Fluke 123B/S Service Manual - Page 32

Channel A, Channel B Measurement Circuits, 2.2 Trigger Circuit

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123/124 Service Manual 3.2.1 Channel A, Channel B Measurement Circuits The Channel A and Channel B circuit are similar. The only difference is that Channel A can do all measurements, whereas Channel B does not provide resistance, diode, and capacitance measurements. Volts, and derived measurements (e.g. current with optional probe) The input voltage is supplied to the C-ASIC, via the LF and HF path. The C-ASIC converts (attenuates, amplifies) the input signal to a normalized output voltage ADCA/ADC-B, which is supplied to the Analog to Digital Converters (ADC-A and ADC-B) on the DIGITAL part. The D-ASIC acquires the digital samples to build the trace, and to calculate readings. For the HF and LF attenuation section of the C-ASIC some external components are required: the HF DECade ATTenuator and LF DECade ATTenuator section. Resistance, continuity, and diode measurements (Input A only) The T-ASIC supplies a current via the Ω/F relays to the unknown resistance Rx, connected to the Input A and the COM input jacket. The voltage drop across Rx is measured as for voltage measurements. Capacitance measurements (Input A only) The T-ASIC supplies a current via the Ω/F relays to the unknown capacitance Cx, connected to the Input A and the COM input jacket. Cx is charged and discharged by this current. The C-ASIC converts the charging time and the discharging time into a pulse width signal. This signal is supplied to the T-ASIC via the C-ASIC trigger output TRIG-A. The T-ASIC shapes and levels the signal, and supplies the resulting pulse width signal ALLTRIG to the D-ASIC. The D-ASIC counts the pulse width and calculates the capacitance reading. When the capacitance function is selected no other measurement or wave form display is possible. There is only a numeric readout of the capacitance value. Frequency, pulse width, and duty cycle measurements The input voltage is measured as described above. From the ADC samples to built the trace, also the frequency, pulse width, and duty cycle of the input signal are calculated. Miscellaneous Control of the C-ASIC, e.g. selecting the attenuation factor, is done by the D-ASIC via the SDAT and SCLK serial communication lines. An offset compensation voltage and a trace position control voltage are provided by the D-ASIC via the APWM bus. The C-ASIC's also provide conditioned input voltages on the TRIG-A/TRIG-B line. These voltages can be selected as trigger source by the T-ASIC. 3.2.2 Trigger Circuit The T ASIC selects one of the possible trigger sources TRIG-A (Input A) or TRIG-B (Input B). For TV triggering the selected trigger source signal is processed via the Sync(hronization) Pulse Separator circuit (TVOUT-TVSYNC lines). Two adjustable trigger levels are supplied by the D-ASIC via the PWM FILTERS (TRIGLEV1 and TRIGLEV2 line). Depending on the selected trigger conditions (- source, - level, - edge, - mode), the T-ASIC generates the final trigger signal TRIGDT, which is supplied to the D-ASIC. 3-4

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123/124
Service Manual
3-4
3.2.1 Channel A, Channel B Measurement Circuits
The Channel A and Channel B circuit are similar.
The only difference is that Channel A
can do all measurements, whereas Channel B does not provide resistance, diode, and
capacitance measurements.
Volts, and derived measurements (e.g. current with optional probe)
The input voltage is supplied to the C-ASIC, via the LF and HF path.
The C-ASIC
converts (attenuates, amplifies) the input signal to a normalized output voltage ADC-
A/ADC-B, which is supplied to the Analog to Digital Converters (ADC-A and ADC-B)
on the DIGITAL part.
The D-ASIC acquires the digital samples to build the trace, and to
calculate readings.
For the HF and LF attenuation section of the C-ASIC some external
components are required: the HF DECade ATTenuator and LF DECade ATTenuator
section.
Resistance, continuity, and diode measurements (Input A only)
The T-ASIC supplies a current via the
Ω
/F relays to the unknown resistance Rx,
connected to the Input A and the COM input jacket.
The voltage drop across Rx is
measured as for voltage measurements.
Capacitance measurements (Input A only)
The T-ASIC supplies a current via the
Ω
/F relays to the unknown capacitance Cx,
connected to the Input A and the COM input jacket.
Cx is charged and discharged by
this current.
The C-ASIC converts the charging time and the discharging time into a
pulse width signal.
This signal is supplied to the T-ASIC via the C-ASIC trigger output
TRIG-A.
The T-ASIC shapes and levels the signal, and supplies the resulting pulse
width signal ALLTRIG to the D-ASIC.
The D-ASIC counts the pulse width and
calculates the capacitance reading.
When the capacitance function is selected no other measurement or wave form display is
possible. There is only a numeric readout of the capacitance value.
Frequency, pulse width, and duty cycle measurements
The input voltage is measured as described above.
From the ADC samples to built the
trace, also the frequency, pulse width, and duty cycle of the input signal are calculated.
Miscellaneous
Control of the C-ASIC, e.g. selecting the attenuation factor, is done by the D-ASIC via
the SDAT and SCLK serial communication lines.
An offset compensation voltage and a trace position control voltage are provided by the
D-ASIC via the APWM bus.
The C-ASIC’s also provide conditioned input voltages on the TRIG-A/TRIG-B line.
These voltages can be selected as trigger source by the T-ASIC.
3.2.2 Trigger Circuit
The T ASIC selects one of the possible trigger sources TRIG-A (Input A) or TRIG-B
(Input B).
For TV triggering the selected trigger source signal is processed via the
Sync(hronization) Pulse Separator circuit (TVOUT-TVSYNC lines).
Two adjustable
trigger levels are supplied by the D-ASIC via the PWM FILTERS (TRIGLEV1 and
TRIGLEV2 line).
Depending on the selected trigger conditions (- source, - level, - edge,
- mode), the T-ASIC generates the final trigger signal TRIGDT, which is supplied to the
D-ASIC.