N2PK VNA in combination with a hybrid combiner and attenuator can be used for measuring active devices such as amplifiers. VNA gives +4 dBm at the RF Output (front panel) and -2 dBm at he Auxiliary output (rear panel LO OUT for detector 1 in my configuration). In generator mode we can set different frequencies for both outputs of VNA. To achieve the same signal level we need to insert 6 dB attenuator to the RF output. This output level usually is not enough to measure passive circuits with high IP3. At the output of the hybrid combiner we will get maximum level abt. -8 dBm, which in combination with simple spectrum analyzer usually does not lead to the measurement result (high level of noise floor, low level of IMD products...). Signals from the DDS need to be intensify for such measurements.
What are the basic requirements for different parts of the measuring system?
- Independent signal sources with sinusoidal outputs
- As high as possible reverse isolation of both RF signal sections
- Maximum possible linearity of RF amplifiers
- Maximum suppresion of the second harmonics of both signals at the combiner input
- As high as possible directivity of the combiner
- Best impedance matching of all components of the kit
- Respect the dynamic range of Spectrum Analyzer
Block diagram of the measuring set was designed to meet these requirements as possible.