Specifications
After the filter-amplifier chain, the signal entersa0to30dBswitched at
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tenuator for gain ranging prior to detection. A comparator tree is used to
supply the control voltages required by this attenuator. While the attenu
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ator appears to have quite repeatable and stable attenuation values, the
absolute values from attenuator to attenuator vary substantially. This is
not surprising based on the manufacturer’s accuracy specifications of ap
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proximately ± 0.2 to 1 dB (depending on attenuation setting). Since the
attenuation value is used in insertion gain calculations, the absolute ac
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curacy can be somewhat important, particularly for slightly lossy DUTs.
It is estimated that an attenuation accuracy of 0.1 to 0.2 dB is required to
meet system accuracy expectations, hence some additional calibration is
required.
A factory BACK-END ATTENUATOR CAL has been developed to per
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form this task and is similar to an ALC calibration in how it is performed
and stored in nonvolatile memory.
During this calibration, a throughline is connected from Port 1 to Port 2
and the system is automatically tuned to generate an IF of 25 MHz. The
level of this signal is adjusted using the ALC and the step attenuator to
get it into an appropriate range for the noise detector. The attenuator is
then moved a single 2 dB step and the difference in detected power is
measured. The sinusoidal power is then again adjusted for the detector
dynamic range and the process is repeated for all attenuator steps. These
attenuator values are stored for later use in insertion gain calculations.
The power detector is a simple Schottky diode. Because of dynamic range
concerns, the detector is driven current mode and impedance transforma-
tion (50 ohms to a few kilohms) circuitry is added. A bias circuit is used to
turn on the main detector while keeping it temperature stable.
Shielding is important in this circuit, as might be expected because of the
small signal levels involved and the large amount of gain present on the
chain (up to 90 dB).
2-24 MS462XX MM
OPTION MODULES THEORY OF OPERATION