"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

Phil Hobbs wrote:
> 
> On 01/27/2013 11:28 PM, Bill Bowden wrote:
> > On Jan 27, 7:37 pm, Phil Hobbs
> > <pcdhSpamMeSensel...@electrooptical.net>  wrote:
> >> On 1/27/2013 9:28 PM, Bill Bowden wrote:
> >>
> >>
> >>
> >>
> >>
> >>> On Jan 27, 5:42 pm, John Larkin
> >>
> >>> <jjlar...@highNOTlandTHIStechnologyPART.com>  wrote:
> >>>> On Sat, 26 Jan 2013 21:13:59 -0800 (PST), Bill Bowden
> >>
> >>>> <bper...@bowdenshobbycircuits.info>  wrote:
> >>>>> 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
> >>
> >>>> 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.
> >>
> >>> So, why does the s/n ratio double if you go through a transformer?
> >>> Doesn't the transformer double everything going into it?
> >>
> >> 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
> 
> 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.)


   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.


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


   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.

On 01/27/2013 11:28 PM, Bill Bowden wrote:
> On Jan 27, 7:37 pm, Phil Hobbs
> <pcdhSpamMeSensel...@electrooptical.net>  wrote:
>> On 1/27/2013 9:28 PM, Bill Bowden wrote:
>>
>>
>>
>>
>>
>>> On Jan 27, 5:42 pm, John Larkin
>>
>>> <jjlar...@highNOTlandTHIStechnologyPART.com>  wrote:
>>>> On Sat, 26 Jan 2013 21:13:59 -0800 (PST), Bill Bowden
>>
>>>> <bper...@bowdenshobbycircuits.info>  wrote:
>>>>> 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
>>
>>>> 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.
>>
>>> So, why does the s/n ratio double if you go through a transformer?
>>> Doesn't the transformer double everything going into it?
>>
>> 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

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

On Sat, 26 Jan 2013 21:13:59 -0800, Bill Bowden wrote:

> 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?

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

On Sun, 27 Jan 2013 18:28:34 -0800 (PST), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

>On Jan 27, 5:42&#4294967295;pm, John Larkin
>
><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>> On Sat, 26 Jan 2013 21:13:59 -0800 (PST), Bill Bowden
>>
>
>> <bper...@bowdenshobbycircuits.info> wrote:
>> >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
>>
>
>> 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.
>>
>
>So, why does the s/n ratio double if you go through a transformer?
>Doesn't the transformer double everything going into it?

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

On Jan 27, 7:37=A0pm, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> On 1/27/2013 9:28 PM, Bill Bowden wrote:
>
>
>
>
>
> > On Jan 27, 5:42 pm, John Larkin
>
> > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> >> On Sat, 26 Jan 2013 21:13:59 -0800 (PST), Bill Bowden
>
> >> <bper...@bowdenshobbycircuits.info> wrote:
> >>> 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
>
> >> It would deliver four times the voltage if you run it through a 2:1 st=
ep-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 inpu=
t of the
> >> transformer stops looking like a high impedance. The best place to sto=
p 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 act=
ually
> >> 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?
>
> As the old saying goes, "One man's noise is another man's data." =A0If yo=
u
> 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 amplifie=
r.
>
> 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. =A0At 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. =A0So 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. =A0It'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

On 1/27/2013 9:28 PM, Bill Bowden wrote:
> On Jan 27, 5:42 pm, John Larkin
>
> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>> On Sat, 26 Jan 2013 21:13:59 -0800 (PST), Bill Bowden
>>
>
>> <bper...@bowdenshobbycircuits.info> wrote:
>>> 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
>>
>
>> 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.
>>
>
> So, why does the s/n ratio double if you go through a transformer?
> Doesn't the transformer double everything going into it?
>

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

On Jan 27, 5:42=A0pm, John Larkin

<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Sat, 26 Jan 2013 21:13:59 -0800 (PST), Bill Bowden
>

> <bper...@bowdenshobbycircuits.info> wrote:
> >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
>

> 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 o=
f the
> transformer stops looking like a high impedance. The best place to stop i=
s 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 actual=
ly
> match impedances. And atmospheric noise usually dominates LF reception, s=
o
> 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?

> --
>
> John Larkin =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Highland Technology Incwww=
.highlandtechnology.com=A0 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 =A0analog, thermocouple, LVDT, synchro, tachometer
> Multichannel arbitrary waveform generators

On Sat, 26 Jan 2013 21:13:59 -0800 (PST), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

>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

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