S/N Ratio dependent on impedance match?

"Bill Bowden"

** No.

FET ( source follower) pre amps for ferrite antennas usually have no gate resistor - cos it only reduces the Q and the signal level.

.... Phil

Absolutely..

Jamie

It would deliver four times the voltage if you run it through a 2:1 step-up transformer before you go into the hi-z amplifier, which would double the s/n ratio. Keep extending that idea, more and more step-up, until the input of the transformer stops looking like a high impedance. The best place to stop is when the transformer impedance matches the source impedance.

But as Phil says, you don't want to kill the Q, so may not want to actually match impedances. And atmospheric noise usually dominates LF reception, so getting the best s/n in the electronics usually doesn't matter.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

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So, why does the s/n ratio double if you go through a transformer? Doesn't the transformer double everything going into it?

.highlandtechnology.com  jlarkin at highlandtechnology dot com

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As the old saying goes, "One man's noise is another man's data." If you were looking at sferics, for instance, you'd want a low noise front end so as to make sure you were measuring the atmosphere and not the amplifier.

At low signal levels, jacking up the input amplitude with a transformer is a win, until the transformer or the transformed amplifier input impedance starts to load down the signal. At MF, the input of a follower made from a BF862 looks like about 1/(j*omega*3.5 pF) to ground, with very little real part. So in principle you can go a fair way before that becomes a limitation.

That approach is commonly used with FETs at lowish frequency, since their noise temperature is so very low, but their noise resistance is so very high. It's generally limited by the nonideal behaviour of the transformer.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 USA 
+1 845 480 2058 

hobbs at electrooptical dot net 
http://electrooptical.net

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If you're talking about noise that the antenna picks up from the world, the transformer multiplies that and the stuff that you consider to be "signal". In that situation, the transformer doesn't improve s/n. That's the usual case for LF radio.

If the noise is inherent to the amplifier, as it tends to be many situations, then the s/n is optimized by proper impedance matching. That tends to be the case for microwave and exotic instrumentation and a lot of audio stuff.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

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Yes, thanks Phil. I think you are saying that jacking up the signal with a transformer is a good idea until it degrades the Q due to non- ideal conditions of the transformer? I guess any reduction in S/N ratio would result from a lowered Q and wider bandwidth? So, if you can maintain the Q, there will be no change in S/N ratio? Is that a correct statement?

-Bill

So: kinda what Phil said, unless you're working with some bizarre non- voice radio or no-tune thing that would get screwed up by a too-high Q.

And kinda what John said; if it's a loopstick antenna then you're working at MF, and at MF the atmospheric dominates all but the worst radios.

But while it was mentioned that the best S/N ratio isn't to be had at a perfect impedance match, no one said where it _can_ be found.

Amplifiers -- most specifically RF amplifiers, but audio ones, too -- have an optimal impedance for the best noise performance. It varies by the amplifier, but it's basically the the noise voltage of the amplifier reflected to the input, divided by the noise current reflected to the input. If you can present the amplifier with that impedance without losing any power in the coupling stages, then you're doing pretty good.

But, you get back to the fact that you're working at MF, and your signal is dominated by atmospheric noise, so it doesn't matter so much.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

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It depends on the relative contributions of the background noise at the antenna and the RF amp. With a quiet input or a relatively noisy RF amp, a transformer is a win because the signal goes up and the noise basically doesn't. Once the antenna noise dominates the RF amp's noise, increasing the transformer ratio stops helping.

Along the way, the transformer contributes reactance as well as both loss and noise. (The two are related by the fluctuation-dissipation theorem, which shows that any process that can dissipate power will also introduce noise. You can derive this from statistical mechanics in a page or two, or from the second law of thermodynamics in about three lines of algebra.)

The loss will show up as a reduction of Q, just as you say, but also the reactance will detune the antenna some. Tapping the antenna down on a tank circuit is one possible method, but the last time I used a loopstick was when I was about 14, so I don't have a good feel for their characteristics. (It was a JW Miller part that cost me a whole week's allowance, $5.)

Inductors used to be the biggest ripoff in electronics, bar high-end audio and scientology. $1 for a radial-lead Delevan choke, forsooth.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

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A tap, or seperate winding with less turns was used for bipolar transistor radios to match it to the llower impedance. Some loopsticks were adjustable with a moving core, and others allowed you to move the secondary coil for best perfomance.

EBAY has some Delevan, but mostly surface mount.

I have some axial Delevan axial on hand, but I'll have to find a datasheet for them.

"Tim Wescott"

** Equal source/load impedance matching is virtually never used in audio - with the exception of long cable runs where the characteristic impedance of the cable may be matched with a resistive load at the receiving end to neutralise the undesired effects of cable inductance and/or capacitance.

With low noise sources (ie mics and other passive transducers) the practice is to make the load 5 to 10 times the source impedance. This is most easily done with FET and tube inputs and also BJT stages where local or loop feedback makes the actual load impedance quite high.

But as Tim said, it is highly desirable to match the "noise impendence" of the amplifier to the source impedance.

FETs and tubes have optimum noise impedances in the megohms range while BJT stages can be tailored to give much lower values - down to a few ohms for MC pickups and ribbon mics.

** Not so quickly - a ferrite loop antenna is VERY inefficient so the noise generated by the loop can dominate over atmospheric sources. A low noise FET makes an excellent ferrite loop pre-amp as it adds only about 2dB to the theoretical noise for source impedances in the 10k to 100 kohms range.

... Phil