DDS purity

[Andre Kesteloot]

Andrew Talbot wrote:

> In my latest LF Receiver project, I use a 9850 DDS clocked from a low phase
> noise 10MHz source, to directly generate the LO of 455 to 605kHz for mixing
> 0 - 150 up to 455kHz IF.  The DDS filter is a customised Chebyshev design
> passing just the wanted band and, together with the DDS overclocking, means
> there are no spurious products at frequencies that could cause problems with
> the frequency upconversion plan.
>
> The image in the MF broadcast band (910 - 1060kHz) my main real design
> problem, and that was overcome with a high order LPF on the input giving
> over 100dB rejection in this region.  When used with a resonant antenna for
> LF, this rejection level proved sufficient to make image signals inaudible.
> I can bring this Rx along to the HF Convention.
>
> One use for a DDS I intend to look at shortly, is using the image response
> for multiplying up to microwaves.  By clocking an AD9851 at 150MHz, and set
> for 42MHz output, but instead of the normal Low Pass FIlter, use a bandpass
> filter at the first image response of Fclock - Fout, ie 108MHz in this case.
> Then by multiplying by 96 in a conventional multiplier stage that will give
> a signal at 10368MHz, with a very low phase noise determined only by that of
> the clock oscillator multiplied up.  A good quality VHF Butler oscillator as
> used on uWaves is more than adequate for that.
>
> Andy  G4JNT
>
> > -----Original Message-----
> > From: Alan Melia [mailto:Alan.Melia@btinternet.com]
> > Sent: 2001-09-25 11:22
> > To: LF-Group
> > Subject: LF: re DDS purity
> >
> >
> > Hi Andy thanks for the comment and resoring my faith. I was a little
> > confused after the discussion about using a PLL to 'clean a
> > DDS up' earlier
> > this year and a coment from Dan Tayloe on the hoembrew
> > newsgroup which I
> > past for interest.
> > -----excerpt--------------------------------
> > DDSs are kind of dirty as they are really designed for transmitters
> > (certainly clean enough for that) or for cellular phone reception for
> > signals of with limited dynamic range (such as a cellular basestation
> > using power control of all the phones in its area).
> >
> > --------------------------------
> > He is talking about what is virtually a DC receiver, using a
> > DDS as the LO.
> >
> > Cheers de Alan G3NYK
> > Alan.Melia@btinternet.com
> >
> >