Definition of amp noise

Hi,

Could someone so kindly tell me exactly what 1.3nV / Sqrt(Hz) @ 1KHz means on a differential amp spec? What would the noise be between

37.00 Hz and 37.01 Hz? Would it be 1.3nV / Sqrt(37.00 - 37.01) or 1.3nV / Sqrt(37)

Thanks, Paul

Reply to
pmlonline
Loading thread data ...

Sorry, that should have been 1.3nV * Sqrt(37.00 - 37.01) or 1.3nV * Sqrt(37)

Reply to
pmlonline

That's 1.3nV times sqrt 0.01Hz. But one has to ask how the 0.01Hz bandwidth is obtained; it's likely that other factors are involved.

--
 Thanks,
    - Win
Reply to
Winfield Hill

Linear Technology has some outstanding tutorials on this. Note that op amp noise depends very strongly on frequency band and source impedance.

Bottom line: 1.3 is "pretty good" but not spectacular.

--
Many thanks,

Don Lancaster
 Click to see the full signature
Reply to
Don Lancaster

Source impedance, very interesting. Thanks for the information Don. I'll have to read up on that. I'll be dealing with high impedance.

Paul

Reply to
pmlonline

Thanks, it makes sense. So at 0.01 Hz bandwidth it comes to 130pV. Where does the noise come from? Is it purely thermal or just a byproduct of the amplification process?

Here's my problem. I am trying to get a rough guestimate of the values to see if it is even plausible. I am estimating that a signal will be picking up ~ 1pV of RF at 37Hz on a coil. I guess it would take a differential amplifier fed into another differential amplifier. If the gain of each amplifier is 1E+6 than that's a total gain of 1E+12. Wow! The device will be out and away from the city and will contain a second nearby coil to cancel as much RF noise. So there would be two coils-- the RF coil and the noise cancellation coil. I believe that if the output from noise cancellation coil is properly adjusted / calibrated that it can cancel out the RF noise from the output coil. So if we can get the RF noise low enough then what is left? Thermal noise? Also I would know the phase of the 37Hz signal. Could that be an aid? The end goal is to design a device that can tell if the 37Hz

1pV signal is on or off. Is this doable? I think time is on our side. That is, if we build a filter with high enough Q then it could detect the signal. The problem is that the Q would probably have to be so high that it may take a long time for the signal to rise above the noise. Hopefully some clever math technique, possibly taking advantage of the known signal phase, can bring the detection time under a minute. I've worked with FT functions before, but off hand I don't see how the phase would help that much. What if the 37Hz signal could be modulated at say 3.7Hz? Could that help somehow?

I really appreciate any tips, Paul

Reply to
pmlonline

Sensitivity of a rf pickup system is swamped by terrestral noise below about 50 MHz.

Unless they are apallingly and bad and mesmerizingly awful, you will observe no difference whatsoever in noise between a high noise and a low noise amplifier picking up rf at audio frequencies.

One approach to narrow bandwidths is to use phase lock loop or synchronous demodulator technologies.

Sounds to me like the fundamental physics of what you are trying to do is not very well thought out.

--
Many thanks,

Don Lancaster
 Click to see the full signature
Reply to
Don Lancaster

Neither.

Since the units in the specs are V/Sqrt(Hz), you have to *multiply* by the sqrt(bandwidth) to get the RMS voltage.

V = 1.3 * sqrt(0.01) nV = 0.13 nV RMS

Since you only know what it is at 1 kHz, which is not too close to 37Hz, it is a bit of an assumption to make that they will be the same, but it is not too far off.

Reply to
Dave

These 1.3nV/rtHz apply to 100 Ohms or so. It'll be much higher with high impedance.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Reply to
Rene Tschaggelar

He is trying to receive a RADIO signal at 37 Hertz.

By a conservative estimate, the manmade and terrestrial noise at this frequency will be something like 170 decibels ABOVE the ktb noise.

It does not matter in the least which device he uses for his input stage.

Unless everything is in an anechoic fully shielded room.

--
Many thanks,

Don Lancaster
 Click to see the full signature
Reply to
Don Lancaster

Beware of the noise figures in the data sheet. They may not apply at such low frequencies where 'flicker noise' often predominates. (A Nuvistor can sometimes out-perform modern semiconductors for low LF noise)

If the option is open to you, it is often more cost-effective to improve the level of signal pick-up, rather than trying to get ever nearer to perfection in the amplifier.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
 Click to see the full signature
Reply to
Adrian Tuddenham

Hey everyone thanks for the great information. I found one site that listed terrestrial noise down to 100KHz

formatting link

Don, I get a power ratio of 1e+17 and voltage ratio of 3e+8 for 170 dB. What exactly does it mean to have a 170 dB above ktb noise? I understand that ktb noise is receiver input noise. I see the noise graph in above link also uses that terminology; e.g. over ktb. So 170 dB over ktb means the terrestrial noise is 3e+8 times greater than receiver input noise? The noise graph shows dB increasing as frequency decreases. Shouldn't it be the opposite? Perhaps those numbers are negative; e.g., -120dB. According to the graph your 170 dB seems reasonable. Have you seen any graphs that come close to 37Hz?

Last, how do they figure the terrestrial noise values? Is it based on a dipole antenna at a fixed length or what? For example, wouldn't a

200-meter dipole pick up more noise than say a 10 cm radius 100-turn coil?

Thanks, Paul

Reply to
pmlonline

A parabolic reflector with a 60 decibel gain might do the trick.

Tiny problem with zoning, though.

--
Many thanks,

Don Lancaster
 Click to see the full signature
Reply to
Don Lancaster

His budget and effort would be best put into making the aerial more directional.

At that frequency, spaced aerials and a correlation technique might be the way to go.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
 Click to see the full signature
Reply to
Adrian Tuddenham

LOL. A 60dB parabolic at 37Hz .... Lambda is around 10'000km ... Drop a wire into the ocean.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Reply to
Rene Tschaggelar

Don, after further studying, I believe that I can cancel out nearly all the terrestrial noise with a second nearby coil since the 37Hz signal would be at a local small area. The second coil, that would be away from the 37 Hz signal, would be the terrestrial noise cancellation coil. I would expect the terrestrial noise to cancel out fairly well since the RF wavelength at 37 Hz is over 8 million meters.

Ken Smith wrote:

Thanks Ken. I found a great pdf file on noise,

formatting link

In considering the JFETs as an amplifier, wouldn't I have to use several resistors? It seems that resistors are a definite source of noise, which would add to the 1/4 nV/sqrt(Hz) JFET.

Yes, great idea. I was thinking about that yesterday. Although I didn't know that it was also the sqrt() of total parallel devices. Isn't paralleling equal to multiple sampling? Consider a computer program that does averaging. Say we take 10 samples, sum up all 10 samples, and take average signal. I thought that 10 averages would decrease random noise by a factor of 10. BTW, the program would be in sync with the 37Hz signal so that averaging would not cancel it out like the noise.

Does anyone have any input on the following idea? According to the above pdf, Inductors are considered fairly noise free. Perhaps that pertains to well made L's. I have not verified that. If that is true then a transformer should also be fairly noise free-- say a nice toroid. Why not amplify the signal to say a few hundred times with the transformer and then use a differential amplifier? That's presuming that I am able to cancel out enough of the terrestrial noise. For example, lets say we have a 200pV 37Hz signal with no noticeable noise. After the transformer, we get a 20nV 37Hz signal with no noticeable noise. Now our amp should have no problem with the 20nV signal. Lets say the amp adds 1nV of noise. So that's a 20:1 signal to noise ratio. Without the transformer it would be a 0.2:1 signal to noise ratio.

Thanks for the input, Paul

Reply to
pmlonline

Don, after further studying, I believe that I can cancel out nearly all the terrestrial noise with a second nearby coil since the 37Hz signal would be at a local small area. The second coil, that would be away from the 37 Hz signal, would be the terrestrial noise cancellation coil. I would expect the terrestrial noise to cancel out fairly well since the RF wavelength at 37 Hz is over 8 million meters.

Ken Smith wrote:

Thanks Ken. I found a great pdf file on noise,

formatting link

In considering the JFETs as an amplifier, wouldn't I have to use several resistors? It seems that resistors are a definite source of noise, which would add to the 1/4 nV/sqrt(Hz) JFET.

Yes, great idea. I was thinking about that yesterday. Although I didn't know that it was also the sqrt() of total parallel devices. Isn't paralleling equal to multiple sampling? Consider a computer program that does averaging. Say we take 10 samples, sum up all 10 samples, and take average signal. I thought that 10 averages would decrease random noise by a factor of 10. BTW, the program would be in sync with the 37Hz signal so that averaging would not cancel it out like the noise.

Does anyone have any input on the following idea? According to the above pdf, Inductors are considered fairly noise free. Perhaps that pertains to well made L's. I have not verified that. If that is true then a transformer should also be fairly noise free-- say a nice toroid. Why not amplify the signal to say a few hundred times with the transformer and then use a differential amplifier? That's presuming that I am able to cancel out enough of the terrestrial noise. For example, lets say we have a 200pV 37Hz signal with no noticeable noise. After the transformer, we get a 20nV 37Hz signal with no noticeable noise. Now our amp should have no problem with the 20nV signal. Lets say the amp adds 1nV of noise. So that's a 20:1 signal to noise ratio. Without the transformer it would be a 0.2:1 signal to noise ratio.

Thanks for the input, Paul

Reply to
pmlonline

For those who are very interested in low noise at lowish frequencies:

Take a look at the company called "interfet" in Texas. They make JFETs with a noise of about 1/4 nV/sqrt(Hz).

You can also get that level by putting 16 LSK170s in parallel.

--
--
kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

In article , Adrian Tuddenham wrote: [.. 37Hz ..]

Yes, using the 1PPS pulse from a GPS you could keep the systems in sync to a fraction of a degree. If you spinkle the recievers over the entire surface of the earth, the result would be fairly directional and reject local noise quite well.

--
--
kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

My prediction is that the S/N ratio at the input noise terminals is unlikely to be changed much no matter what circuitry follows it.

But fully balanced, shielded, and guarded systems are a good starting point.

--
Many thanks,

Don Lancaster
 Click to see the full signature
Reply to
Don Lancaster

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.