# Tag Info

21

To express a ratio in dB, the ratio must be unit-less, since the logarithm of the ratio must be taken, so I'm not sure I understand why you're puzzled that we use dB. dB is often used to express unit-less ratios precisely because of the properties of logarithm. For example, multiplication becomes addition, division becomes subtraction. Also, since the the ...

21

The only way that a signal that is "buried in the noise" can be detected is if you can run the signal + noise through some filter that attenuates the noise more than it attenuates the signal. At which point the signal is no longer buried in the noise, so "buried in the noise" was just a hasty assumption. In radio carrying an audio (or Morse code) signal in ...

17

Ask yourself which cable is likely to have the lowest loop area: A large loop area has greater inductance and can emit more EM interference. It can also receive more EM interference. If each forward conductor has its own return wire then this potentially minimizes each circuits loop area.

15

It's simple: manufacturers make what customers will buy! It's the same reason why Ferrari won't put a trailer attachment at the rear of their cars... Price is a very important part of this, of course, and price is tied to silicon area, process, yield, and of course packaging. For example, an opamp with +/-12V supplies and 1A output current will dissipate ...

10

Decibels isn't a "unit" in the sense of meter, Netwons, seconds, etc. It is like percent, dozen, parts per million, and the like. Those are all ways of expressing dimensionless numbers. Decibels happens to be a way to express values on a logarithmic scale, but that doesn't change the fact that there is nothing wrong with having various "units" for ...

10

Imagine a perfect noiseless signal transmitted down two wires and you looked at what was received at the other end. Along the way, noise impregnates the two wires. What you might see at the receiving end is a noise voltage that exists between the two wires (differential). You will likely also see a noise voltage that affects both wires exactly the same with ...

10

In your example, there is absolutely a correct answer, and that answer is current. Current provides the highest possible signal-to-noise ratio. Here is why: Johnson–Nyquist noise, commonly referred to simply as thermal noise. All conductors experience this noise, and is the result of thermal agitation of charge carriers within the conductor itself. In ...

9

Your amplifier has average noise characteristics, the problem is that your signal is very, very weak. The amplifier is responsible only of the noise part of the SNR, so it has not a "poor SNR", but a "poor input referred noise". To obtain a better SNR you can either amplify your signal, without adding noise, or reduce the noise. Since amplifying without ...

9

The general concept of detecting a signal in noise is knowledge of something about the signal, and ideally something about the noise. The easiest thing to use is spectral knowledge. If you know the signal occupies some part of the spectrum, then you can safely filter out noise in other parts of the spectrum without losing signal. This is taken to its ...

7

The spectrum of interest is important : some otherwise very good amplifying devices have extra high noise at frequencies below 10Hz. Two options are worth considering : the first is bipolar transistors to provide useful gain before an opamp second stage. Why not go straight to an opamp? They are pretty noisy, very few have input noise voltage below 1 nV/...

7

Common mode changes slowly, signal changes fast. What you thus need is a high-pass filter, which filters out the DC component. In the easiest case: that's a capacitor in series with your signal source, with a resistor to ground to "short" low frequency content. here's an easy-to-use RC high-pass design tool. Start with something like C=10nF. You seem to be ...

7

Another technique that might be useful is autocorrelation Autocorrelation, also known as serial correlation, is the correlation of a signal with a delayed copy of itself as a function of delay. Informally, it is the similarity between observations as a function of the time lag between them. The analysis of autocorrelation is a mathematical tool ...

6

Similarly, radians should not have a unit, but are still expressed as rad for clarity. More specifically, SNR is measured in dB, because dB are convenient for the situation. dBs are convinient for the situation, as the differences of signal and noise can have a large dynamic range, that is, to be small or very large. So the SNR of 100000V signal with 1V ...

6

It sounds like you are missing pull up or pull down resistors on your inputs. If you leave your inputs floating you will get unreliable results. http://en.wikipedia.org/wiki/Pull-up_resistor <== read this for more info If your input is normally off you'd use a pull down resistor and if normally on you'd use a pull up resistor. This is a simplified way ...

5

Digital inputs which may become open should have a pullup/pulldown resistor so that their state becomes defined. Otherewise, they will have an undetermined voltage between [0,5V], leading to sporadic digital readings (as you've noticed). The microcontrollers used by Arduinos have internal pullup resistors which can be enabled. I'm not entirely sure which is ...

5

Your signal is buried in the noise. Assume you band limit your input in a 200Hz bandwidth. $$V_{n(rms)}=5 \times 10^{-4}V/\sqrt{Hz} \times \sqrt{200Hz} \approx 7mV$$ Your input SNR $$V_{in} = 8mV\\ SNR = 20log(\frac{V_{in}/\sqrt{2}}{V_{n}}) = -1.9$$ When you amplify your signal, the noise will be amplified too. You may need a lock-in-amplifier. ...

5

If you were transmitting something of the order of a 10s to 100s of kHz, number 1 may be perfectly adequate, but number 2 does have the advantage that each signal has a return path that is closer and therefore reduces the total current loop. This is useful in minimising radiated emissions (and radiated susceptibility too), to say nothing of crosstalk. I ...

5

FET input amps don't suffer from the same noise sources as resistors, which is how they can still have <100nVpp noise with input resistances in the teraohm range. Analog devices makes a "32" bit ADC w/preamp with input noise of <100nVpp, you could average out many samples to try to improve the noise floor (5sps for an hour should give you a couple ...

5

And then there is the idea of averaging multiple independent observations. In overly simplified terms, the aim here is to increase the signal strength while letting the noise cancel itself out, i.e., the desired signal increases faster than the noise, and the more samples you average the better.

4

Decibels are sometimes a more convenient "unit" to work with. The same question applies to voltage gain of an op-amp - the tendency is to state open-loop gain in decibels. Ditto closed-loop gain. Same with filters - low pass filters(for example) have a "gain" reduction with an increase in frequency and this is usually expressed as "so many" dB per octave ...

4

Reading the question and the comments, there may be a conceptual misunderstanding : the attenuator WILL attenuate any noise presented on its input (even from just a 50 ohm source impedance), to the same extent it attenuates the signal. However it also generates noise of its own, which may be represented as the noise from a perfect resistor equal to its own ...

4

It seems you don't know how to even start. I'll try to guide you a bit. Matlab will be really useful to you. First, you need to understand the basics. You can find lots of tutorials, for example, this one: http://users.ece.gatech.edu/bonnie/book/TUTORIAL/tutorial.html Then, once you know how to work with the software, you need to create a script that does ...

4

As you plainly stated, decibels are used to quantify the relationship between two signals. They are relative, not absolute. Saying that a transmitter has 1dB of output is meaningless. Therefore it must be referenced to some other unit. For example, 1dBm is 1dB with respect to 1 miliwatt. In the case of Signal to Noise ratios, the dB is the only thing that ...

4

I'm going to take a stab at this though I don't know if it applies to sonar. If you have a signal received on a transducer there will be both signal and noise. If you have two transducers (in an array) it can be presumed that the same signal is received on both but the noise on each is likely gaussian and not coherent. The upshot of this is that when you ...

4

The second alternative provides far better shielding. Since the current in signal N and return N flows in opposite directions, their EM fields cancel each other out. So in the first example, when signal 1 is active, signal 2 is more exposed to crosstalk as it is in the second example. As the picture title says, a single return path creates unequal (and much ...

4

The critical reason for the low frequency blocking capacitor is avoiding large DC currents to flow. In any practical application, "ground" is only at zero potential in a very small region; the "ground" potential can vary across a circuit board, equipment chassis, or or larger assembly of equipment (telecom, AC power, building) due to current flow and non-...

4

As mentioned by Barry, this has got to do with beamforming. If you consider what occurs when a signal hits the two antennas, as shown in the following figure: The signal received by the antennas will have a time delay between the antennas. This time delay is related to the angle theta that the signal is arriving from. When adding the signals together at ...

4

It's not obvious at all to me that 'a few' nV/sqrt Hz noise swamps your signal as you have said nothing about bandwidth. If your bandwidth is very low then there may not be a problem. Note that it is bandwidth not maximum frequency. Note that the quoted nV/sqrt Hz noise is above the 1/f corner frequency and if your frequency is low then you may have ...

4

A possible solution, is to use a slew-rate-limited amplifier on this signal, in combination with AC coupling. The step size, then, would be diminished while slower-slewing ripple is passed through. Some external-compensation-capacitor op amps can do this with an oversized compensation capacitor. Another would be synchronous gating, so that a series of ...

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