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:[email protected]]
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
[email protected]
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