Impedance Matching with an unknown load

Hello,

1) I somewhat know the answer to this question but just want to clarify. In order to design the impedance matching, I will need to know the load impedance and the source impedance to begin with. Correct?

2) Let's say I have a circuit designed which is the "load" circuit of my setup. How would I go about measuring its input impedance since this is what is necessary to design the impedance matching circuit?

3) I would also like to know of any easy to obtain software that may be useful for impedance matching. I am presently trying to get my hands on ADS from my university but if there are any other software which are helpful that you know off, that would be great.

Regards

Pheezy said:

How would I go about measuring its input impedance

Click to expand...

Use a vector network analyzer.

Pheezy said:

software that may be useful for impedance matching

Click to expand...

For a simple single frequency matching network is a printed Smith chart and a pen all that is needed. Even if you intend to use a software, is how to manual use a Smith-chart very important knowledge.
If it is a more complex problem, wide band matching, using non ideal inductors and capacitors, check AnTune software. All calculations are done automatically but it do assume that you know how to set up a VNA. AnTune reads live data from a network analyzer or imports Touchstone files and deliver a proposed schematic for matching components. As a VNA is a complicated instrument, spend some time to learn how to calibrate measurement object as final result is just as good as the setup.

For a primitive impedance plot if you have a VCO, DMM and frequency counter to generate sweep then use directional coupler (DC3) or cable splitter or MiniCircuits splitter to match generator or source (75 or 50 Ohm) then look at reflected signal or return loss with hot carrier or pin diode and DVM. Use an impedance divider to create source impedance for test purposes to minimize reflected signal or design stripline or microstrip directional coupler for in circuit monitor.

Then use trial and error on PCB stripline or microstrip impedance transformer, as 10-20% error is common on first try due to over-estimate of dielectric constant at higher f or , etch-back factor in track width.

Pheezy said:

Hello,

1) I somewhat know the answer to this question but just want to clarify. In order to design the impedance matching, I will need to know the load impedance and the source impedance to begin with. Correct?

2) Let's say I have a circuit designed which is the "load" circuit of my setup. How would I go about measuring its input impedance since this is what is necessary to design the impedance matching circuit?

3) I would also like to know of any easy to obtain software that may be useful for impedance matching. I am presently trying to get my hands on ADS from my university but if there are any other software which are helpful that you know off, that would be great.

Regards

Click to expand...

all you need ia an oscilloscope and a signal generator and an impedance bridge.
however if you have not got an impedance
bridge you can just use a variable resistor
connect the resistor in series with the signal
and your load. set the generator for the desired frequency and voltage.
measure the signal generator output voltage
no load.
adjust the resistor
measure the voltage across you load.
remove the resistor and measure it. or you
could use a fixed resistor.
calculate the load Z.

now you circuit sees RS=R+RS
the resistor was included to enable
easy measurements.

so now we will call R+RS, Rs

Vs=generator OC voltage,
Vm = measured voltage
Rl = load

RL= (Rs*Vs/(Vs-Vm))-Rs.

or you can make an impedance bridge
which can measure R,XL and Xc. at
frequency of interest
or hire a network analyzer

and remember you match is only correct at one
frequency.

also see this:
https://www.fourier-series.com/rf-concepts/smithchart.html

1. It depends on types of matching, ie. your matching goal.
2. Use Thevenin equivalent ckt thoery to measure the Zin.
3. A software called RFOffice (check it at https://www.wave-dimension.com) can do all types of impedance matching easily. Very simple and useful.