Re: PLL Lock to an Offset Frequency

On Fri, 14 Oct 2005 16:00:06 -0700, Jim Thompson
<thegreatone@xxxxxxxxxxx> wrote:

>On Fri, 14 Oct 2005 18:39:50 -0400, Phil Hobbs
><pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
>[snip]
>>
>>Perhaps we were starting from different points.
>>
>>Instead of using a SSB mixer, use an offset PLL as suggested by the OP,
>>running at 1:1. This is an alternative to the 64x PLL with the 125-kHz
>>reference frequency, that avoids the phase noise penalty of frequency
>>multiplication (in this case, 20*log(64) or 36 dB). Something like this:
>>
>> USB/LSB----------------------------------------------+
>> |
>> |
>> |
>> 125 kHz ref--------------------------------+ |
>> | |
>> _ _V__ _ _V__
>>8 MHz carrier------\\---\ RC=700 ns | | | X |
>> )) >---RRRR---+--->| PFD |---| +-1|---------+
>> +-//---/ | |_____| |____| |
>> | R
>> | (xor) CCC R
>> | | +-CCC--RRR--+ R
>> | GND ___ | /| | |
>> | / \ | /-|----+---+
>> +-----------<----------| VCO |<--+---< |
>> | | \+|----+
>> \___/ \| |
>> GND
>>
>>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. It would be almost
>>as cheap to generate all three signals and switch the outputs with a mux.
>>
>>Thus instead of needing to use fancy things like SSB mixers, narrow
>>filters, or 500 MHz divide-by-Ns, or accepting the 36-dB phase noise
>>penalty of multiplication, you can do it all with a PFD, one or two op
>>amps, and a few gates, none of which will need any adjustment. This is
>>all a consequence of the nice phase properties of offset PLLs, as
>>described previously. Which works the best depends on the VCO design
>>and other constraints--e.g. an RC VCO is probably noisy enough that the
>>36-dB phase noise added to the reference signal is a drop in the bucket.
>>
>>
>>Cheers,
>>
>>Phil Hobbs
>
>OK. Now I follow you. Somehow I missed the "offset loop".
>
>I like it!
>
>I've seen similar things done using a D-Flop as a harmonic mixer.
>
> ...Jim Thompson

Phil,

The more I study your scheme the more I like it! I have a 1056MHz ±
132MHz situation that would be a perfect application of your
configuration, because it's clearly superior for high ratios of
carrier to offset.

...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
    ... > Instead of using a SSB mixer, use an offset PLL as suggested by the OP, ... that avoids the phase noise penalty of frequency ... > phase noise added to the reference signal is a drop in the bucket. ...
    (sci.electronics.design)
  • Re: PLL Lock to an Offset Frequency
    ... >Instead of using a SSB mixer, use an offset PLL as suggested by the OP, ... >reference frequency, that avoids the phase noise penalty of frequency ... >a lowpass filter on the loop control voltage--using all jellybean parts ...
    (sci.electronics.design)
  • 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)
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