I'm studying op-amp circuit combined with rf mixer.
While I was reading articles on the circuit,I found that the term like
"50 ohm load"
or "50 ohm  (  )"   occurs frequently.
I also remember that one of the input options of Digital oscilloscope
was 50 ohm load.

Because I'm a newcomer to analog electronics, I don't know
significance of that term.

Why should we think about the specific value 50 ohm?
How do we get such value?

s...@hanmail.net wrote:
> 
> I'm studying op-amp circuit combined with rf mixer.
> While I was reading articles on the circuit,I found that the term like
> "50 ohm load"
> or "50 ohm  (  )"   occurs frequently.
> I also remember that one of the input options of Digital oscilloscope
> was 50 ohm load.
> 
> Because I'm a newcomer to analog electronics, I don't know
> significance of that term.
> 
> Why should we think about the specific value 50 ohm?
> How do we get such value?


   RF circuits are designed for 50 ohms, because the coaxial cable (used
most of the time) has a 50 ohm characteristic impedance.  The exception
is TV antennas, cable TV and sat TV equipment.


-- 
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Right, however it is recommended you read and understand the difference 
between resistance, capacitance, and inductance. Each of these variables 
will affect the overall impedence of a circuit and the circuits overall 
effeciency. Having a missmatch in overall impedence can, and has, rendered 
many circuit designs inoperable.
Find a good tutorial on electronics and have at it. Don't be afraid of 
asking questions, others will respond, some in a helpful manner, some with 
raw datum, others with theory, and yet others with left field opinions. 73's
"Michael A. Terrell" <m...@earthlink.net> wrote in message 
news:4...@earthlink.net...
> s...@hanmail.net wrote:
>>
>> I'm studying op-amp circuit combined with rf mixer.
>> While I was reading articles on the circuit,I found that the term like
>> "50 ohm load"
>> or "50 ohm  (  )"   occurs frequently.
>> I also remember that one of the input options of Digital oscilloscope
>> was 50 ohm load.
>>
>> Because I'm a newcomer to analog electronics, I don't know
>> significance of that term.
>>
>> Why should we think about the specific value 50 ohm?
>> How do we get such value?
>
>
>   RF circuits are designed for 50 ohms, because the coaxial cable (used
> most of the time) has a 50 ohm characteristic impedance.  The exception
> is TV antennas, cable TV and sat TV equipment.
>
>
> -- 
> Service to my country? Been there, Done that, and I've got my DD214 to
> prove it.
> Member of DAV #85.
>
> Michael A. Terrell
> Central Florida 

On Tue, 24 Jul 2007 01:36:13 -0700, s...@hanmail.net wrote:

>I'm studying op-amp circuit combined with rf mixer.
>While I was reading articles on the circuit,I found that the term like
>"50 ohm load"
>or "50 ohm  (  )"   occurs frequently.
>I also remember that one of the input options of Digital oscilloscope
>was 50 ohm load.
>
>Because I'm a newcomer to analog electronics, I don't know
>significance of that term.
>
>Why should we think about the specific value 50 ohm?
>How do we get such value?

---
A 1/4 wave whip exhibits about a 50 ohm resistive impedance at
resonance so, in order to keep from having reflections in the
transmission line, the transmission line's impedance is made equal
to the load (antenna) impedance and the source impedance is made
equal to the load impedance as well.

That also guarantees that with the system "matched",  maximum power
will be transferred to the antenna from the source.

  
-- 
JF

AJ wrote:
> 
> Right, however it is recommended you read and understand the difference
> between resistance, capacitance, and inductance. Each of these variables
> will affect the overall impedence of a circuit and the circuits overall
> effeciency. Having a missmatch in overall impedence can, and has, rendered
> many circuit designs inoperable.
> Find a good tutorial on electronics and have at it. Don't be afraid of
> asking questions, others will respond, some in a helpful manner, some with
> raw datum, others with theory, and yet others with left field opinions. 73's
> "Michael A. Terrell" <m...@earthlink.net> wrote in message
> news:4...@earthlink.net...
> > s...@hanmail.net wrote:
> >>
> >> I'm studying op-amp circuit combined with rf mixer.
> >> While I was reading articles on the circuit,I found that the term like
> >> "50 ohm load"
> >> or "50 ohm  (  )"   occurs frequently.
> >> I also remember that one of the input options of Digital oscilloscope
> >> was 50 ohm load.
> >>
> >> Because I'm a newcomer to analog electronics, I don't know
> >> significance of that term.
> >>
> >> Why should we think about the specific value 50 ohm?
> >> How do we get such value?
> >
> >
> >   RF circuits are designed for 50 ohms, because the coaxial cable (used
> > most of the time) has a 50 ohm characteristic impedance.  The exception
> > is TV antennas, cable TV and sat TV equipment.


   Who are you replying to?


-- 
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Michael A. Terrell wrote:
>    RF circuits are designed for 50 ohms, because the coaxial cable (used
> most of the time) has a 50 ohm characteristic impedance.  The exception
> is TV antennas, cable TV and sat TV equipment.
> 

Actually, a coax cable can have pretty much any value for characteristic 
impedance, it's just that the most common values are 50 ohms and 75 
ohms.  (The characteristic impedance of a coax line is proportional to 
the natural log of the outer radius of the coax to the inner radius. 
See any undergraduate physics or EE text on electromagnetics.  If you 
want to change the characteristic impedance of a coax, just change the 
ratio, assuming you're keeping the coax's dielectric material unchanged.)

"It can be shown" that 75 ohm coax gives minimum loss, and 50 ohm coax 
is a good compromise between minimum loss and maximum power handling 
capability.  See, for example, Thomas Lee, "The Design of CMOS 
Radio-Frequency Integrated Circuits", 2nd edition, pp. 229 - 231.

Bob Pownall

Sorry to follow up my own post, but I just realized I owe Michael an 
apology.

My first reading of his post made me think he was saying all coax had 50 
ohm impedance.  A second, more careful, reading caused me to realize he 
was just referring to the coax cable used most of the time for RF purposes.

The rest of my post still stands, of course.

Bob Pownall

Bob Pownall wrote:
> 
> Sorry to follow up my own post, but I just realized I owe Michael an
> apology.
> 
> My first reading of his post made me think he was saying all coax had 50
> ohm impedance.  A second, more careful, reading caused me to realize he
> was just referring to the coax cable used most of the time for RF purposes.
> 
> The rest of my post still stands, of course.
> 
> Bob Pownall


  Thank you.  The largest 50 ohm coax I've worked with was 6" copper and
brass hardline on a 25 KW UHF TV transmitter I moved. ;-)


-- 
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

On 24 Jul, 09:36, spere...@hanmail.net wrote:
> I'm studying op-amp circuit combined with rf mixer.
> While I was reading articles on the circuit,I found that the term like
> "50 ohm load"
> or "50 ohm  (  )"   occurs frequently.
> I also remember that one of the input options of Digital oscilloscope
> was 50 ohm load.
>
> Because I'm a newcomer to analog electronics, I don't know
> significance of that term.
>
> Why should we think about the specific value 50 ohm?
> How do we get such value?

Because it is conveniently close to "the impedance of free space" - a
universal constant.

To get as much energy into and out of an aerial, it needs to closely
match "air" which is almost the same as a vacuum i.e. the impedance of
free space.

So the 50 ohm value is a sort of "RF thing" (as opposed to "audio
thing").

See this rather good link about it:-
http://www.setileague.org/askdr/imped.htm

Robin

On Tue, 24 Jul 2007 01:36:13 -0700, in message
<1...@d30g2000prg.googlegroups.com>,
s...@hanmail.net scribed:

>I'm studying op-amp circuit combined with rf mixer.
>While I was reading articles on the circuit,I found that the term like
>"50 ohm load"
>or "50 ohm  (  )"   occurs frequently.
>I also remember that one of the input options of Digital oscilloscope
>was 50 ohm load.
>
>Because I'm a newcomer to analog electronics, I don't know
>significance of that term.
>
>Why should we think about the specific value 50 ohm?
>How do we get such value?

I'll add to the already excellent answers, and we'll assume 50 ohms as the
Value of the Gods.  Transmitters and other signal sources will have an
output impedance of 50 ohms.  It is expected that they will drive a 50 ohm
load, as, after all, that is the Value of the Gods.  As John Fields hinted,
when the source impedance matches the load impedance, maximum power is
transferred.  There is an engineering theorem addressing that phenomenon,
and is called, appropriately, the Maximum Power Transfer theorem.

Another topic that was hinted at was "reflections."  A reflection is a
return of the transmitted signal back into the transmitter.  If a
reflection happens, it indicates that the power transfer is not perfect,
and hence that source and load impedances are not perfectly matched.
Reflections can be measured and compensated for.

Here's an interesting phenomenon, and one that bears learning: if you
disconnect the 50 ohm load and drive an RF or audio source into practical
infinity, the observed voltage at the output will double.  That might seem
like a good thing on the surface, but it's impractical and potentially
damaging to the source.  One thing that happens with such a driven source
is that reflections are maximized (this in fact is one part of a reference
test for a transmission line and antenna - driving an open line, and
driving a shorted line).  When reflections are maximized, all sorts of
harmonic distortion can be introduced on the waveform.

When testing RF transmitters, a "dummy load" is always attached to the
output if there is no antenna path into free space, in order to prevent
reflections and possible damage to the transmitter.  

The concept of matching source to load applies at lower audio frequencies
as well, such as when fitting loudspeakers to a stereo system.  The best
arrangement is to match source and load as precisely as possible.  The
common example is 4-ohm auto systems vs. 8-ohm home systems - ideally, one
shouldn't mix components between the two.