Patent ID: 7521939

Claim:
A circuit to be inserted in a signal path between a signal generator and a test port of a vector network analyzer (VNA), the circuit enabling the VNA to make scattering parameter measurements of a load (RL) connected to the test port when the signal generator generates signals having frequencies that are below a low frequency limit of an actual dual directional coupler, the circuit comprising: a pi-attenuator including three resistors R 1 , R 2 and R 3 , the resistor R 2 connected in the signal path between the signal generator and the test port of the VNA, and the resistors R 1 and R 3 forming first and second arms of the pi-attenuator; a first op-amp having a non-inverting (+) input, an inverting (−) input and an output, the non-inverting (+) input of the first op-amp being connected to ground, the inverting (−) input of the first op-amp being connected to the first arm of the pi-attenuator, and the output of the first op-amp being connected to the inverting (−) input of the first op-amp by a feedback resistor have a same value as the resistor R 1 of the first arm of the pi-attenuator; a second op-amp having a non-inverting (+) input, an inverting (−) input and an output, the non-inverting (+) input of the second op-amp being connected to ground, the inverting (−) input of the second op-amp being connected to the second arm of the pi-attenuator, and the output of the second op-amp being connected to the inverting (−) input of the second op-amp by a feedback resistor have a same value as the resistor R 3 of the second arm of the pi-attenuator; a third op-amp having a non-inverting (+) input, an inverting (−) input and an output, the non-inverting (+) input of the third op-amp being connected to the output of the second op-amp by a resistor R 6 F , the non-inverting (+) input of the third op-amp being connected to ground by a resistor R 7 F , the inverting (−) input of the third op-amp being connected to the output of the first op-amp by a resistor R 4 F , and the output of the third op-amp being connected to the inverting (−) input of the third op-amp by a resistor R 5 F ; and a fourth op-amp having a non-inverting (+) input, an inverting (−) input and an output, the non-inverting (+) input of the fourth op-amp being connected to the output of the first op-amp by a resistor R 6 R , the non-inverting (+) input of the fourth op-amp being connected to ground by a resistor R 7 R , the inverting (−) input of the fourth op-amp being connected to the output of the second op-amp by a resistor R 4 R , and the output of the fourth op-amp being connected to the inverting (−) input of the fourth op-amp by a resistor R 5 R ; wherein a signal (VF) output by the third op-amp is indicative of a signal incident on the load (RL); and wherein a signal (VR) output by the fourth op-amp is indicative of a signal reflected from the load (RL).