Using Tips for balanced VNA test(1)

The traditional vector network analyzer (VNA) usually uses the so-called "virtual" method when measuring balanced/differential devices: the network analyzer uses a single-ended signal to excite the device under test, and then it is measured. Balance (unbalanced) parameters, and then the network analyzer converts the unbalanced parameters into balanced parameters through mathematical calculations. This method is sufficient for analyzing active/passive devices in the small signal (linear) state. But when the device is in a large signal (non-linear) working state, the accuracy of the test result of this method is limited. Although people have come up with many methods to overcome this problem: such as using "ideal" broadband power dividers or couplers, these methods are unable to perform full system calibration.

Microwave monolithic integrated MMIC (monolithic-microwave-integrated-circuit) amplifier. It can be seen that in the case of small signal (linear), the measurement results of the two methods are the same, but when the amplifier is in a compressed state (large signal), the measurement results of the two methods are significantly different. The gain measured by the true differential excitation is compressed 4dB earlier than the result of the virtual method, and the measurement result of the maximum gain is also 0.5dB lower.

The improvements (advantages) of this new technology are as follows:

1. At present, differential amplifiers are widely used in mobile phones, smart phones, data cards and other mobile devices. However, most of these devices are currently tested using virtual methods (because there was no true differential testing technology before). This means that the device characteristics measured so far are not correct.

2. If the actual compressed power of the device is lower than the manufacturer's mark (because the manufacturer currently uses the virtual method to test), it means that many amplifiers now work in a compressed (overload) state, and their actual intermodulation products It is much higher than the design value.

3. Manufacturers that use virtual methods to design and produce mobile phones must currently use "power back-off" technology to achieve ideal linear power performance.

However, the use of "power back-off" technology means that more (or higher output power) active devices are required to achieve the specified output power, and the entire      transmitter part may need to be redesigned.

Of course, if the characteristics of the balanced device can be tested more accurately, the device and system manufacturer can design the ideal performance and working conditions before the product leaves the factory (rather than after a problem occurs in use).

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