Ultra low frequency VCO

Phase noise and frequency accuracy are two different parameters. For example it=92s possible to have a phase noise spec of great than 100 db=92s at x number of hz of the carrier and have over 10KHz or more of frequency in accuracy. Jitter is the parameter that has a direct correlation to phase noise.

Is the goal of the project to lock to 60 Hz line,(it's the ref for the loop), and provide a clean 60 Hz out?

That's really the question, too many times people want a solution when they can't define the problem. He thinks he needs XXX, but we can't know until we know what he wants to do with it.

My guess was that he wants to feed something back into the AC line, such as excess power from solar panels, and hence needs it to match the existing 60Hz power grid. That is just a guess, but obviously that actually defines the problem rather than the specs.

One thing, since he actually needs 60Hz rather than a wide range of frequency, he actually is in better shape than some of the responses suggest. There is a big difference between the best choice being LC circuits at that "low" a frequency and having a wide frequency shift, and starting with a crystal controlled oscillator and just enough frequency control to ensure it is really at 60Hz. Since one knows exactly what frequency one needs, you can start with something that is highly on frequency as it is, and then just enough control to bump it exactly where it belongs.

Michael

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Yeah, as said, unfortunately so many of these questions are so poorly framed, it=92s almost impossible to give any meaningful help.

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What is done at RF of 60 MHz, 600 MHz, or 6 GHz to make the measurement easier, does not make it faster. Sure, it takes a second or so at 600 MHz for 100 dBc. And a megasecond or so at 60 Hz to measure it in the RF traditional way. Simple scaling. That is why i recommended changing to jitter and wander specifications, which you have a decent shot at measuring in an hour or so.

Or maybe you are on to something, but for my and OPs sake, please flesh it out a lot more, with some calcs please.

=20

=20

Who needs to calculate? Mix it down to DC in quadrature. You'll have naught but a residual at DC, and all the phase noise will appear around DC (amplitude noise will be nulled out, 'cause it's in quadrature).

Now amplify the snot out of it with a really low noise, low corner frequency AC-coupled amp, and apply it to a really low frequency spectrum analyzer (you may have to use a dynamic signal analyzer to see anything useful, I dunno).

You'll only need to measure for hours if you want to resolve frequencies in the mHz (that's m for milli, not M for mega).

Note that you could get total noise by notching out the carrier, but if you just want _phase_ noise you'll be screwing your measurement with amplitude crap.

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.

You might even be able to do it with all analog--the OPA378 has 20 nV/sqrt(Hz) all the way down to DC. With a 5V sine wave at 60 Hz, that's something like 1800 V/s, so 20 nV gives you something like 10 picoseconds per root hertz. You probably lose a factor of sqrt(2) in there, but that ought to be good enough. Your ALC network would contribute more than that, almost for sure.

Cheers

Phil Hobbs

Cheers

Phil Hobbs

do=20

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Alternatively,=20

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?

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And that would be on the order of 30 dBc measurement of phase noise? How long do you have to measure to get to 100 dBc phase noise? Or do you claim that is already 60 dBc phase noise measurement, then how many days/weeks to get to 100 dBc phase noise measurement?

Minus 100 dBc phase noise is an inappropriate measurement for the fundamental frequency involved. Jitter and wander are more to the point.

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

Eh? I'd think it's N**0.5 (the multivibrator has cumulative but random errors).

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.

It=92s 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 it=92s 60 Hz or several GHz=92s 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 doesn=92t matter. But I=92d like to know more about the application and derive solutions from there.

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

could do

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Alternatively,

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.

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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.

d do

own

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.

Hey Phil! How come no comment on conservation of charge and energy? You have a dog in this show ?:-) Weenie! ...Jim Thompson

It's the modulation frequency that's relevant, not the carrier frequency. Measurements get slower when you reduce the bandwidth.

(You can see why this doesn't work if you imagine running it backwards--mixing or multiplying up to some very high frequency doesn't allow you to make a measurement with 1 Hz bandwidth any faster.

Cheers

Phil Hobbs

Modulation frequency isn't affected by heterodyning or frequency multiplication and division. If you take a 60 MHz sine wave and FM it at 1 Hz modulation frequency and 1 MHz peak frequency deviation (M=1E6), then divide it by a million, you get a 60-Hz sine wave modulated at 1 Hz with a 1-Hz peak frequency division (M=1).

Cheers

Phil Hobbs