Definition of amp noise

Don, that would be great, but are you sure that's the correct part #? I looked it up and found a switch.

Paul

But it was poorly distributed and severely in need of editing.

That's correct, it's a switch, as Don intended.

A cd4066 is better than a 4016, and a 74hc4053 is even better yet.

The fact that the signal phase is known suggests that you have access to the generator of the signal, which would make your problem a piece of cake for a commercial lock-in amplifier.

robert

Well, I wonder how you can make a single 4016 switch chip into a lock-in amplifier or synchronous demodulation.

Paul

Easy as pie, piece of cake.

Don't you just hate rumors like that? ;-)

You're going to make me think? I guess you make a mixer, but not sure how you'd make a lock-in amp. For mixer I would put switch directly across signal output and an oscillator would control the switch. So the 37Hz signal with short the output half the cycle. LOL, that's a really cheesy mixer though, no?

Paul

No.

But clearly you didn't read the 74hc4053 datasheet. All early lock-ins used balanced signals with reversing switches operated by a synchronous square wave. Tens of millions of these are still made, and work well. This approach has the advantage of being highly linear as well as being very inexpensive.

More advanced lock-in amps use multipliers with synchronous sine-waves, thereby avoiding the demodulation of harmonics. Even more sophisticated lock-in amps use a DSP to perform the multiplication, with a digitally- generated sine wave.

How much money would it free up for social programs, space exploration, and international relations, if we dismantled the nuke subs and used the material to make stuff like power plants and other peaceful stuff? Y'know, the old "beat your swords into plowshares" thing?

Thanks, Rich

...

One Demerit to Winfield Hill, for one extraneous apostrophe in an otherwise possessive "its", and FIVE Punitive Demerits, because Win Hill is one of our Gurun.

Right. I know well the correct usage, and an improper usage grates on me when I see it. Yet somehow my typing mode sticks an extraneous apostrophe in every now and then, which I don't see it until it's too late, if at all.

The switch changes the gain of the opamp from +1 to -1 and does so in synchronization with the positive and negative signal peaks, thus performing full wave rectification.

This used to be called a lockin amplifier. A more modern term is synchronous demodulation.

Signals other than the synchronized one decorrelate with time.

Why don't you try to find out who wrote that book on the 4016 and then read it?

Only went seven languages and one and a quarter million copies so far.

Sigh. You use the 4016 to configure an opamp as a +1 -1 gain. Then synchronously switch in phase with the positive and negative signal peaks.

One of the CMOS cameos.

How do you edit a rumour?

To a very high degree of accuracy, thermal noise power is

P = k T B.

(where k is Boltmann's constant and T tempeature in Kelvin, B is bandwidth in Hz).

There are corrections you need to apply at very high frequencies (over 1 THz) or very low tempeatures (1 K or less), but that is the thermal noise power. Contry to popular belive, even at absolute zero there is thermal noise, but it is insignficant except at opetical frequencies.

The amplifier will add some of its own noise.

As somone else has said, 37Hz is audio not RF. If the system is RF modulated at 37Hz, you will need to demodulate first, but a lock-in can still be used, but I suggest a dual phase unit is purchased.

Others have given you links to articles on lock-ins, but here are a few commments of mine. They are no doubt in the articles too, but they are important so I will restate a few.

• You need to decide if you need a single or dual phase lock-in amp. If you know the phase of the detected signal, a single phase unit is OK. But for a lab instrument, I would *seriously* look at buying a dual phase unit. They are a lot more useful and remove a lot of the uncertainty that you get with a single phase unit.

• Single phase lock-in amps are often on eBay with a 'buy it now' of under 0. Needless to say, dual phase ones are more expensive. The dual phase units tend to be more modern too, which again adds to their price.

• EG&G and Stanford are the two big manufactuers of them. There are a few more.

• I would suggest if you are demodulating RF to get the 37Hz, a dual phase one will be safer, as you don't need to consider what phase shifts might have occured in the demodulation process.

• Most moden lock-ins will have GPIB and/or RS232 serial so you can read the data down. Old ones will lack this.

• You obviously need a source of modulation to drive your signal with. Some lock-ins have a signal generator built in, which you can use if you wish, although you can always use an external one. Some units have no internal signal geneator.

• One of the big lock-in manufacturers (not sure if EG&G or Stanford) make a single channel unit with no controls on the panel. It is all done via a PC. I've not used one.

• There may well be a book written about how to build a lock-in by switching an op-amp to have a gain or +1 or -1, but there is certainly an old paper on it written in India. I don't have a reference to it off-hand, but could find it. That used FETs across the resistors in the op-amp configuration, effecitvely making the resistors shorts or of the required resistance. It did work. I built it and got a dynamic reserve of around 70 dB.

• Systems using a square wave are suseptable to odd harmonis of the signal. So if there is any 3*27=81Hz around, then that will be detected too. Systems working on sine waves don't have that problem.

• Stanford is the only manufacturer of a lockin that works above 3MHz.

You will not avoid that with the lock-in. The lock-in is basically a high Q tunable bandpass filter. The output is converted to a DC level. T o reduce the noise, you must make the filter have a low cutoff frequency. The time constant of the filters is set on the lock-in. Normally (assuming a 6dB/octave filter) you have to wait 5x the time constant for the measurement to settle to make an accurate reading (that gets it to 99%). In your case, you can do a bit less than that, as you only need an on/off status.

There are some errors/details not expalined too well there. When that article talks about tempeature, it does not define what it means. It is not as obvious as you think.

P = k T B

is the noise power from a conductor (be it 1 or 1,000,000 Ohms). P is in Watts, k is Boltzmann's constant, T is the *source* temperature (Kelvin) and B bandwidth (Hz).

Often the source is at about 298k (room tempeature), but sometimes it is a lot lower. Point a highly directional microwave antenna at the sky and you get an average tempeature around a few K (not sure of the exact number). At some parts it will be lower.

Point it towards the moon or Sun, and you will get a lot more. Point it at the earth, and it will be about 290 K (average tempeature of the earth).

For a good understanding of noise figure, noise tempeature, noise factor and how to measure them, take a look at an Agient application note or two. AN 57-1 is the most useful.

but -2 and -3 are worth reading too.

I would not put too much faith in them. Man made noise depens very much on location and will chage over time. Bluetooth, mobile phones etc will put it up very much as these technologies develope.

I've no idea what it would be like at 37Hz. But at that, you wil not be able to make a high gain antenna.

If you can cool your detector, and all what it sees (which is basically everthing) you can reduce the kTB bit. Liquid nitrogen is commooly used.

Yes, but it might not scale the way you think. A 20 dB (relative to a dipole) gain Yagi will probaby pick up no more/less than dipole, which by definition (of dBd) has 0 dB of gain. The reasona being the high gain anteann will pick up more from one directiona than another. So you gain 100x more power from one directiona, but make the direction ony a few degrees, rather than your 360 degree.

I just looked it up. It is ELF in the radio band frequencies. Audio is reference to the vibrations of entire atoms. RF is a range EM frequencies in the Radio spectrum which include ELF, SLF, ULF/VF, VLF, LF/LW, MW, HF/SW, VHF, UHF, SHF, EHF

Check it out on wiki pedia

Paul