Re: phase/frequency noise from voltage noise

clicliclic · 2005-07-27T06:39:08+00:00

Many thanks for the comments made so far, concerning an estimation ofthefrequency noise for this circuit (capacitance labels corrected):John LarkThe 1/f noise from channel surface defects (?) is a low-frequencyaffair;for most small-signal J-FETs it dissapears into the white channel noisearound 1kHz or 10kHz - below my f0 of 60kHz. So the question is: towhatextent does low-frequency noise modulate f0? Judging from thedependenceof f0 on supply voltage (1ppm from VCC=3D8 to 10V) this effect should bevery small. Could it still be more important than white noise from thevicinity of f0?Did I notice the millikelvin drift effects! With temperaturecoefficientsaround +200ppm/deg for my L and -200ppm/deg for my C, a 1mKdifferentialmeans a frequency shift of 1*10^-7. However, these effects were theresultof erratic air convection and have been dealt with by styrofoamencapsulation with good thermal contact between L and C. (BTW, I thinkintrinsic thermodynamic temperature fluctuations can be neglected for10^23 or so atoms.)Dave Platt wrote:Errrhhh, the order-of-magnitude numbers were just to express myignorance:I hoped they might induce someone to come up with a calculation. Theyareby no means a design goal. (Actually, I do have some suspicion wherethetrue frequency-noise level of this oscillator is.)Yes, the oscillator dissipates about 10mW, and the temperaturedifferentialresulting from the heat flow manifests itself in a turn-on drift. Ihavecompensated the temperature coefficients to about 1ppm/deg; this shouldmake ambient temperature variations unnoticeable over observationalperiodsof 10 seconds.BTW, never use polystyrene caps for temperature compensation: theirvariation from moisture uptake can easily exceed their variation withtemperature between the Antarctic and the Sahara! Use polypropyleneinstead.Just take me for a curious guy: I simply want to know how to estimatethefrequency noise of this oscillator, i.e. what the general principlesare,and what the dominant mechanism...

Jul 27, 2005 23 Replies

Rate this thread:

Ken Smith 20 years ago

In article , wrote: [...]

I think you want to look up the LSK170 "improved" version of the 2SK170 for the frequencies you are working at.

IIRC:

White noise comes in as:

Y = A(B^2 + f^2)/f^2

Where: Y is the phase noise in radians RMS at a given frequency offset B is the 3dB bandwidth of the tuned circuit f is the frequency offset from the carrier in radians A is the noise at the input to the perfect part of the FET model

-- kensmith@rahul.net forging knowledge

Vote

clicliclic 20 years ago

I knew the 2SK117 and 2SK170, but not this one. From the datasheets (mostly read from the graphs and converted, hopefully correctly):

2SK117: 15mS 2.7nV/rtHz @ 10Hz 1.1nV/rtHz @ 1kHz (0.5mA, 10V) 2SK170: 22mS 3.0nV/rtHz @ 10Hz 1.0nV/rtHz @ 1kHz (1mA, 10V) LSK170: 22mS 2.5nV/rtHz @ 10Hz 0.9nV/rtHz @ 1kHz (2mA, 10V)

I guess the LSK170 to be hard to obtain (particularly so in Europe); the

2SK117 and 2SK170 are no problem.

Ken Smith wrote:

I'm not sure how to make use of this formula; it appears to be independent of the oscillation amplitude, in contrast to my naive expectation. Let me therefore propose the following derivation of frequency noise from white amplifier noise near the oscillator frequency f0:

Assume again an observational interval dt = 10s. Coherent "pulling" action over this interval can only be expected from a narrow range of noise with f0-df/2 < f < f0+df/2. Such noise has an approximate coherence time dt =

4/dw = 2/(pi*df). Thus, for coherent action over 10s, we have df = 64mHz.

Guesstimating a white noise density of 6 nV/rtHz for my BF245A at Id=1mA, the coherent noise amplitude is B = 6nV/rtHz * sqrt(64mHz) = 1.5nV. On the other hand, my (measured) oscillation amplitude is A = 0.6V (across each of the tank capacitors).

If the noise happens to have the optimum 90deg phase for pulling, we may write

A*sin(w0*t) + B*cos(w0*t) = sqrt(A^2+B^2) * sin(w0*t+arctan(B/A)).

That is, an extra phase shift dp = arctan(B/A) = B/A = 2.5*10^-9 is introduced (for B

Vote

Ken Smith 20 years ago

[...]

You are right there is an error. "A" was supposed to be noise/signal.

[....]

How non-linear is the FET being with a 0.6V swing. Remember that frequencies near the harmonics will be mixed down to near the carrier by any non-linear action.

[..]

Colin may well be right he may also be wrong and just lucky. If you change JFETs you may be able to settle the matter.

-- kensmith@rahul.net forging knowledge

Vote

clicliclic 20 years ago

No serious objections were raised, and I consider the question settled:

Over a 10 second interval, frequency deviations df0/f0 of the order

10^10 can be expected for my simple 60kHz oscillator from each, JFET flicker noise at low frequency and JFET white noise near f0.

Thanks to everybody for having seen me through my labors with this problem,

Martin.

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required