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Input return loss of Amplifiers

Amr Wael

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Hello ,
I would like to ask a very basic question regarding the input return loss (S11) of any amplifier.
I understand S11 describes how much power is reflected back to port 1 ; therefore , most designers tend to keep the magnitude in dB less than -10 dB (the lower the better) over the desired frequency of operation.
My question is why the return loss drops at certain frequencies before it rises again ? I understand that this might indicate that the amplifier is perfectly matched at those frequencies but it seems like repeated oscillations , so why does this happen? does it repeat every quarter wavelength for example due to the repetitive nature of the wave? I thought about that but separation of frequencies between the oscillations aren't uniform.
Thank you very much
Amr
 

spudboy488

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It will be flat if the amplifier input is purely resistive. Any reactance will vary with frequency. It will dip when the reactance is at the match impedance.
 

BigBoss

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These repeating peaks and dips are coming from the cable of the VNA.
Probably bad quality cables or/and bad calibration.
 

Amr Wael

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These repeating peaks and dips are coming from the cable of the VNA.
Probably bad quality cables or/and bad calibration.
Thank you Big boss but I am using ADS to simulate , haven't done any HW measurements yet.
 

Amr Wael

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This phenomena may appear in simulations too if there are some transmission lines.
Yes I am using microstrip Lines & Coplanar waveguides but I am trying to find out a physical explanation for this Phenomena.
 

FvM

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Your question isn't very clear. Are you asking about effects that only occur with a connected transmission line, or also directly at the amplifier ports? In the former case, expect that the transmission line has characteristic impedance different from reference impedance (e.g. 50 ohms).
 

Amr Wael

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Your question isn't very clear. Are you asking about effects that only occur with a connected transmission line, or also directly at the amplifier ports? In the former case, expect that the transmission line has characteristic impedance different from reference impedance (e.g. 50 ohms).
I designed a PCB using ADS Layout
The microstrip lines are impedance matched to the IC of the amplifier.
I performed EM simulation on the PCB and found those dips in the S11.
 

BigBoss

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I designed a PCB using ADS Layout
The microstrip lines are impedance matched to the IC of the amplifier.
I performed EM simulation on the PCB and found those dips in the S11.
So that's means the Characteristic Impedance of the Transmission Line is not exactly 50 Ohm therefore you see ripples on S11.
 

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