Ultra low frequency VCO
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From: j on 9 Jul 2010 13:22 Resolution of noise vs frequency, (as in bw), is the issue in phase noise measurements. The OP never stated the offset from the carrier nor bandwidth. Or maybe I just missed it. Its not clear to me why JosephKK thinks this would be either a time consuming or difficult measurement to make. Assuming the appropriate measurement system is in hand 100 dBc numbers are easily achievable. Whether its 60 Hz or several GHzs the global issues are the same in making a phase noise measurement. But having said the above, without the OP responding I guess it really doesnt matter. But Id like to know more about the application and derive solutions from there.
From: Phil Hobbs on 9 Jul 2010 14:08 whit3rd wrote: > On Jul 8, 12:29 pm, Phil Hobbs > <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > >> I don't know that -100 dBc/Hz is that hard at 60 Hz. I bet you could do >> that by running a bog standard multivibrator at 1024*1024*60 Hz and >> dividing down. You'd need a sine shaper, but the phase noise goes down >> by N**2 > > Eh? I'd think it's N**0.5 (the multivibrator has cumulative but > random errors). The time jitter of the edges stays the same, but the resulting phase error goes down by a factor of N due to the division. Phase is like amplitude, so you have to square it to get the noise power--hence N**2. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: JosephKK on 10 Jul 2010 07:16 On Fri, 09 Jul 2010 11:56:28 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 7/9/2010 8:59 AM, JosephKK wrote: >> On Thu, 08 Jul 2010 15:37:28 -0400, Phil Hobbs >> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >>> Phil Hobbs wrote: >>> >>>> I don't know that -100 dBc/Hz is that hard at 60 Hz. I bet you could do >>>> that by running a bog standard multivibrator at 1024*1024*60 Hz and >>>> dividing down. You'd need a sine shaper, but the phase noise goes down >>>> by N**2, so you'd get 100 dB improvement just from that. Alternatively, >>>> you could make an LC VCO and divide that down. >>> >>> 120 dB. Can't count today. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Sure, you can mathematically "predict" it, but how do you measure it? >> Or do you switch to another metric which can be both predicted and >> measured? > >Let's keep the math bashing to the other thread, okay? > >Although it isn't highly relevant to the OP's problem, it wouldn't be >very difficult to measure the residual FM--use MOSFET buffers to drive >two divider strings running from independent power supplies, and >cross-correlate their outputs, exchanging them periodically to get rid >of the drift in the correlator. For the correlator design, see Hanbury >Brown and Twiss, circa 1963--and they did it with discrete bipolars. > >There are hard measurements, but this isn't one of them. > >Cheers > >Phil Hobbs My issue was not so much the direct difficulty of the measurement, there are several fairly straight forward setups. But with the _time_ it would take to make the measurement using many of those setups. The elapsed time seriously aggravates other measurement issues, notably including calibration.
From: JosephKK on 10 Jul 2010 07:26 On Fri, 9 Jul 2010 10:22:34 -0700 (PDT), j <jdc1789(a)gmail.com> wrote: >Resolution of noise vs frequency, (as in bw), is the issue in phase >noise measurements. The OP never stated the offset from the carrier >nor bandwidth. Or maybe I just missed it. > >Itâs not clear to me why JosephKK thinks this would be either a time >consuming or difficult measurement to make. Assuming the appropriate >measurement system is in hand 100 dBc numbers are easily achievable. >Whether itâs 60 Hz or several GHzâs the global issues are the same in >making a phase noise measurement. > >But having said the above, without the OP responding I guess it really >doesnât matter. But Iâd like to know more about the application and >derive solutions from there. OK. For a carrier of 60 MHz. Pick an instrument or test setup of your choice, state the model[s]. Clearly explain just what is going on in the measurement and the time it takes to accumulate sufficient data to make the measurement. Explain why it takes that much time to reach a reliable measurement of -100 dBc phase noise at that carrier frequency. Now see how well it scales to one million times lower fundamental frequency without a similar scaling in measurement time.
From: whit3rd on 10 Jul 2010 12:48
On Jul 9, 11:08 am, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > whit3rd wrote: > > On Jul 8, 12:29 pm, Phil Hobbs > > <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > > >> I don't know that -100 dBc/Hz is that hard at 60 Hz. I bet you could do > >> that by running a bog standard multivibrator at 1024*1024*60 Hz and > >> dividing down. You'd need a sine shaper, but the phase noise goes down > >> by N**2 > > > Eh? I'd think it's N**0.5 (the multivibrator has cumulative but > > random errors). > > The time jitter of the edges stays the same, but the resulting phase > error goes down by a factor of N due to the division. Phase is like > amplitude, so you have to square it to get the noise power--hence N**2. With an LC oscillator (class C transistor drive) the jitter in one edge (as determined by the transistor conduction) would be random, and only a small fraction of the circulating energy would respond to the edge error. So, the jitter in the LC output is a sequence of random errors. For a multivibrator, however, the internal state resets each cycle; the jittery time of cycle N becomes the new zero, and the jitter in cycle N+1 is the sum of those two values. This kind of timing error is the accumulating kind. The jitter is an arithmetic (sum) sequence of randoms. So, for an LC oscillator you can get the N**2 behavior after squaring; for a multivibrator oscillator only expect N**1. I think this is why serious timing eschews the multivibrator. |