What sort of circuit has this response generally???

S

strokebow

New Member
Hi Guys,

I have a Real Impedance Vs. Frequency Response and it shape is pretty much a sharp bell shape. The peak occurs at 70MHz and it is positive at all times. Initially the real impedance is ~20 Ohms and rises to ~ 200 Ohms at the peak before falling back to ~20 Ohms.
It looks like a resonant point.

Additional info, the Imaginary Impedance Vs Frequency starts at 0 Ohms, slopes up to a peak ~80 Ohms and then falls at a steep gradient (passing through 0 at ~70MHz) to ~-80 Ohms and then starts to slop back up again.

Please check out the attached responses and tell me what you think it could be?

Thanks for your help!!
 

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  • Responses.jpg
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Looks like homework?
 
MikeMl said:
Looks like homework?
Click to expand...

Its the flippin 19th of August!!! Get a grip

It looks like someone trying to get a equiv cct. for an electric component to me. Who is not doing any such homework.

Anyone who actually wants to help care to comment please do.

Thanks!!
 
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I dont know why I am helping a grouch, but you didn't give enough info to answer the question. That is why I suspected it was homework.

Since you "measured" the impedance, you should have stated how you measured it, and how the measurement might have effected the network you are testing. Chances are that you used a signal generator, or some sort of vector impedance meter, which drive the circuit under test from a 50Ω source. Unfortunately, the 50Ω source impedance effects the network under test...

Here is a sim of a simple circuit which "sort-of" behaves like you describe. I'll leave it to you to iterate on it until it matches your conditions....

Note that LTSpice plots Im( ) as a Magnitude and angle, so when the angle is less than zero degrees, the value is negative.
 

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Last edited:
Another similar network:
 

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  • DF60a.png
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Hey Thanks for your help!

I did, however, use a network analyzer - and properly calibrated it over the range taken.

So, can I ask, how did you come to that conclusion? what was your method of thinking? Please teach me.
 
MikeMl said:
Another similar network:
Click to expand...

Also, without being cheeky, the imaginary impedance i measured actually went negative at the 'resonant' point. Sort of inflected which is not shown by your cct.

But please, explain your method, if you dont mind, so I can have an idea for future fiddlings.
 
The LTSpice sim is showing that the Im() part has an inflection; it just cant plot it that way. Look at the "Phase" of the Im() part, note it changes by 180deg at the inflection point. Look at the faint green dashed line on the plot.

Look at the specs and use model for your Network analyzer again. It will "load" the circuit under test...
 
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Hi,

After a quick look i was going to say that the second plot from the OP was the phase characteristic, not the imag part, but that's just a guess, because it looks like a typical phase plot rather than imag part plot. I havent run through any numbers yet though.
 

That was the way I looked at it also. The upper plot as a bandpass filter response and the lower as the phase response.

EDIT: Got to thinking about the lower plot and I'm not so sure now. I'm doing battle in my head, and now thinking it is the inverse plot of the phase.
 
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Hey guys,

I am still perplexed by this data.

The second plot is actually imaginary impedance versus frequency.

The data was taken from the network analyzer smith chart.


Any ideas?

[perhaps this sketch provides more info. apologies for any confusion].
 

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Last edited:

Here are some curves showing the relevant quantities for a circuit consisting of a 1 µH inductor in parallel with a 5.1695 pF capacitor and also in parallel with a 200 Ω resistor.

The blue curve is the magnitude of the impedance, the black curve is the real part of the impedance, the red curve is the imaginary part of the impedance, and the green curve is the phase of the impedance (in degrees).
 

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