Amplifier impedance measurement

Starting with the EMRFD Fig 2.57 amplifier as a base, you 
can measure input returnloss using a return loss bridge.

Return loss can be translated into VSWR and impedance.

If the return loss is large (or the VSWR is low) the impedance is near 50 ohms.

Low return loss means a higher VSWR so the impedance must be removed from 50 ohms.

For example. a 9.5 dB return loss is a 2:1 VSWR.
A 2:1 VSWR could be caused by either a 100 or a 25 ohm resistance,
A 2:1 VSWR could also be caused by a resistance of 40 ohms and a capacitive
or inductive reactance of 30 ohms. 

If you look at a Smith Chart and draw a circle centered at the center
with a radius (Reflection Coefficient) of .33, that is a circle of 
constant VSWR of 2:1 so any number of resistances and reactances could
cause an SWR of 2:1.    The phase component of the impedance is difficult
to measure with an return loss bridge but that does not prevent us from 
using the return loss bridge as an indicator of a good match in general.


See the following for returnloss to VSWR conversion:

  http://users.easystreet.com/w7zoi/retloss.html
  http://www.minicircuits.com/pages/pdfs/dg03-111.pdf
  http://www.rfcafe.com/references/electrical/vswr.htm



I put its spice file with my other spice models.  
If you click on the spice button to the left you can see the other models.  

The circuit worked as designed.  I didnt use exact values of resistors 
as I have more of  some flavors that I want to use up instead.   
I got +18 dB gain flat through HF with +21dBm (125mW) out.


I have been interested in trying to experimentally determine input
and output impedance of amps to see if what I see in spice really 
happens in the real world.    


You can measure input return loss in Spice by adding a second voltage
source to the input of your spice model.

Take a look at the spice models at the bottom of this page:

http://users.easystreet.com/w7zoi/fbapad.html

Notice AC 1 and S11.

from:
http://www.sss-mag.com/spara.html#spar

S11: input reflection coefficient of 50 ohm terminated output.
S21: forward transmission coefficient of 50 ohm terminated output.
S12: reverse transmission coefficient of 50 ohm terminated input.
S22: output reflection coefficient of 50 ohm terminated input. 

S11 is the input reflection coefficient.  

The magnitude of the
  reflection coefficient goes from 0 to +1 volts given the 2 
  and 1 volt AC sources used.  If you consider the phase component
  of the reflection coefficient then the reflection coefficient
  can range from -1 to +1 which is not coincidentally the range
  of the horizontal axis of a Smith Chart.

If you express S11 in dB then S11 is return loss

Input and output impedance can be measured by using a return loss bridge to 
determine return loss which is related to VSWR and impedance.

The input return loss can be measured by the following recipe:
 
  Attach a 50 ohm dummy load to the amplifier output
  Attach a siggen to the RLB "RF in" port
  Attach a power meter/oscope to the "Det" detector port
  Turn on the amplifier, siggen and power meter.
  Note the power reading on the power meter ("Unknown open").
  Attach the RLB "Unknown" port to the amplifier input and note the power.
  The return loss is the difference between the two power readings.

 
The output return loss can be measured by the following recipe:

  Attach a 50 ohm resistor to the amplifier input -important-
  Turn on the amp.   With a scope, insure that it is not oscillating.
  Attach a siggen to the RLB "RF in" port
  Attach a power meter/oscope to the "Det" detector port
  Turn on the amplifier, siggen and power meter
  Note the power reading on the power meter ("Unknown open").  
  Attach the RLB "Unknown" port to the amplifier output and note the power. 
  The return loss is the difference between the two power readings.

  

That is it!  I couldnt believe it was that easy

I understood that you could place a return loss bridge at the input of 
an amplifier to determine input return loss/VSWR.   What I didnt understand
is that you could use the same return loss bridge on the output of an amp
to determine output return loss.   In addition to not understanding that 
this was possible at all, I was concerned that I might blow up my siggen
by attaching it to the amp output.  That turned out not to be an issue
at the 100 mW power level.  At higher power levels it may be necessary 
to put a 20 dB pad betwen the siggen and the RLB to protect the siggen.
Same goes for the power meter.  It may need to have a 20 dB pad between
it and the detector output.
  
I think that as long as the amp is quiet and not oscillating and putting
out more power than the siggen/powermeter/RLB can handle then you should
be ok directly measuring output return loss/VSWR.

It is kind of wierd to me to be applying low level power to the output
of an amplifier in order to determine "reverse SWR" but it works.  

I saw 18 dB input return loss which is something like a 1.3:1 VSWR
across HF.  


This is a big deal for me.  To actually be able to measure output
impedance gives me confidence that what Im doing is actually working. 

Im going to do some work at this power level to get some more 
experience then am going to try to graduate to higher power levels
and see what happens.