How has he making it 50 ohm??

[samy555]

Hi
From
http://www.talkingelectronics.com/projects/Ultima/Ultima.html
I got:

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I decided to analys and understand the final stage:

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I use the following link to calculate the values of inductors:
http://www.66pacific.com/calculators/coil_calc.aspx
The 8 turns coil = 50 nH
The 5 turns coil = 30 nH, notice that 3mm diameter
Then I use MultiSim10 to measure the output impedance Zo with the 50 ohm antenna was disconnected:

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I am surprised!! How could he makes it almost 50 ohm for maximum power transfer?
Now I'm going to ask a direct question and I hope that getting a straight answer, who knows.
The question is: What are the parameters that mainly control the value of the Output Impedance?
With my sincere wishes to you.

 

[Audioguru]

Again, Multisim is WRONG! How can the output current be 46mA p-p when there is no load??
Without the antenna, why isn't the high impedance collector voltage swing very high and current swing very low??

Without an antenna, in LTspice the voltage swing is very high and the current swing is very low. In LTspice, the voltage and current swings are about the same as your simulation when the antenna is connected.

The antenna impedance is 50 ohms. But the output impedance of the circuit is much higher than 50 ohms, especially since it has a parallel resonating LC circuit.
The circuit reduces its voltage swing and increases its current swing when its load is 50 ohms.

 

[samy555]

OK
First I would like to thank you Audioguru for quick response
I read your response yesterday night but I did not find what to say.
Second: Yes, I agree with you on what you said, but what occupies my thinking is if someone wants to match that final stage to another stage using any matching networ (like transformer, pi or T), Here he/she should know all of the input and output impedances of the two stages. My question is how can I calculate the Output Impedance of that final stage?
Also why the circuit designer connect a very light load, such as 50ohm-antenna with a very large signal source impedance? There is no maximum power transfer?
Thank you.

 

[FvM]

It's an AC circuit with complex impedances and Vpp/Ipp (or Vrms/Irms) usually don't give a real impedance, at best an impedance magnitude |Z| which is meaningless for impedance matching calculation.

Again, Multisim is WRONG! How can the output current be 46mA p-p when there is no load??
Without the antenna, why isn't the high impedance collector voltage swing very high and current swing very low??

The secret of the simulation results is, that the LC tank doesn't resonate at 100 MHz, thus there's a considerable reactive current into the LC circuit. Secondly the output transistor is heavily overdriven which will be hardly achieved with the original oscillator circuit.

Output impedance can be determined in simulation in a small signal analysis, or by performing load pulling with an external load impedance in transient analysis.

 

[samy555]

It's an AC circuit with complex impedances and Vpp/Ipp (or Vrms/Irms) usually don't give a real impedance, at best an impedance magnitude |Z| which is meaningless for impedance matching calculation.



The secret of the simulation results is, that the LC tank doesn't resonate at 100 MHz, thus there's a considerable reactive current into the LC circuit. Secondly the output transistor is heavily overdriven which will be hardly achieved with the original oscillator circuit.

Output impedance can be determined in simulation in a small signal analysis, or by performing load pulling with an external load impedance in transient analysis.

Thanks for trying to help,
I have more questions:
1 - Is this last stage work as a buffer? If the answer is yes, I know that buffer matches high impedance source with low impedance loads, and this means that the input impedance of the buffer is high to not loading the oscillator, and its Output Impedance be small to fit the light loads like the antenna.
2- What is the function of C2=1n in the third stage?
3- Is it useful to connect the antenna to a tag on L2, i.e to make L2 an autotransformer to improve matching? If tht is true, Why dong that experimentally?
Why not to use the famous formula:
Turns ratio = sqr root( source resistance/load resistance)
But you know we can not use that equation before we calculate Output Impedance of the third stage!!

One final word: Why there is no way to calculate or even measure Output Impedance of any stage contains tank circuit?
Why books, as well as the Web sites ignore to talk about it
Why I did not find so far, and for a long time anyone answer me this question?? Is it a dangerous secret??!!!

Even if there were no answer to my question, does anyone tell me that I was thinking the wrong way and that I wasted my time on a subject circuit designers do not care about.
Thank you

 

[zorro]

The subject has several issues (how to calculate output impedance, accuracy of simulations, matching, etc.).
But in this moment I would remark only one: don't care about 50 ohms.
If my understanding is right, the antenna is a piece of wire of 1.65 m. This is far to present a 50 ohms resistive impedance; if placed over a ground plane it could be rather like a monopole of about lambda/2, so quite reactive and with low series R (or a high parallel R).

Z

 

[BradtheRad]

One final word: Why there is no way to calculate or even measure Output Impedance of any stage contains tank circuit?
Why books, as well as the Web sites ignore to talk about it
Why I did not find so far, and for a long time anyone answer me this question?? Is it a dangerous secret??!!!

Output impedance consists of (a) ohmic resistance, and (b) reactive impedance at the frequency being applied. These determine the amount of current oscillating back and forth in the tank circuit.

Each LC tank is different, both in its ohmic resistance, and in its reactive impedance.

A 1H and 1F tank resonates at the same frequency as 10H and 0.1F. Yet they have different impedances. There are formulae for these calculations, but it gets tedious to do them individually, in order to test all combinations of L, C, freq.

Generally speaking, if you want higher impedance (or low current)...
Select a high Henry value, and small capacitor.

To obtain low impedance (or high current)...
Select a low Henry value, and large capacitor.

These facts are not always brought out in the open by tutorials. They are not necessarily a dangerous secret.

This does not rule out the possibility there are secrets still to be discovered, in the fields of magnetism and electricity, which will allow us to defeat gravity, or tap a source of unlimited power, etc.

 

[FvM]

I have more questions

The only simple point is about the 1 nF capacitor. It's a bypass capacitor and working together with L1 as a supply filter, to prevent RF output spreading over the Vcc node.

The problem with the other questions is that your simplified view on electronic circuits isn't sufficient to describe RF circuits. You don't consider complex transistor in- and output impedances and finite power gain. The circuits can't be calculated as if they where audio amplifiers. So you are possibly looking in the wrong books.

 

[samy555]

Thanks to zorro, BradtheRad and FvM

 

[samy555]

The only simple point is about the 1 nF capacitor. It's a bypass capacitor and working together with L1 as a supply filter, to prevent RF output spreading over the Vcc node.

Why 22.5MHz? the oscillating freq = 100MHz
thanks

 

[FvM]

Its's a filter, no resonant circuit. 8 turns for the choke inductor is just a ballpark figure, preferably you'll use a higher inductance choke in this place.