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In message <kt7o77+jld6@...>, dated Tue, 30 Jul 2013, christianvierck <christianvierck@...> writes:

I thought it was right to simulate it with a single resistor, because the only thing I knew about antennas was that they are oscillating circuit.

No, they MAY be resonant circuits but, being passive, they cannot be oscillating circuits.

So I removed the imaginary content and only the resistor was left. I will improve it as you told me.

It looks like your power supply voltage, V1, is not set to any DC voltage.
Its AC value should be 0. Its DC value should be 6.

I know it, because the schematic told me it is a DC voltage. But I had to change it due to the AC analysis for the frequency.

No, the SUPPLY voltage is always DC. For AC analysis, normally the INPUT voltage would be AC, but in this case the only input voltage you have is from the microphone, which doesn't help much to find out if the UHF part of the circuit is working. If the first stage is indeed oscillating, it provides its own input voltage in practice, but for simulating an AC sweep you need to add a generator somewhere in the UHF circuit, and in series between the 1 - 4 pF trimmer and earth looks a good place. I suggest you set the sine wave signal to 50 mV initially and sweep from 300 MHz to 3 GHz. Look at the output voltage at the collector of the second transistor.

If the circuit really is oscillating, the output voltage will go crazy as the added generator sweeps through the oscillating frequency.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
Why is the stapler always empty just when you want it?

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

If the circuit really is oscillating, the output voltage will go crazy
as the added generator sweeps through the oscillating frequency.

I don't know what I awaited, but I see a clear maximum. And it gets closer to 900MHz. Nice.

I simulated it with .TRAN and it worked well. The simulation through the first transisitor was congruant to the oscillation at the antenna.

Relating to the frequency in the circuit, I have to do an AC analysis. But
I understand, that I have to use the .TRAN to simulate the oscillation.

Why do you say you have to do an AC analysis?

An AC analysis tells you nothing about what the oscillator is doing. It
won't tell you the frequency where it oscillates. It won't tell you even
IF it is oscillating. You MUST use a .TRANsient analysis to check for (a)
whether it oscillates, and (b) at what frequency.

The antenna load doesn't do anything.
...


I thought it was right to simulate it with a single resistor, because the
only thing I knew about antennas was that they are oscillating circuit. So
I removed the imaginary content and only the resistor was left. I will
improve it as you told me.

That's correct to use the resistor. At the antenna's resonant frequency,
the antenna looks like a resistor, to ground. (A quarter-wave antenna over
a ground plane probably looks like about 35 ohms, BTW.)

However, a resistor with the other end floating, or shorted across the
resistor, is just an open circuit (i.e., no load) because zero current
flows to it.

It looks like your power supply voltage, V1, is not set to any DC voltage.

Its AC value should be 0. Its DC value should be 6.

I know it, because the schematic told me it is a DC voltage. But I had to
change it due to the AC analysis for the frequency.

This doesn't make sense. What you have is a 0V power supply with about 6V
of AC ripple. I can't figure out why you would want to do that, nor what
you would get from your AC analysis. Effectively, your AC analysis shows
you how much the AC ripple on the power supply feeds through to the output.
Also, with 0V DC applied, the transistors are non-functional.

I can't emphasize enough that you really need to be doing a TRANsient
analysis. I think you don't understand yet the purpose of an AC analysis.

Regards,
Andy

I thought it was right to simulate it with a single resistor, because
the only thing I knew about antennas was that they are oscillating
circuit.

No, they MAY be resonant circuits but, being passive, they cannot be
oscillating circuits.

I think maybe he meant to write "the only thing I knew about antennas was
that they are resonant circuit."

If the circuit really is oscillating, the output voltage will go crazy

as the added generator sweeps through the oscillating frequency.

In an AC analysis, the oscillator will NOT be oscillating. AC analysis
finds the DC operating point, then linearizes all nonlinear devices
(transistors and diodes) at that point. Then it looks at your circuit as a
fully passive network and see how much of the applied AC propagates through
the circuit.

In a TRANsient analysis, if you also applied a small sine-wave signal and
swept its frequency, you may or may not see interaction between the applied
signal and the oscillator's own frequency. Chances are the applied signal
would "pull" (or is it "push"?) the oscillator and force it to lock up with
the applied signal.

Andy

I think maybe he meant to write "the only thing I knew about antennas was
that they are resonant circuit."

Exactly! Thanks.


I red a tutorial about the analysis modes and understand your point now.

My knowledge about this was, if you have to analyse the frequency response do a AC and if you have to analyse the time response do a tran.

This is totally my fault, that I was ill-informed. I trusted in the words of someone without refering to another source.