# Tag Info

## Hot answers tagged noise-spectral-density

10

The problem in your reasoning is that you do not show the complete path of the signal. More specific the impedance level of the signal. You are right in that you cannot have both a high impedance and a low noise. If you want low noise you must keep the impedance low. Simple as that. In the two circuits you have drawn it is unclear what the impedance is of ...

9

At lower frequencies less common events become part of the signal,on scales of seconds hearbeats and footsteps on a scale of weeks there are electrical storms, on a scale of months there are seasonal effects, on a scale of years earthquakes etc... At $2.3\times10^{−18}Hz$ one must include the big bang :)

6

There is a trick to estimate noise amplitude of your display if you have a two-channel oscilloscope. First, turn off any triggering mechanism. Apply the same noise signal to both channels so that you see something like this: [ Now slide one trace closer to the other. As they approach, the dark space between the two traces gets brighter, but you still see two ...

5

The $\mathrm{W/Hz}$ may be a bit confusing as it looks like it refers to a single frequency. But that's just the dimension, it actually refers to a bandwidth, which is also expressed in Hz: maximum frequency - minimum frequency. So it's the power over a given bandwidth. If you divide power by the load's resistance you get voltage squared. So for a given ...

5

What I've been trying to promote is a more nuanced classification system. While it all can be classified as "noise" - i.e. an unwanted signal most noise specialists view things the following way: Interference: Signals from other parts of the circuitry or from outside sources that are unwanted. Best characterized by the possibility of removal through ...

5

Adding to @Kaz's answer, noise is normally modeled as a random process. Even an interfering RF carrier or hum can be modeled as a random process (random phase sinusoid). The theory of random processes is very elegant and allows for simple analysis of systems with "noise". Moreover, even deterministic components such as quantization or rounding errors are ...

4

However, dBc/Hz is the power referenced to the carrier and I'm not sure what that is in this case. I suspect the carrier in this case is the average optical power, which they may be thinking of as a many-terahertz carrier. some authors present system noise floor measurements in units of dBc/Hz. Is this wrong since in this case there's no carrier? It's ...

4

This analysis assumes that the circuit is a standard transimpedance amplifier and you are varying its gain by changing the feedback resistor value. The signal at the output of the amplifier is proportional to the feedback resistor. However, the feedback resistor also contributes its own thermal noise, which scales with the square-root of the feedback ...

4

Noise is any difference between the pure signal that is desired by the engineer, and the actual signal. Well, not any difference, but a difference not related to that signal. For example, an unwanted attenuation of some band of frequencies isn't noise, and neither is distortion. Noise does not have to be random. An unwanted periodic signal, like power line ...

4

Any quantized signal has a certain minimum quantization noise level, which is related to the number of bits per sample as you have noted. Even a delta-sigma converter cannot reduce the noise below this theoretical floor. The noise reduction (noise shaping) that is talked about in these converters relates to the fact that they use a very coarse quantizer (...

4

Noise is continuous spectrum in this case and measured here in $\mu V/Hz^{0.5}$ Thus for a given ratio, the voltage also goes to zero as Hz =>0. In practice we cannot make a band pass filter with zero bandwidth, so it is academic.

4

As you say, peak is not well defined for noise. That's why an opamp, anything really that defines noise, measures it as RMS. Look up Normal distribution, wikipedia for instance. When people want to know how noise peaks behave, they usually plot the Cummulative Distribution Function. It will give figures like (these from memory will be wrong in detail, but ...

4

The 6.6 presumably is some number related to statistical probably of the resulting peak to peak voltage When you have gaussian noise you have a picture like this: - The RMS value of the noise occurs at 1$\sigma$ (one standard deviation) and that means the signal is constrained as a peak-to peak signal to this RMS number for 68% of the time. That's not ...

4

First of all, here is a noise diode model for a PN junction from sourceforge It consists of the capacitance of the diode $c_d$, the conduction (or resistance $g_d = \frac{1}{r_d}$) and a noise current source $i_d$. The resistance is a standard thermal noise source: $S_{r_d} = 4 k T r_d$ k is the Boltsmann constant T is absolute temperature and \...

4

$Does ~f ~go \to 1/\infty ?$ unknown , unproven but close to it $\dfrac{1V}{\sqrt{Hz}} @10^{-14} Hz$ equals.... wait for it = 31,709.8 centuries .. now that's a little flicker but which century? Is this the probability of gamma wave hitting electrons out of orbit? In audio it is called "Pink Noise" and it exists everywhere in nature. The ...

3

This is undoubtedly all to do with the quality of the charger, as well, possibly your hands. You see, what a cheap 5V supply does (which is 99.9999999% of all USB chargers), is make a "floating-ish" voltage, which has only a very weak relevance to power earth. If at all. If it is a simple 2 prong adapter it may even relate the signal to one of those prongs ...

3

Too much gain. The amp is open-loop so its gain will be about 100,000. Any difference between the source voltages, even 1mv, will cause the output to clip (settle as close as it can get to one of the supply rails). Look for examples how to set the opamp gain to something more reasonable (say 100 or 1000) such as the "non-inverting" amplifier here. With Rf= ...

3

Remember that you are connecting this amplifier to a signal source, so this 200M impedance is in parallel with the source impedance. Measure the amplifier's noise with the input open circuit and you will see your predicted noise. (plus a contribution from any electric fields at the input; you may need screening to measure this properly) Measure the ...

3

If you only have DC, then you're not allowed to change it. Ever. As soon as you change it, you have a bandwidth that corresponds to the rate that you want to keep up with that change. Also, your bandwidth may include whatever frequencies you're likely to detect as unwanted noise, even if all you want is a solid reference.

3

The circuit you reference actually does work by generating white noise and then filtering it to make it pink. Capacitor C2 (according to the description in the text on the same page as the schematic) provides the -3dB/per octave low pass needed to make pink noise out of white noise - R3 and C2 together form a lowpass filter with a cutoff of around 300Hz. Q1 ...

3

I have a bunch of observations that I decided to make into an answer and please note that I'm quite happy to spend 30 minutes doing a simulation of this if someone can precisely state what the test circuit was that produced the large anti-resonant peaks. Firstly, I'm not sure that I follow the precise circuit of what was described by Ott. Are the 15 nH ...

3

As @brhans said, most of opamps become unstable in unity-gain configuration if they drive capacitive load. That is the case, when you put large 10uF C2. According to TL072 datasheet, there is 128 Ohm output resistor, which makes RC-circuit in conjuction with C2. It makes additional phase shift, which at some frequency converts negative feedback into positive,...

3

A Windows PC can do this for free with Audacity. You still need the coax cables, Faraday shield box if necessary and an AC coupled 50 ohm load. View into a scope and amplify as needed from 0.1 V to 1 Vpp Then measure for 1 minute for a smooth response as shown below. Then feed the noise into your audio input and calibrate with a sine wave. Some PC's have ...

3

You need something with low noise that can do FFT's, and can sample up to ~2x your measurement frequency. What you need depends on how much noise there is. A switchmode or a poor linear regulator is relatively easy. A low noise regulator is likely to need a low noise instrument. Below 100Hz, and especially below 10Hz, the cmos inputs of many devices will be ...

3

You're correct in thinking that the noise can be evaluated like that. The ideal L1 and C1 in the model are noiseless, all the noise originates from R1 and R2. For an inductor these resistors will have quite a low value so their noise contribution is small compared to other elements in a typical LNA circuit. I am quite sure your formula \\$\overline{v^2_n} =...

3

I want to edit @Bimplerekkie's answer, but cannot. From a noise perspective, the coil will act like the equivalent circuit schematic. This means that at very low frequencies R1 will be shorted out by the equivalent coil -- I would say that at very high frequencies it would be shorted out by the equivalent cap, but I suspect the model breaks down above ...

2

Taking a crude stab at it, the TI (Nat Semi) LMV1012 has four fixed gain configurations. The highest gain (24dB) one is likely to be the quietest (by analogy with other low noise amp designs) : its output noise level is specced at -82 dBV. Thus its noise referred to input level is: N(in) = N(out) / Gain = -82 dB - 24dB = -106 dBV = 5 uV. (Re-running for ...

2

These 2 figures have a different meaning. The 1st one is the input-referred noise density, the last one the output noise. The reason for the √ is that thermal noise power is independent from the input resistance of the amplifier which eases comparing specs. P = U²/R so in terms of density U²/(R*f). Or U/√ f if you ignore the input resistance. Therefore V/√ ...

2

TI is quoting a spec based on the total noise in the audio range (20 Hz-20 kHz, presumably), adjusted based on a standard A-weighting curve (see http://en.wikipedia.org/wiki/A-weighting for more, as well as figure 22 of the TI data sheet.) -89 dBv is what you'd see if you looked at the noise on a scope, after passing it through a filter with that particular ...

2

In addition to the above answers regarding bandwidth, there is another consideration : the noise specification shown - at least for the amplifier - is likely to be broadband noise, with some rider in the specification that it is measured at 1kHz or above. Read it more carefully... There are separate noise considerations at very low frequencies - variously ...

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