Hi The only "gotcha" is that in the upper octave, the DDS can't produce falling edges that are 90 degrees apart. It can indeed produce sampled waveforms that will eventually re-construct to the needed signals. Filtering prior to slicing is important.
------------------------------- With no filtering a DDS will indeed produce a *lot* of output frequencies. With a 75 MHz clock, tuned to 30 MHz you also get (for no added charge) 45, 105, 120 MHz and so on. I confess to having feed the first one I built into a spectrum analyzer to check that out. Feed the full output, unfiltered into something like a diode ring mixer and you will down convert them all pretty much equally well. Bob On Jan 24, 2010, at 8:26 PM, ni9n wrote: > > You guys had me worried for a few minutes, there! Had to test it on 10 meters > just to make sure. The two synched AD9834's do indeed produce signlas 90 > degrees apart, even at frequencies above 1/4 of the master clock. > > I think some of you are having a little trouble understanding bandpass > sampling. Nyquist doesn't really say the signal has to be bandlimited to less > than half the sample rate, that's just the case for a "baseband" or lowpass > signal, such as sampled audio. If you sample a bandpass signal, the signal > must have BANDWIDTH less than the sample rate. > > If a baseband voice signal is limited to the usual 3 kHz (that is, no energy > to speak of above 3 kHz) it has a two-sided spectrum from -3 kHz to +3 kHz. > When it is translated up to a carrier (suppressed or not) frequency of, say, > 10 MHz as double sideband, it has a bandwidth of 6 kHz because both sides of > the two-sided spectrum are translated together. If you pass the RF through a > brick-wall filter centered at 10 MHz and having a bandwidth of 6 kHz, you can > use a sample rate of 10 kHz and theoretically lose nothing. This is because > the bandpass antialiasing filter (that is, the brick wall filter) rejects the > image responses at 9.990 and 10.010 MHz. As a matter of fact, without the > bandpass filter you would have an image response every 10 kHz, but the > bandpass filter rejects all the unwanted ones. > > The symmetry of the sampling waveforms (the I and Q LOs) does not matter, > their duty cycle can be anything you like. All that matters is that the > falling edges are 90 degrees apart. > > By the way, what does the "AW:AW:AW:..." mean? > > --- In [email protected], Bob Camp <li...@...> wrote: > > > > Hi > > > > A full wave rectifier can also be used as a simple broad band doubler. > > > > The problem with the output of the DDS is that without filtering it is far > > from symmetrical in the upper octave. Some use bandpass filtering to get > > around the problem, but that usually is not an option on a wide band radio. > > > > Bob > > > > > > On Jan 24, 2010, at 5:08 PM, Rotten Robbie wrote: > > > > > For this application Nyquist does not matter. It is just used as a > > > clock.And > > > what you need for the detector is a square(rectangular) waveform. > > > > > > If the waveform is reasonably symetrical you can use both edges for > > > frequency doubling. It just takes a few inverters for a time delay and an > > > exclusive OR. > > > > > > Bob Macklin > > > K5MYJ > > > Seattle, Wa. > > > "Real Radios Glow In The Dark" > > > ----- Original Message ----- > > > From: "Dave Wade" <g4...@...> > > > To: <[email protected]> > > > Sent: Sunday, January 24, 2010 1:56 PM > > > Subject: Re: AW: AW: AW: AW: [soft_radio] Re: LD-1 Discussion on > > > Garage-shoppe.com Blog > > > > > > > > > > > > > > > k5nwa wrote: > > > >> At 12:04 PM 1/24/2010, you wrote: > > > >>> > > > >>> Thanks for your explanations, Bob! > > > >>> > > > >>> I understand the principles of driving the mixer with a squarewave > > > >>> signal and that you use a comparator to change sine to squarewave. > > > >>> > > > >>> Where I'm failing at is the trick how to receive 30 mhz with a DDS > > > >>> that make 37 MHz max without getting problems with nyquist. Can you > > > >>> use a comparator to double your VCO frequency without getting > > > >>> problems at the duty cycle? > > > >>> > > > >>> > > > >>> Stephan > > > >>> > > > >> > > > >> This is a mixer, you are not trying to digitize the 30MHz signal > > > >> where you would need a minimum of two samples, instead you are mixing > > > >> it down to the base band. > > > > > > > > No but the DDS chip is synthesising a sine wave from samples, and > > > > Nyquist bite both ways. However some DDS chips (I don't know about this > > > > one have on-board frequency multipliers... > > > > > > > > > > > >> > > > >> > > > >> Cecil > > > >> k5nwa > > > >> www.softrockradio.org www.qrpradio.com > > > >> < http://parts.softrockradio.org/ > > > > >> > > > >> Never take life seriously. Nobody gets out alive anyway. > > > >> > > > >> > > > >> > > > >> ------------------------------------ > > > >> > > > >> Yahoo! Groups Links > > > >> > > > >> > > > >> > > > >> > > > > > > > > > > > > ------------------------------------ > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > > > > > > > > > >
