Wrong measurement in dual-band power amplifier

BigBoss said:

Have you ever simulated this circuit in EM simulator? Can you share the results with us??
How could you separate the bands?

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vfone said:

Just looking to your PCB layout, I don't understand why you have different widths for the 50 ohms transmission lines.
OK, if a microstrip line is thinner than its width for 50 ohms, the line will behave as an inductance, but I don't think this was your intention in this design.

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volker@muehlhaus said:

How good is the radial stub decoupling of supply lines at 1.4 and 3.5 GHz?
Layout looks like a narrowband solution, but these two frequencies are far apart.

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albbg said:

As already pointed out in post #2, more detail are required to answer you question. For instance how did you designed the matching network, what about the substrate, ecc.
In any case its very difficult that a circuit at these frequencies, even if simulated, will behaves as expected without an on-board tuning.
If you are quite sure about your design, you should have a look at the gain (by means of a network analyzer using an attenuator on the output to protect the instrument) over the whole band, let's say from 1 to 4 GHz, to see if it could be mistuned.
Then you can use the small metallic tip of a plastic tuning tool to touch each single microstrip. Rather in general, if after touching the gain increases, then the line should by a little bit larger or a small capacitor (tenth of pF) has to be added, otherwise the line should by reduced. However this requires a good amount of experience in tuning RF boards.
Also the input and output impedences may play a role and should be measured. In this case be very careful because in measuring the output impedance the network could be damaged if an input signal is present (the input connector have to be terminated onto 50 ohms) or if the design is not stable a self-oscillation starts.

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dangan1993 said:

Thank you for your reply. Do you think the coupling effect between radial stubs and supply lines is reason??? But at low band, the performance is quite good. !!!

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volker@muehlhaus said:

I am not worried about coupling effects, but the fundamental design of your supply lines. Radial stub is something that we use for narrowband designs, and it looks wrong to me in this dual band context.

In circuit simulation you terminated them with an ideal voltage source at the EM pin/port, which creates an indeal short for all frequencies. You need to check real world response when adding much series inductance there to represent your DC supply lines. Does that have an impact on RF path response?

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Regarding bad results when looking at EM results: when used properly, EM results are more accurate than schematic models.

Looking at your ADS design, one big mistake is that you simulated microstrip models at schematic level (bottom ground only) but in layout you have top metal ground pour very close to all lines. That looks more like coplanar design, and will have a strong effect. If you want a layout that agrees with microstrip models, don't use ground pour on top metal!

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vfone said:

I didn't go deep to measure the impedances vs line widths, and so on.
I assume the 34.12mil is the width of the 50ohms microstrip lines, but what is the reason using different widths (14.44mil and 18.95mil) at the input and output 50 ohms SMA connectors?

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BigBoss said:

Volker is right. Since the whole bottom layer is ground, you don't have to define a second layer being as GND. Single layer and standard GND layer of Momentum is enough.
Distances are very close between microstrip elements and ground pour on TOP layer. You should clean around them. Otherwise, they will act as Co-Planar components.

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BigBoss said:

Don't zip the whole project directory as is, instead use "Archive Project" utility then zip this archived project to post here. And don't forget to include models (and other libraries) for forum users.
And use more appropriate organization for your project tree.

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dangan1993 said:

Thank you for your comment. The radial stub is intended to be short at 1.4 GHz and the overall supply line is a Dual-Band Biasing network that means it provides open at both fundamental frequency band. An ideal voltage source can provide short at all frequencies,

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volker@muehlhaus said:

Yes, but you don't have that ideal short there in real world hardware!
You really really need to check if your bias lines are properly decoupled with arbitrary impedance at that pin!

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dangan1993 said:

I also have added two bypass capacitors that can too yield short circuit at all frequencies.

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volker@muehlhaus said:

Yes, these 2.4nF are ideal in schematic simulation. Real world response at GHz frequencies is another topic ...

Good luck with your design!

Volker // Keysight Certified Expert EDA

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volker@muehlhaus said:

I am not worried about coupling effects, but the fundamental design of your supply lines. Radial stub is something that we use for narrowband designs, and it looks wrong to me in this dual band context.

In circuit simulation you terminated them with an ideal voltage source at the EM pin/port, which creates an indeal short for all frequencies. You need to check real world response when adding much series inductance there to represent your DC supply lines. Does that have an impact on RF path response?

~~

Regarding bad results when looking at EM results: when used properly, EM results are more accurate than schematic models.

Looking at your ADS design, one big mistake is that you simulated microstrip models at schematic level (bottom ground only) but in layout you have top metal ground pour very close to all lines. That looks more like coplanar design, and will have a strong effect. If you want a layout that agrees with microstrip models, don't use ground pour on top metal!

Click to expand...

volker@muehlhaus said:

Try again with your EM substrate modified so that bottom boundary is directly below the TLY substrate.

No layer names shown in my screenshot because your project references an external library that was not included. That also means we can't see your EM port layer definitions, and can't see if you defined your EM ports to use the explicit reference layer. By removing the bottom metal from EM stackup, we ensure that bottom boundary is used for port reference, and that should be perfect here because you have no cutouts in ground plane anyway.

Click to expand...

volker@muehlhaus said:

Try again with your EM substrate modified so that bottom boundary is directly below the TLY substrate.

No layer names shown in my screenshot because your project references an external library that was not included. That also means we can't see your EM port layer definitions, and can't see if you defined your EM ports to use the explicit reference layer. By removing the bottom metal from EM stackup, we ensure that bottom boundary is used for port reference, and that should be perfect here because you have no cutouts in ground plane anyway.

Click to expand...

volker@muehlhaus said:

Try again with your EM substrate modified so that bottom boundary is directly below the TLY substrate.

No layer names shown in my screenshot because your project references an external library that was not included. That also means we can't see your EM port layer definitions, and can't see if you defined your EM ports to use the explicit reference layer. By removing the bottom metal from EM stackup, we ensure that bottom boundary is used for port reference, and that should be perfect here because you have no cutouts in ground plane anyway.

Click to expand...