Loadpull Compression point impedance using xdb analysis in Harmonic Balance

Hi SkkyLee,

I'm just curious, but what does the Pout vs. Pin plot look like if you were to pick a purely real value of 10 for the load impedance?

David Webster

This is the interesting phenomenon, when the real part impedance is bigger than the optimum load's real part, the xdb loadpull contour is fit to compression point sweep result very well, when the load impedance is 10Ω(it's bigger than the real part of the optimum load which is about 7Ω), the OP1dB is 30.39dBm, which fit to the xdb contour result.

Dear SkkyLee,

SkkyLee said:

But I don't perform transient analysis, the result 30.416dBm OP1dB was gotten when I perform hb analysis's sweep function(as screenshot shown below), so I want to confirm that when perform pss/hb simulation's sweep function (for my instance, pin sweep from -20 to 30dBm), a set of transient analyses are executed?

I now understand that the power transfer function plot is a result of an HB sweep of the input power and not a set of transient analyses. Thank you for this clarification SkkyLee.

SkkyLee said:

(But I don't know how to verify this assumption, because I don't understand what the small-signal transfer function means exactly, does it mean the Power Amplifier's gain when the input power is very small?(Also my teacher's advice, he says the small-signal conception which used in the sp/ac analysis, the power level is always below -50dBm, but my PA simulation, the minimum input power is -20dBm, I don't know can this power level be called "small-signal"). And I don't know why the slope value is set as 1.05 the 2nd time(as the screenshot shown below), because of the "least squares algorithm"?

To determine the X dB compression point, one must know the small-signal gain of the amplifier. The small-signal gain is the gain when the output shows essentially no harmonic distortion. The actual input power range that can be used to measure the small-signal gain is totally dependent on the gain characteristics of the amplifier under study. There is not a single range of input power levels that can be used to reliably provide a good estimate of the small-signal gain. In your plot of input to output power, I found that the slope and intercept used to compute the 1 dB compression point has a slope of 1.00 and an intercept of 12.166 dBm if one uses a range of input power levels between -20 dBm and -13 dBm . This produces your 1 dB compression point at an output power of 30.39 dBm.

However, if I use a smaller range of input powers to compute the linear fit (-20 dBm to -19 dBm), the slope and intercept are 1.05 and 12.984 dBm. When I used this curve to estimate the 1 dB compression point output power, I obtain 28.497 dBm. Hence, my hypothesis is that the small-signal gain of your amplifier in the sweep analysis is different than the small-signal gain of your amplifier in the Xdb analysis.

Unless you know the distortion characteristics of your amplifier output signal as a function of ihe input power. I think you want to use the smallest range of input power possible. I do not know how Cadence's tool chooses the range of input powers to use in its curve fit to your sweep. It may use the gain of "1.0" you provided in the form:

But, perhaps, you might try reducing the input power sweep to start at something much less than -20 dBm to see if the 1 dB  compression point output power is changed from 30.39 dBm.

SkkyLee said:

This is the interesting phenomenon, when the real part impedance is bigger than the optimum load's real part, the xdb loadpull contour is fit to compression point sweep result very well, when the load impedance is 10Ω(it's bigger than the real part of the optimum load which is about 7Ω), the OP1dB is 30.39dBm, which fit to the xdb contour

One of the reasons there is a discrepancy between your two values of 1 dB compresion point output power is the peaking in the power gain curve. This makes the 1 dB compression point output power very sensitive to the small-signal gain used in its estimate. When you change the load impedance from its matched value to use a real part of 10 ohms, it is likely the peaking in the power gain curve is no longer present. Intuitively, with the larger output impedance real part, my guess is the output current of your amplifier is less and hence it does not show the non-linear peaking in gain with input power. Therefore, if the output impedance changes the relationship between input and output power to eliminate the peak, I would expect the 1 dB compression power to be far less sensitive to the small-signal gain value and , as a result, show consistent results in your Xdb and HB sweep analyses.

Shawn

Dear SkkyLee,

SkkyLee said:

But I don't perform transient analysis, the result 30.416dBm OP1dB was gotten when I perform hb analysis's sweep function(as screenshot shown below), so I want to confirm that when perform pss/hb simulation's sweep function (for my instance, pin sweep from -20 to 30dBm), a set of transient analyses are executed?

I now understand that the power transfer function plot is a result of an HB sweep of the input power and not a set of transient analyses. Thank you for this clarification SkkyLee.

SkkyLee said:

(But I don't know how to verify this assumption, because I don't understand what the small-signal transfer function means exactly, does it mean the Power Amplifier's gain when the input power is very small?(Also my teacher's advice, he says the small-signal conception which used in the sp/ac analysis, the power level is always below -50dBm, but my PA simulation, the minimum input power is -20dBm, I don't know can this power level be called "small-signal"). And I don't know why the slope value is set as 1.05 the 2nd time(as the screenshot shown below), because of the "least squares algorithm"?

To determine the X dB compression point, one must know the small-signal gain of the amplifier. The small-signal gain is the gain when the output shows essentially no harmonic distortion. The actual input power range that can be used to measure the small-signal gain is totally dependent on the gain characteristics of the amplifier under study. There is not a single range of input power levels that can be used to reliably provide a good estimate of the small-signal gain. In your plot of input to output power, I found that the slope and intercept used to compute the 1 dB compression point has a slope of 1.00 and an intercept of 12.166 dBm if one uses a range of input power levels between -20 dBm and -13 dBm . This produces your 1 dB compression point at an output power of 30.39 dBm.

However, if I use a smaller range of input powers to compute the linear fit (-20 dBm to -19 dBm), the slope and intercept are 1.05 and 12.984 dBm. When I used this curve to estimate the 1 dB compression point output power, I obtain 28.497 dBm. Hence, my hypothesis is that the small-signal gain of your amplifier in the sweep analysis is different than the small-signal gain of your amplifier in the Xdb analysis.

Unless you know the distortion characteristics of your amplifier output signal as a function of ihe input power. I think you want to use the smallest range of input power possible. I do not know how Cadence's tool chooses the range of input powers to use in its curve fit to your sweep. It may use the gain of "1.0" you provided in the form:

But, perhaps, you might try reducing the input power sweep to start at something much less than -20 dBm to see if the 1 dB  compression point output power is changed from 30.39 dBm.

SkkyLee said:

This is the interesting phenomenon, when the real part impedance is bigger than the optimum load's real part, the xdb loadpull contour is fit to compression point sweep result very well, when the load impedance is 10Ω(it's bigger than the real part of the optimum load which is about 7Ω), the OP1dB is 30.39dBm, which fit to the xdb contour

One of the reasons there is a discrepancy between your two values of 1 dB compresion point output power is the peaking in the power gain curve. This makes the 1 dB compression point output power very sensitive to the small-signal gain used in its estimate. When you change the load impedance from its matched value to use a real part of 10 ohms, it is likely the peaking in the power gain curve is no longer present. Intuitively, with the larger output impedance real part, my guess is the output current of your amplifier is less and hence it does not show the non-linear peaking in gain with input power. Therefore, if the output impedance changes the relationship between input and output power to eliminate the peak, I would expect the 1 dB compression power to be far less sensitive to the small-signal gain value and , as a result, show consistent results in your Xdb and HB sweep analyses.

Shawn