Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 98

In some VNA designs, a step attenuator is added between the reference coupler and the test coupler to allow a greater change in the source power setting beyond that which the source ALC circuit can produce. This step attenuator has the additional benefit of providing a good match to the test port. In cases where the power source‐match and ratio source‐match are not the same, the step attenuator reduces the difference between these values based on twice the attenuation value. Reducing the difference between power source‐match and ratio source‐match allows one to compute the error in source power from the ratio source‐match, which is determined as part of the normal calibration process. In general, the power source‐match is not characterized during any normal calibration.

Another issue to be concerned about with step attenuations is their effect on the quality of the measurement when the attenuation value is changed. In most newer VNAs, the nominal attenuator value is known, and the effective value of the reference receiver is compensated for when the attenuator value is changed. The source ALC power is also changed, so that changing the attenuator value causes only a slight change in the value of the power coming from the test port; the nominal attenuator value is usually within 0.25–0.5 dB of the actual attenuator value. Since the port power stays the same, the internal source power must be raised by the amount of increase in the step attenuator. Since the reference receiver comes before the step attenuator, it will see a larger signal value; as it is desired to have the reference receiver power display the same value as the port power, its reading is also decreased by the value of the step attenuator.

Placing the step attenuator at this point in the block diagram has a distinct advantage in that it allows a large signal in the reference channel even when a small signal is needed at the test port, providing a low noise signal.

The loss of the step attenuator is well compensated for, but its effect on the match is not. If the preset condition has the step attenuator set to 0 dB, the match of the port is terminated back in the test port switch. When even a single stage of test port attenuation is used, the predominate source and load match characteristic is set by the match of the attenuator, which is typically quite good. Thus, it is good practice to use some source step attenuation if the maximum test port power is not required for the measurement. And since the raw match is better for any attenuator setting other than the 0 dB step, the effect on a calibrated measurement if the attenuator is switched to a different, non‐zero, value is smaller if the calibration is performed with some attenuation applied.

Some older VNAs did not allow changing the attenuation value after calibration and did not compensate for the nominal value of an attenuator change; error correction in these VNAs is often turned off for a step attenuator change.

In general, changing a step attenuator will change all the raw error terms on the port that has the step attenuator. Techniques are discussed in Chapter 3 that can compensate for much of this change.