapping PFD outputs, negating them with an extra XOR each, or using a +-unity gain amp. ...
    (sci.electronics.design)
  • Re: PLL Lock to an Offset Frequency
    ... I want to design a PLL which must lock to a frequncy which is a bit different from input frequency. ... don't even have to use an SSB mixer, because the unwanted null is unstable. ... one null is unstable because of the sign of the loop gain. ...
    (sci.electronics.design)
  • Re: PLL Lock to an Offset Frequency
    ... I want to design a PLL which must lock to a frequncy which is a bit different from input frequency. ... On each sideband, one null is unstable because of the sign of the loop gain. ...
    (sci.electronics.design)
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