RF S parameters measurments
i have a chip , with RF output , i need to measure the output impedance of the chip by using VNA ,
when i connect the VNA to the output ans measure S11 , it shows negative resistance do to that the device is transmitting RF power
i need to know to over come this issue , and how to get the actual impedance is it just the absolute value of the impedance or what
khouly
I think this chip is not conjugate matched, but maximum power matched,
the output impedence maybe is meaningless.
if this chip is already matched to 50 Ohm, you just need to check the linearity maximum output power and PAE.
if it is not matched to 50Ohm, you should using Slide Screw Tuner to find the optimum impendence.
it is not a power amplifier , it is a driver amplifer , it should be matched to 50 ohm , coz the power amplifier i will add will be 50 ohm input impedance
khouly
This is related to the topology of the input and output matching networks.
In general a High Pass topology at the input and a Low Pass at the output it will avoid to get this kind of trouble.
Also the gate/base bias adjustment would affect the S11 vs. Frequency response.
If this is a driver the best would be to tune its output match with the input of the PA connected, because in reality this one could be different than a real 50 ohms.
i need to know how to measure the ouput impedance correctly
khouly
whether it is an driver or PA, you can all use a Slide Screw Tuner to find the optimum impendence.
then you can build up you match circuit
because the input impendence next stage is 50Ohm
correct?
Search google for "hot S22" and there are a few app notes. If the output power is a few dB below the damage limit to VNA. Can you measure slightly below and above frequency enough that the VNA is not affected by the drive? Interpolate between the two measurements to get your center frequency. You can also a stub tuner as a notch filter.
varying(reducing) the power level of the VNA may be an option