Mike Feher wrote: > Christophe - > > While all of the mathematics to prove the 20logN behavior can be found in > most elementary texts, the concept of what is really happening may not be so > obvious from the math, even though the math of course supports the 20logN > behavior. You, yourself said it in your assumption in 2a. When a carrier is > phase modulated by a single frequency you do get the +/- sidebands about the > carrier as stated. The sidebands have two items contributing to the > sidebands location and their amplitude. Their location is due to the > frequency of modulation, and their amplitude to the deviation of that > frequency about the carrier. When doubled, the carrier is doubled but the > modulation frequency remains the same, yet the deviation is also doubled, > hence the 6 dB increase relative to the carrier of said sideband amplitude, > or the level it was before. This was a very common way to get increased > deviation in WWII military radios. They started out at a low frequency and > FM modulated it one way or another, however with a resultant low deviation. > After numerous multiplier stages to get to the final frequency, the > deviation was sufficient to recover adequate audio in the receivers > discriminator, yet the audio frequency was the same as initially injected. > Regards - Mike > > > > Mike B. Feher, N4FS > 89 Arnold Blvd. > Howell, NJ, 07731 > 732-886-5960 > > Mike Such analysis only applies when the modulation index of the multiplied signal is also small. In most cases this is true, but the above simplified analysis doesn't apply when the multiplication factor is sufficiently large that the carrier collapses. Carrier collapse is a significant problem when multiplying OCXO outputs to the TeraHertz region. The mathematical approach still works as long as previously neglected terms are taken into account. Bruce

Bruce - I am just trying to keep things practical for us. - Mike Mike B. Feher, N4FS 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 -----Original Message----- From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On Behalf Of Bruce Griffiths Sent: Wednesday, January 23, 2008 7:20 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] 20logN was Re: phase noise questions (long) Mike Feher wrote: > Christophe - > > While all of the mathematics to prove the 20logN behavior can be found in > most elementary texts, the concept of what is really happening may not be so > obvious from the math, even though the math of course supports the 20logN > behavior. You, yourself said it in your assumption in 2a. When a carrier is > phase modulated by a single frequency you do get the +/- sidebands about the > carrier as stated. The sidebands have two items contributing to the > sidebands location and their amplitude. Their location is due to the > frequency of modulation, and their amplitude to the deviation of that > frequency about the carrier. When doubled, the carrier is doubled but the > modulation frequency remains the same, yet the deviation is also doubled, > hence the 6 dB increase relative to the carrier of said sideband amplitude, > or the level it was before. This was a very common way to get increased > deviation in WWII military radios. They started out at a low frequency and > FM modulated it one way or another, however with a resultant low deviation. > After numerous multiplier stages to get to the final frequency, the > deviation was sufficient to recover adequate audio in the receivers > discriminator, yet the audio frequency was the same as initially injected. > Regards - Mike > > > > Mike B. Feher, N4FS > 89 Arnold Blvd. > Howell, NJ, 07731 > 732-886-5960 > > Mike Such analysis only applies when the modulation index of the multiplied signal is also small. In most cases this is true, but the above simplified analysis doesn't apply when the multiplication factor is sufficiently large that the carrier collapses. Carrier collapse is a significant problem when multiplying OCXO outputs to the TeraHertz region. The mathematical approach still works as long as previously neglected terms are taken into account. Bruce _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.

Mike Feher wrote: > Bruce - > > I am just trying to keep things practical for us. - Mike > > > > Mike B. Feher, N4FS > 89 Arnold Blvd. > Howell, NJ, 07731 > 732-886-5960 > Mike I realise that, but some caveats are necessary, otherwise someone will try locking a low frequency VCO to the rubidium resonance at (6.83?) GHz by using a frequency multiplier chain to generate a 6.83GHz carrier. They will then wonder why it doesnt work, not realising the frequency multiplied carrier may have collapsed. If one is aware of the carrier collapse phenomenon, then the required VCO phase noise spec to avoid it can be worked out beforehand. In most other cases the simplified analysis works well. Bruce

All, I like everybodies so answers so far, as it helps to form a more thorough understanding... I ll revist the math over the weekend. Thanks for yr time. Another unrelated topic, I seem to get emails from time-nuts only sporadically, and with sender time-nuts-bounces. However I don t have any bounce status... Does anybody else see this phenomenon. It is a bit irritating to have to go to the archive page to read up. Thanks, Christophe Christophe Huygens wrote: > Hi John, Steve, et al, > > While I am not a phase noise buff at all, in talking to many on this > subject > I feel that this is not well understood. When I ask where the 6db/Hz for > doubling or 20logN in general comes from, I very often get an > unsatisfying > answer and I have seen strange notes on this mailing list on this > subject as > well... > > For me to understand what happens in a simplified way 2 things are key: > > 1. Phase noise is subject to FM theory - you can think of the carrier > being FM modulated with a very low modulation index, with > modulation frequency the offset from the carrier. This is easy > enough to accept for most. The noise phasor sits on top of the carrier. > This give amplitude noise, that can be limited away, and well... > phase noise. The actual modulation index in our case is always > very small I guess, except when you looking real close to the > carrier, but then still - if the oscillator is good, the deviation will > still be small hence low modulation index theory still applies.. > > 2. What happens with an FM signal when applied to an ideal doubler - > this is a bit of a trickier. Say I have a narrowband (low modulation > index) signal of 200Hz, modulated by 20Hz. > > a. The spectrum is: > sideband 1 (180) - carrier (200) - sideband 2 (220). > AFTER the doubler the spectrum is: > sideband 1 (380) - carrier (400) - sideband 2 (420). > > I have a hard time to find an intuitive explanation for this, > but it only takes 20 lines of octave/matlab code to verify... > I am getting too old (or I m too young) to get into the Bessel > functions myself. > > So no need to multiply the offset also by N as sometimes seen. > > b. The amplitude of the sidebands does grow with respect to > the carrier (all this for small modulation indexes) by about > 6 db. Also easy to show in a a simulation. > > The duality of multiplication in the time domain and convolution > in the frequency domain also explains this I think, like it > can explain a. > > Maybe somebody on the list can step in and give a clear and > concise explanation for the above. > > > > Christophe > > > > > > > > > > > > > > John Miles wrote: >> Doubling your clock frequency adds 6 dBc/Hz to whatever the noise >> level was >> at the input, at all offsets within the doubler's bandwidth. Only if >> the >> input noise level is near or below the multiplier's own residual >> noise floor >> will the increase be worse than 6 dBc/Hz. >> >> That will not happen when ordinary crystal oscillators and conventional >> Schottky-diode multipliers are used together; high-performance active >> multipliers are needed only when working with exceptionally clean >> inputs. >> At input noise levels higher than -155 to -160 dBc/Hz, ordinary diode >> multipliers will not usually contribute any additional noise. >> >> -- john, KE5FX >> >> >> >>> Hello, >>> >>> I followed with some interest a discussion about a NIST doubler circuit >>> using matched FET's and I was wondering if you could get similar >>> results >>> using an analog multiplier chip from Analog Devices. It would seem that >>> they take some care about device matching and have parts that work up >>> to pretty high frequencies. Of course there would need to be some >>> filtering >>> employed. Oh, and I think those parts do pretty well with temperature. >>> >>> Also, when using a doubler that is rated in dBc how do you apply that >>> number to get an expectation from a given starting dBc oscillator. So >>> if my 10 MHz clock is -125dBc and I use the NIST circuit, what would >>> I see at 20 MHz in dBc? >>> >>> thanks in advance, >>> steve >>> >>> >>> > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.

Christophe Huygens said the following on 01/24/2008 04:55 AM: > Another unrelated topic, I seem to get emails from time-nuts only > sporadically, and with sender time-nuts-bounces. However I > don t have any bounce status... Does anybody else see this > phenomenon. It is a bit irritating to have to go to the archive > page to read up. time-nuts-bounces is the sender for all messages on the list, so that's not an issue. If you're only getting messages sporadically, I'd check your mail system and especially its spam trap; I haven't had any reports of problems. The list system (Mailman) does have fairly sophisticated bounce detection and removal algorithms, and if too many messages from a subscriber bounce, they will be automatically unsubscribed. But short of unsubscription, there's nothing other than normal mail system issues that would cause some messages not to make it to an individual subscriber. John

The following paper on the symmetry and correlation between the USB and LSB components of PM and AM may be of interest: http://tf.nist.gov/timefreq/general/pdf/1326.pdf Where among other results it is shown that a phase detector has the same response to coherent PM sidebands and PM noise sidebands. Bruce

The following paper details a method used by NIST for PM and AM noise measurement system calibration. http://tf.nist.gov/timefreq/general/pdf/1000.pdf An accuracy of 0.14dB or better is possible. Unlike the beat frequency method a phase detector with a sinusoidal response isnt required. The effect of the PLL is also accounted for, this is particularly important at low frequency offsets. Bruce

On Sat, 26 Jan 2008, Bruce Griffiths wrote: > The following paper on the symmetry and correlation between the USB and > LSB components of PM and AM may be of interest: > > http://tf.nist.gov/timefreq/general/pdf/1326.pdf > > Where among other results it is shown that a phase detector has the same > response to coherent PM sidebands and PM noise sidebands. > > Bruce As seen in the acknowledgements, it was I who initially proposed this idea to Fred Walls. We exchanged letters and phone calls then I visited him in Boulder to discuss the measurements. Jeffrey Pawlan

Bruce Griffiths wrote: > The following paper on the symmetry and correlation between the USB and > LSB components of PM and AM may be of interest: > > http://tf.nist.gov/timefreq/general/pdf/1326.pdf > > Where among other results it is shown that a phase detector has the same > response to coherent PM sidebands and PM noise sidebands. > > Bruce > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > Yet another paper on USB and LSB symmetry: http://tf.nist.gov/timefreq/general/pdf/1249.pdf Bruce