Using Tips for balanced VNA test(2)

When measuring a differential (balanced) device with a traditional network analyzer, the instrument can only generate a single-ended excitation. Through mathematical calculation, the measured single-ended S-parameter is converted into a differential S-parameter. The instrument does not use a differential signal to excite the device under test, but treats it as a single-ended device for measurement. Then use the measured single-ended S-parameters to calculate the mixed-mode S-parameters. Since no real differential signal is used to excite the DUT, the accuracy of this virtual method is difficult to guarantee. The accuracy of this method is acceptable in a small signal (linear) state, but it is difficult to guarantee in a large signal (non-linear) state. [VNA Test Cable Assembly]

When an active device is excited by a large signal, its non-linear characteristics are gradually revealed (usually measured by the 1dB or 3dB compression point). At this time, it is difficult to obtain an ideal result when the traditional virtual method is used to measure the active device. For example, using the virtual method to measure the 1dB compression point of an amplifier is higher than the actual value. If this parameter is used to guide the design, the designed amplifier may be in an overload state, resulting in many non-linear products. However, in the past, network analyzers can only provide virtual methods, because the technology of the network analyzer to control the amplitude and phase of the two output signal sources is extremely complicated.

The working principle of the traditional virtual method is as follows: At each frequency point, the 1 port of the network analyzer outputs a single-ended excitation, the transmission component is measured at the 2, 3, and 4 ports, and the reflection component is measured at the 1 port, and then 2 3, 4 ports output single-ended excitation signal, repeat the above test. Can get 16 single-ended S parameters (S11 to S44), and then use these 16 parameters to calculate the mixed mode S parameter Sxxyy. But for non-linear devices, the 1 port and 2 ports of the instrument cannot output excitation signals, so the performance of the DUT under actual working conditions cannot be reproduced.

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