S/N Ratio dependent on impedance match?

[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?

-Bill

 

[Phil Allison]

"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

 

[Jamie]

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?

-Bill
Absolutely..

Jamie

 

[John Larkin]

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

 

[Bill Bowden]

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?

--

John Larkin                  Highland Technology Incwww..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

 

[Phil Hobbs]

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

 

[John Larkin]

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

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

 

[Bill Bowden]

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

 

[Tim Wescott]

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

 

[Phil Hobbs]

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

 

[Michael A. Terrell]

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.

 

[Phil Allison]

"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