Filters And Amps, order?

[deth502]

quick question, is there a preferred method of filtering and amplifying a signal?

what i have is basically a noisy line coming in. i want to have a bandpass filter weed out a certain frequency, and have that frequency amplfied to activate a circuit.

should i filter the raw signal, then amplify, or should i amplify, then filter? pros and cons?

[gremlinsa]

Well first Question is what Freq. ??

What Freq is your signal and what Freq is the noise...

However most times its Pre-Amp - Filters - Amp..

[deth502]

so, both? lol, ok.

right now what im working on is very low frequency stuff. under 150hz.

[gremlinsa]

That is low... In the Audible range...

the trick normally is the Pre-amp amplifies around the freq you need, then filter off outside noise, and amplify to the level you require...

[CWB]

but what kind of "noise" is present on the line ?
do you need to band-pass or low-pass ?
what is the incoming level ?

[deth502]

it will be an antenna, so, a lot of very low signals coming in, im guessing. looking to detect just one very precise specific frequency. im starting to look into acticve filters, to get a more narrow response.

[CWB]

heh ... that would be one helluva long dipole !
there is some stuff around 19-40KHz ... the navy runs it .

maybe a further explanation of what you are attempting is in order .

[deth502]

basically, a super-duper, high performance, high reliability non-contact ac voltage detector with a minimal chance for false alarms, that would attach to a relay to activate a circuit if you are in the area of a live line.

the idea of the filters is because, if you get a ncvd and rub it on something, you will see it will flash as though you are on a live line. i dont want a screwdriver sliding along a dead case to generate some random signal like this on a dead line and lock the tech out.

being kind of vague about this cuz i have an idea that afaik has never been tried before, a safety device. hoping to maybe make some $$$ on it, and have this hobby i keep sinking money into maybe turn a profit for once. or at least break even.

problem im finding is im having to design it from scratch, because every single other circuit on the entire planed is specifically designed to REJECT the 60hz "hum", im trying to isolate and amplify it.

[Sch3mat1c]

So it's like those live wire detect pens?

It's worth noting, if you want very high certainty of detecting exactly 60Hz, you have to wait many cycles to do it. An analog bandpass filter with a Q of 10 will detect 57-63Hz and take almost 0.2 seconds to do so. It won't detect 50Hz all that well, nor any other line current (25Hz is still in use in very few places, I think some other oddball frequencies related to trains and subways, and also DC). You can go faster with an arbitrary amount of digital, but there's only so much you can guess at until you have recorded a couple cycles of waves.

A magnetometer might be handy too -- it tells you if there's current flow, which doesn't help on an unloaded line, but does work at DC.

Tim

[deth502]

yes, exactly! the time really isint an issue, thats why im looking to have it very sensitive. so, the way im planning it, the operaor would be "locked out' before he actually got close enough to a live wire to touch it.

the more sensitive i can get it, the longer it would be able to take and still be acceptable. ie., say your walking toward an exposed live wire. if you got a warning when you were 2 steps away, the warning would have to be fairly quick, or you would walk into it, if it warned you at 10 steps away, taking 3 seconds to do so would not be a problem, as you would still be amply far away.

[Sch3mat1c]

Ok. Then your problem is going to be pervasive: inside any building, you're basically ten steps away from any wire. It's so nonspecific and generic that it's completely useless. If your application requires that all such lines also be deenergized, then I suppose that would work.

Note that conduits and raceways put wires inside grounded metal casings, which you'd have to open to "see" inside.

Tim

[CWB]

from a consumer/end user standpoint (and maybe myself and others here are not what one would call "average") ...

dealing with mains or similar lines :
i would not trust *something* that did not actually make physical contact (think meter with two probes) with the lines in question ... especially when one is working with 440 and above .


in the case of an "early warning system" ...
i agree with tim ... there is a flood of signals of interest at 60Hz ... just expose the center conductor (two feet or so) of a high impedance amplifier line and you will discover how pervasive this field is .
also ... what about 3 phase ? yes , there are three 60HZ "components" to it but would it not want to "look like" 180 Hz ?

not to be a wet blanket here ... but ... i can see liability issues with such a device .
uuuggghhh ... "legal disclaimer city" .

[deth502]

the 10' was an analogy to describe what i was trying to get at about the slow time.

i hadnt really thought about 3 ph appearing as something differnt. i wonder how that would work. the family has a business w/ 3ph power, i suppose i could try to use that to confirm somehow.

i realize that it wont be 100% foolproof, but in most cases, something is better than nothing. its no replacement for common sense. the problem today is that common sense dosent seem to be so common, and beung responcible for your own actions is even less so.

[CWB]

yep ... common sense aint all that common .
and don't get me started about those damnable sorry-arsed weenies that *somehow* have been instilled with a "sense of entitlement" .
yeah buddy ... i got yer entitlement right here (points to crotch) .

now ask me how i really feel .


there is a fix for those that do not want to be responsible for their own actions ...
allow them to be the recipient of ether a genuine darwin award or make the "honorable mention" listings .

[deth502]

agreed 150%!!