Fluke 5720A Wideband Output Attenuators, Wideband Output RMS Sensor and Amplitude Control Circuit

Theory of Operation 2

Analog Section Detailed Circuit Description

2-224. Wideband Output Attenuators

The 10 Hz to 30 MHz sine wave signal output of the power amplifier circuit connects to the 50Ω output attenuator composed of attenuator networks Z1 and Z2 and relays K4, K5, K6, K7, and K8.

Relay K4 switches the 10 dB attenuator into the circuit when activated or bypasses it when not activated.

Relay K5 switches the 20 dB attenuator into the circuit when activated or bypasses it when not activated.

Relays K6 and K7 switch the 40 dB attenuator into the circuit when activated and bypass it when not activated. The attenuator can thereby reduce the signal level in 10 dB steps from 0 to 70 dB.

Power Amplifier output amplitude is continuously variable over a 10 dB range, which when combined with the 0 to 70 dB attenuator, gives a continuous output range of 300 μV to 3.5V rms.

When energized, relay K8 enables the output signal to be connected to the output coaxial connector J2.

2-225. Wideband Output RMS Sensor and Amplitude Control Circuit

The rms sensor circuit is used for:

∙Amplitude control

∙Overload control over the frequency range of 12 kHz to 30 MHz

∙Overload control only over the frequency range of 10 Hz to 12 kHz

The rms sensor and amplitude control circuit is composed of U1, U2A, U2B, U3, U5, U12A, U12B, and associated components. Thermal sensor U1 provides a dc voltage equal to the rms value of the input voltage (at pin 6).

Input voltage to the thermal sensor comes from the power amplifier circuit. DC voltage output from the thermal sensor is connected to U2A configured as an integrator. The output of the integrator is connected to a square-root amplifier configured by U2B U3, and U12A, which keeps the settling time of the sensor constant.

The dc output of this sensor circuit (available at TP1) is buffered by U5. In the 12 kHz to

1.1MHz range, the output of U5 is connected to the RCL line by relay K10 in the set position. In the 1.2 MHz to 30 MHz range, the output of U5 is compared by U12B to

DAC OUT.

The DAC assembly (A11) output, DAC OUT HI and DAC SENSE HI, are tied together by relay K11 to create DAC OUT.

Relay K2A connects capacitor C7 into the rms sensor circuit in the 10 Hz to 11 kHz range to add additional filtering for low frequency signals.

2-226. Wideband Output Overload Control Circuit

The overload control circuit contains comparator U4 (A and B), FETs Q2 and Q3, transistor Q1, and associated circuitry. This circuit protects the rms sensor and attenuators during an overload condition. Comparator U4A detects an overload condition by comparing the dc output of the rms sensor against a reference voltage.

The reference voltage is determined by zener diode VR2 and resistor R21. If the dc output of the rms sensor reaches a voltage 10% greater than the normal full-scale voltage, the output of U4A goes negative. This negative voltage causes a positive voltage at the output of U4B which turns on FETs Q2 and Q3. These FETs protect the rms

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