--- John Winterflood <[EMAIL PROTECTED]> wrote: > R Stiffler wrote: > > ... > > Carbon resistors generate more thermal voltage > > noise than Metal film resistors.... > > This is not really true. We may divide the noise > sources in Carbon > composition resistors into two types: > > 1) True "Thermal noise" (also called "Johnson" or > "Nyquist" noise) which > "is the noise generated by the thermal agitation of > the charge carriers > (the electrons) inside an electrical conductor in > equilibrium, which > happens regardless of any applied voltage." (From > http://en.wikipedia.org/wiki/Thermal_noise). This > noise source is > absolutely fundamental and is completely unvarying > regardless of type of > resistor,
Actually that's not true. Radiation resistance generates no thermal noise. > and its power sourcing capability is > completely unvarying > regardless of value of resistance, number in > parallel/series, size, etc. > It is simply 4kT watts per Hz of bandwidth. (The > bandwidth presumably > goes up to some very high limit determined by the > mean free path of the > electrons being scattered in the resistive > conductor). > > 2) Excess noise (see > http://en.wikipedia.org/wiki/Flicker_noise) - which > is generated by current passing through the resistor > and may well be due > to thermal (or thermally induced microphonic) > effects, but is not > rightly referred to as thermal noise (at least > amongst physicists). It > is readily overcome with better technology. The > excess noise present in > a carbon composition resistor is produced by random > effects driven by > the power fed in and will only be a very small > fraction of this applied > power - ie very far from overunity! Personally I do not adhere to the term, overunity. Nonetheless, the antenna connected to a carbon resistor does indeed radiate more power than an antenna connected to a metal film resistor. > With regard to Johnson noise, if you short or open > the resistor, then > the entire 4kT watts generated is simply dissipated > back into the > sourcing resistor Assuming you disregard black body radiation. > as heat and there is no net power > flow. If you load > it with a matched resistance then you can draw off > half of this power, > but if the resistor you load it with is at the same > temperature, then it > also generates this same power back in the first > resistor and again > there is no net power flow. > > Coupling to it via a transformer is no different to > using a different > value of resistor as the source - the voltage to > current ratio changes > but the power available remains constant. Similarly > connecting many > such resistors in series or parallel simply changes > the impedance (or > voltage to current ratio) without changing the > available power. The idea is connecting it to an antenna, not a transformer. Also more power is indeed radiated by duplicating such devices. Size is irrelevant with respect to power output. So each device could be a nanometer. You could have a trillion of such nano devices. [snip] > > Preface: Radiation resistance generates no thermal > > noise. > > I would guess that the best you could do with any > antenna pointing into > deep space would be to pick up the 2.7 K microwave > background - which > would probably be indistinguishable from 2.7K > thermal noise being > generated in the radiation resistance seen via the > antenna. True, the antenna could pick up anything within its bandwidth. Fact still remains radiation resistance generates no thermal noise. Regards, Paul Lowrance ____________________________________________________________________________________ Sponsored Link Try Netflix today! With plans starting at only $5.99 a month what are you waiting for? http://www.netflix.com/Signup?mqso=80010030
