another common configuration is to use a fixed divide by N ratio then vary the DDS into the reference to move the pll. The DDS can be high frequency then post divided to limit spurs. this configuration uses a wider frequency range in the DDS so it can't be used with a crystal filter. The use of a crystal filter strips off spurs and the DDS provides the fine resolution with a bit of math required.
F6EHJ <[EMAIL PROTECTED]> wrote:
F6EHJ <[EMAIL PROTECTED]> wrote:
Hi Art,
Yes I have already played with DDS and PLL.
Using one or 3 VCO to cover the desired frequencies.
The DDS was usually an AD9850 running at low frequency (2 to 4 MHz) and
lower in my last building (500 to 503 kHz).
This low frequency is used in order to filter the DDS output with a 500
kHz AM Collins filter.
The PLL used is a MC145171.
I have no article about that, only schematics if you are interested.
The purity is very good, the phase noise is directly linked to the PLL
and the VCO.
Good shielding is essential anyway.
The main difficulty is the software - especially the last configuration
- due to the quasi constant DDS output frequency which requires a change
of the PLL's divide ration for each band to do that.
Best regards,
Gérard/F6EHJ
KY1K a écrit :
>Has any amateur ever done a DDS in conjunction with a PLL to clean up
>spurs? I hear chatter about it working well if the PLL is designed
>properly, but have never run across any real life examples of the 2
>technologies rolled into one.
>
>I understand the DDS has advantages, especially in it's ability to
>change frequency quickly. But, if rapid qsy's are not needed, I would
>think a PLL with a slow response time in the filter would be as good
>or better with respect to spectral purity than any DDS available today.
>
>Appreciate any pointers to web resources discussing this topic.
>
>Thanks,
>
>
>Art
>
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>Yahoo! Groups Links
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