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Power detector simulation in ADS

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saad

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Hello everyone,
I am simulating a schottky diode power detector in ADS. The schematic and the diode model are in attachment.

I am interested in computing the extra DC current due to the RF input (due to the square term) to further compute its voltage sensitivity. Hence, I use Harmonic balance simulator to find the difference HB.I_Probe1.i[0]-DC.I_Probe1.i.

From the Taylor expansion of the diode IV equation, it can be found that the coefficient of the 2nd harmonic current term and the extra DC current is identical.

From the simulation, I can get HB.I_Probe1.i[2] to be close to the theoretical prediction but the DC term is very different.

Am I missing something?

Saad
 

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From the simulation, I can get HB.I_Probe1.i[2] to be close to the theoretical prediction but the DC term is very different.
Any even order nonlinearity produce DC components.
 

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Any even order nonlinearity produce DC components.

Exactly. I am actually interested in computing the DC term due to the second order nonlinearity. I do so by calculating the difference HB.I_Probe1.i[0]-DC.I_Probe1.i.

The problem I am facing is that the magnitude of this extra DC current (due to 2nd order nonlinearity) is different from the theoretically predicted value. During expansion of the diode IV curve, the coefficient of the 2nd harmonic and the DC term are the same, as can also be seen by the attached thumbnail of your reply above. The second harmonic current, HB.I_Probe1.i[2] matches well with theory.
 

The problem I am facing is that the magnitude of this extra DC current (due to 2nd order nonlinearity) is different from the theoretically predicted value.
I can not understand what you want to mean at all.

DC current can not be determined by only 2nd order nonlinearity.

HB.I_Probe1.i[0] is a result from all even order nonlinearity.
 

You consider only static I-V characteristics as nonlinearity.

Capacitor also behave nonlinearly
 

You consider only static I-V characteristics as nonlinearity.
Capacitor also behave nonlinearly

I did not get your point. I am using either ideal DC blocks or ideal capacitors from ADS.

Nevertheless, the magnitude of second harmonic is consistent with what I mathematically calculate. I dont see any reason for the DC term to be different.

Also, I am assuming that HB.I_Probe1.i[0] includes the static DC as well as the current due to nonlinearities. **That is why I subtract DC.I_Probe1.i from it to calculate the current due to nonlinearity of the circuit.** Is this assumption fair?
 

I mean capacitor in diode.

Ok but I dont think that *it* could be the cause of huge discrepancy, else it would reflect in the second harmonic term too.

The second harmonic current term is within 10% of what I calculate from diode IV equation. The DC term is orders of magnitude smaller.
 

Also, I am assuming that HB.I_Probe1.i[0] includes the static DC as well as the current due to nonlinearities.
I can not understand what you want mean at all.

**That is why I subtract DC.I_Probe1.i from it to calculate the current due to nonlinearity of the circuit.**
I can not understand what you want mean at all.

Increase maximum order of HB analysis such as 15 or 31.
 

Due to the DC bias voltage, there will be a static DC current through the diode (in the absence of RF input voltage), which I believe is captured by the DC analysis (i.e. the term DC.I_Probe1.i).

Due to the nonlinearity of the circuit, there will be a DC current through the diode (In the absence of the DC bias voltage), which I believe is captured by HB analysis (i.e. the term HB.I_Probe1.i[0])

However, when I have both DC and RF sources in the circuit, the HB current term (HB.I_Probe1.i[0]) will contain the total DC current, i.e. due to both DC and RF sources.

So, if I want to know ONLY the extra DC current term (which appears in the presence of RF input due the circuit nonlinearity), I use the difference term HB.I_Probe1.i[0]-DC.I_Probe1.i

Does it make sense to you now?
 


There will be a static DC current due to the dc source Vb too, right? And I want to characterize only the device nonlinearity by looking at the magnitudes of various harmonic components. How else do I do that?

Increase HB order.
Or try transient analysis.

I did increase the HB order but that does not help.
 

There will be a static DC current due to the dc source Vb too, right?
Not right.
It is not static DC current, if input power is large.

I did increase the HB order but that does not help.
Did you surely increase order such as 15 or 31 ?

Try transient analysis.
 
Last edited:

Not right.
It is not static DC current, if input power is large.

May be I am using the term incorrectly. By static DC, I mean the DC current in the absence of any AC input.


Did you surely incease order such as 15 or 31 ?

I am attaching the results for order 5 and 31. Input power in each case is -50 dBm, which causes a 0.1mV peak ac signal at the diode input.

Try transient analysis.

In case of transient analysis, what result should I specifically look for?
 

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Also, what in your opinion should the term, HB.I_Probe1.i[0]-DC.I_Probe.i, give me when the input power and the resultant ac signal swing across the diode, is quite small.
 

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