Fluke 5720A Amplitude Control Amplifier and X10 Wideband Amplifier, Wideband Oscillator Filters

Theory of Operation 2

Analog Section Detailed Circuit Description

The vco frequency is controlled by varactor diodes, CR1 + CR2, which get their bias voltage from amplifier U2. Amplifier U2, which gets its input from the phase detector outputs (0V and 0R) of U1, changes the bias on varactor diodes CR1 and CR2 until the divided vco frequency has the same frequency and phase as the 50 kHz reference frequency at the input to the phase detector in U1.

Any phase difference is amplified by U2 and filtered by L6, C53, and C17 to bring the loop into lock.

The vco is shut off whenever the Wideband AC module is not in use. To shut off the vco control line WB ON/OFF* is set low and inverted by a comparator in U7 which then turns on Q3.

With transistor Q3 on, transistor Q2 is turned off which removes the -5V supply from the vco to stop the oscillation.

With the vco frequency between 32 and 64 MHz, U4 divides by 2 to give a symmetrical square wave of 16 to 32 MHz, which is the top octave range required for this assembly. Further division by two (pin 15), four (pin 13), eight (pin 4), and sixteen (pin 2) by binary counter U5 gives the other ranges required of 8 to 16 MHz, 4 to 8 MHz, 2 to 4 MHz, and 1 to 2 MHz.

Multiplexer U6, under software control via control lines WB MUXA, WB MUXB, WB MUXC, and comparators in U7, can be programmed to select which of the 5 ranges is needed to give the output frequencies of 1 to 32 MHz.

2-220. Amplitude Control Amplifier and X10 Wideband Amplifier

The square wave output generated by the phase-locked loop and divider circuit (1-32 MHz OUTPUT) is connected to the amplitude control amplifier circuit.

This circuit uses transistor array U9 to form a differential gain-control amplifier. Gain of the amplifier is controlled by the dc signal AMPLITUDE CONTROL, connected to U9 pin 10.

DC signal AMPLITUDE CONTROL is generated by the thermal rms sensor amplitude control circuitry on the Wideband Output assembly (A5) and is discussed in that section.

The gain controlled square wave output of U9 (pin 6) is further amplified by the x10 wideband amplifier circuit. Transistors Q4, Q5, and Q6 are configured as an amplifier with a gain of ten. This amplifier raises the gain-controlled square wave to the level needed to drive output filters. The output of this circuit is a square wave with an amplitude between 1.4V p-p to 6.0V p-p.

2-221. Wideband Oscillator Filters

Each of the five octave frequency ranges has a 5-pole filter to change the square wave input to a sine wave output with a nominal full scale output of 700 mV rms.

The filter switch drive circuit uses control lines from the filter select circuit (that contains transistor arrays U12 and U13) to provide the drive to properly turn on and off FETs and transistors in each of the filters.

The filter inputs and outputs are switched on by FETs and the filter output is applied to output driver Q7. The corresponding filter is selected automatically as each octave range is selected. The operation of the 16-32 MHz filter is described. The other four filters operate in a similar fashion.

The input FET Q101 is turned on via control line 16-32 MHz FILTER and a transistor in U13. This applies the signal FILTER INPUT from the X10 wideband amplifier to follower Q102. Transistor Q102 drives the filter that contains L102, L103, L104, C109

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