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S/N Ratio dependent on impedance match?

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Bill Bowden

Jan 1, 1970
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Why is S/N ratio dependent on impedance match?

It would seem a ferrite loopstick antenna would deliver twice the
voltage if connected to a high impedance rather than matching it to
it's characteristic impedance. But I have heard this is not a good
idea since the S/N ratio would degrade. Any truth to this idea?

-Bill
 
P

Phil Allison

Jan 1, 1970
0
"Bill Bowden"
Why is S/N ratio dependent on impedance match?

It would seem a ferrite loopstick antenna would deliver twice the
voltage if connected to a high impedance rather than matching it to
it's characteristic impedance. But I have heard this is not a good
idea since the S/N ratio would degrade. Any truth to this idea?

** 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
 
J

Jamie

Jan 1, 1970
0
Bill said:
Why is S/N ratio dependent on impedance match?

It would seem a ferrite loopstick antenna would deliver twice the
voltage if connected to a high impedance rather than matching it to
it's characteristic impedance. But I have heard this is not a good
idea since the S/N ratio would degrade. Any truth to this idea?

-Bill
Absolutely..

Jamie
 
B

Bill Bowden

Jan 1, 1970
0
On Sat, 26 Jan 2013 21:13:59 -0800 (PST), Bill Bowden
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 thes/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.

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

Bill Bowden

Jan 1, 1970
0
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

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
 
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Phil Allison

Jan 1, 1970
0
"Tim Wescott"
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.


** 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.

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.

** 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
 
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