On Thu, 27 Sep 2012 08:29:44 -0700, nick cake wrote: (top posting Fixed. Again)> On Thursday, September 27, 2012 3:31:33 AM UTC-4, mnentwig wrote: >> Hi, >> >> >> >> a typical phase noise spectrum may indeed appear in "different shades >> of >> >> pink" , for example between 50k and 50M in a cellphone synthesizer, to >> give >> >> some order-of-magnitude numbers. >> >> >> >> But I meant something different: >> >> >> >> Assume I start with a high-frequency signal, say, 4 GHz. There is a >> given >> >> time jitter on each edge, say 10 ps, in a 250 ps cycle duration. >> >> >> >> Now I divide the signal by 4. I've still got 10 ps jitter, but the >> cycle >> >> duration is now four times as long, 1000 ps. >> >> >> >> Now I convert 10 ps to an angle, relative to the cycle durations 250 ps >> and >> >> 1000 ps. >> >> At 4 GHz, it's equivalent to 14.4 degrees, but at 1 GHz only 3.6 >> degrees. >> >> Effectively, the phase noise (in units of degrees!) is cut in half (-6 >> dB ) >> >> with every division-by-2. >> >> >> >> If the phase _angle_ matters, such as when multiplying with a local >> >> oscillator, this really improves performance. >> >> But it doesn't help if the accuracy of a sampling process is in >> question >> >> (where the error manifests itself in units of seconds, not degrees). >> The >> >> problem here is that the sampled signal moves too quickly, relative to >> the >> >> accuracy of the sampling clock. >> >> >> >> Sampling clock and signal are like Indians and cowboys in an old >> western >> >> movie: Either I run my horse alongside the other guy at about the same >> >> speed. Or, I need to shoot very accurately :o) >> >> >> >> -markus > Hi Markus, > > That's a very vivid analogy and I begin to understand why jitter of Fs, > which for me can be very low compares to the Fc(330MHz), matters to my > phase measurement accuracy. > > But again, after undersampling, if the aliased carrier signal is at 1kHz > for example, my phase accuracy needs to be 1 degree RMS for example, > then the jitter RMS in my Fs needs to be 0.001/360 second, am I right? >The phase jitter from sampling your 330MHz signal will come from the timing jitter of your sampling clock relative to the 330MHz -- not relative to whatever the final signal frequency is. You can figure this out by plotting a sine wave on graph paper, and figuring out the phase vs. time relationship. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Is it possible to use very large undersampling factor (using 5.1kHz to sample 330MHz)?
Started by ●September 23, 2012
Reply by ●September 27, 20122012-09-27
Reply by ●September 27, 20122012-09-27
On Thu, 27 Sep 2012 08:33:25 -0700, nick cake wrote:> On Thursday, September 27, 2012 3:31:33 AM UTC-4, mnentwig wrote: >> Hi, >> >> >> >> a typical phase noise spectrum may indeed appear in "different shades >> of >> >> pink" , for example between 50k and 50M in a cellphone synthesizer, to >> give >> >> some order-of-magnitude numbers. >> >> >> >> But I meant something different: >> >> >> >> Assume I start with a high-frequency signal, say, 4 GHz. There is a >> given >> >> time jitter on each edge, say 10 ps, in a 250 ps cycle duration. >> >> >> >> Now I divide the signal by 4. I've still got 10 ps jitter, but the >> cycle >> >> duration is now four times as long, 1000 ps. >> >> >> >> Now I convert 10 ps to an angle, relative to the cycle durations 250 ps >> and >> >> 1000 ps. >> >> At 4 GHz, it's equivalent to 14.4 degrees, but at 1 GHz only 3.6 >> degrees. >> >> Effectively, the phase noise (in units of degrees!) is cut in half (-6 >> dB ) >> >> with every division-by-2. >> >> >> >> If the phase _angle_ matters, such as when multiplying with a local >> >> oscillator, this really improves performance. >> >> But it doesn't help if the accuracy of a sampling process is in >> question >> >> (where the error manifests itself in units of seconds, not degrees). >> The >> >> problem here is that the sampled signal moves too quickly, relative to >> the >> >> accuracy of the sampling clock. >> >> >> >> Sampling clock and signal are like Indians and cowboys in an old >> western >> >> movie: Either I run my horse alongside the other guy at about the same >> >> speed. Or, I need to shoot very accurately :o) >> >> >> >> -markus(top posting FIXED)> Also what if everything is from the same source, i.e. the 330MHz and my > ADC sampling clock are all generated from the same osc? Will this make a > difference that I don't need to worry the jitter in my Fs? The jitter > spec is only derived from the jitter/phase noise requirement on the > generated 330MHz?You always need to worry about jitter in your sampling. If you derive everything from the same source then you still need to worry about jitter in whatever circuit you use to derive the sampling clock, and you need to worry about sampling jitter within the ADC itself. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●September 27, 20122012-09-27
>> if the aliased carrier signal is at 1kHz for exampleyes, but only if your aliased carrier signal has been "aliased" using a phase noise-free sampling clock :-)
Reply by ●September 28, 20122012-09-28
Totally crazy. You'll only convert noise, and some signal burried into it. My first thought if it comes from the air. Seems fast ADC are kind of too cheap nowadays. filter somewhat in the analogue domain and heterodyne. On Sunday, September 23, 2012 11:15:04 PM UTC+2, nick cake wrote:> Hi sampling gurus, > > > > I'm thinking about undersampling a 150Hz AM signal modulated to a 330MHz carrier, thus the signal bandwidth is 300Hz. I found an ADC has 600MHz analogue bandwidth, and minimum Fs to be 5.1kHz. Suppose the bandpass filter is sharp enough for my my signal at 330MHz, if I use 5kHz to sample the 330MHz, will there be any problem? > > > > One of my colleagues thinks there's a sinc roll-off associates with undersampling thus one cannot use very large undersampling factor. This is true for stealing higher harmonics out of a low Fs driven DAC, but I don't think this is true for ADC. > > > > As I understand, the ADC can be modeled as: > > 1. Sampling: multiply input continuous signal with a series of Diract implues > > 2. Hold： time domain convolution with a rect window, whose width is maximally Ts (and whose freq domain is a sinc, with first null at Fs) > > 3. A-to-D: convert the held stable voltage to digital output using proper coding > > > > The aliasing effect due to undersampling happens in the 1st step, thus the 330MHz has already been "down-converted" to baseband to 601.2Hz, and my two AM bands will be at 451.2 and 751.2 respectively. Then the hold operation simply "mask" the frequency spectrum by a sinc shape and my 601.2Hz signal will be almost intact. > > > > Thus I won't need to do large factor decimation/filtering and save a lot of FPGA resources. > > > > Is this practically feasible? I asked a couple of engineers and they are not very sure..
Reply by ●November 18, 20122012-11-18
On Mon, 24 Sep 2012 22:14:53 +0000 (UTC), glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:>nick cake <nickcake@gmail.com> wrote: > >> I'm thinking about undersampling a 150Hz AM signal modulated to a >> 330MHz carrier, thus the signal bandwidth is 300Hz. >> I found an ADC has 600MHz analogue bandwidth, and minimum Fs to >> be 5.1kHz. Suppose the bandpass filter is sharp enough for my >> my signal at 330MHz, if I use 5kHz to sample the 330MHz, >> will there be any problem? > >Never having actually designed one, wouldn't it be usual to >mix down to a lower frequency for filtering and sampling? > >If needed, I think the LO could be phase locked to some >reference such that phase information was retained. > >-- glenHi glen, I'm with you. nick cake should use analog mixing to translate his 300 Hz signal down toward zero Hz, perform some reasonable analog filtering, *BEFORE* applying any signal to an A/D converter. One of the main problems with such a horribly high sample rate is that *ALL* of the spectral energy, including spectral noise, from the zero Hz to 330 Mhz analog signal will fold down and be contained in his 5 kHz A/D output freq range. It seems to me that nick would need a *VERY* narrowband analog bandpass filter (centered at 330 Mhz) that would be impossible to build. [-Rick-]
Reply by ●December 7, 20122012-12-07
