Fluke 5720A service manual Wideband Output Assembly A5

5700A/5720A Series II Calibrator

Service Manual

and C112. FET Q105 is turned on, in the same manner as FET Q101, to connect the sine wave output of this filter to FILTER OUTPUT. This is applied to output driver transistor Q7.

To obtain the required amount of filtering over the entire octave range, additional capacitors C110 and C111 are switched into the filter by PIN diodes CR102 and CR103 when operating below 70% of the full range (22 MHz in this case).

Diodes CR102 and CR103 are activated by turning on Q103 and Q104, respectively. Transistors Q103 and Q104 are turned on via control line Filter Band Switch and a transistor in U12.

Two additional FETs, Q8 and Q9, are used to isolate the 16-32 MHz filter and the 8-16 MHz filter, from the three other lower frequency filters, whenever the two high frequency filters are used. The isolation provided by FETs Q8 and Q9 eliminates both the input and output load capacity and circuit board capacity of the three lower frequency filters, so that follower Q6 can drive the high frequency filter input without distortion of the waveform. This also eliminates loading the filter outputs, which changes the filter frequency response.

2-222. Wideband Output Assembly (A5)

The Wideband Output assembly (A5) takes the sine wave signal from either the Wideband Oscillator assembly (A6) or the Oscillator Output assembly (A13), and amplifies it to a power level that drives 3.5V into a 50Ω load at the output.

The Oscillator Output assembly generates the sine wave signal from 10 Hz to 1.1 MHz, and the Wideband Oscillator assembly generates the sine wave signal from 1.2 MHz to 30 MHz. This provides a total frequency range of 10 Hz to 30 MHz.

The Wideband Output assembly contains a power amplifier circuit that increases the gain and/or power level of the input signal. Also contained on the assembly are a thermal rms sensor circuit necessary for amplitude control, the 50Ω attenuators needed to reduce the output level through all of the amplitude ranges, an overload control circuit, and digital control circuitry.

Figure 2-37 is a simplified schematic for the Wideband Output assembly.

The Wideband Output assembly has three basic operating ranges:

1.The first range is for Wideband AC module operation at frequencies between 10 Hz and 11.9 kHz. In this frequency range, relay K1A is in the set position so the input signal is from the Oscillator Output assembly with its amplitude is controlled by the Oscillator Control assembly (A12). The amplitude control circuit on the Wideband Output assembly is only used for overload protection. This is done by control line

PB0 which goes high and turns on FETs Q2 and Q3 via comparator U4B. These FETs shunt the rms sensor’s input and output to ground through 200Ω resistors (R2 and R18) to reduce the sensitivity of the sensor.

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