Vertical Channels - Fluke Autoranging CombiScope PM3370B Service Manual

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3.2.2

Vertical channels

There are 4 vertical channels that are mainly identical. A difference is that the 200 MHz oscilloscope
versions have a switchable 50Ω input impedance while the 100 MHz and 60 MHztypes do not have
this feature.
Another difference is between the 4 channel and the 2 channel versions. The 4 channel scopes have
4 identical vertical channels (CH1, CH2, CH3 and CH4) with an extensive range of input sensitivities.
The various input sensitivities are made by combinations of settings of HIGH IMPEDANCE
ATTENUATOR, LOW IMPEDANCE ATTENUATOR and PREAMPLIFIER.
The 2 channel versions have 2 identical vertical channels (CH1 and CH2) with an extensive range of
input sensitivities. Channel 1 operation and the differences with channels 2, 3 and 4 are now
explained.
In the 2 channel versions the channel CH3 is omitted and the EXT TRIG input has 2 switchable input
sensitivities.
The CH1 input signal is applied to the INPUT COUPLING block. Here selection between ac or dc
coupled input is done. Moreover in the 200 Mhz oscilloscopes selection between 50Ω and 1 MΩ input
impedance is done here. A signal 50Ω PROTection signals via the CONTROL CIRCUITS to the
microprocessor if the dissipation in the 50Ω termination resistor gets too high.
The block HIGH IMPEDANCE ATTENUATOR incorporates the attenuation coefficients /100, /10
and /1. This block has a fixed attenuation for the ExtTrig channel in the 2 channel oscilloscopes. The
input signal originating from the AMPLITUDE CALIBRATOR is used to automatically calibrate the
vertical channels. The AMPLITUDE CALIBRATOR is a generator that can deliver 8 different accurate
voltages. The HIGH IMPEDANCE ATTENUATOR also comprises a high-frequency square-wave
adjustment; this is done by adjustment of dc signal CAL.
The IMPEDANCE CONVERTER converts the input signal at high input impedance into an output
signal at a low impedance. This block is followed by a LOW IMPEDANCE ATTENUATOR that makes
the attenuation coefficients /2,5 and /1. This block has a fixed attenuation in the ExtTrig channel of
the 2 channel oscilloscopes.
The PREAMPLIFIER incorporates gain/attenuation coefficients x1, x5, /2 and also continuous GAIN
control via a microprocessor adjustable dc signal. The PREAMPLIFIER is followed by Y-
FUNCTIONS CHANNEL 1. In this block the selection is achieved of vertical display via channel 1,
Main Time Base (MTB) triggering via channel 1 and Delayed Time Base (DTB) triggering via channel
1. Also the filter for the Bandwidth Limiter (BWL), the vertical position control (POS) and an output to
the digitizer (EXT). From Y-FUNCTIONS CHANNEL 1 the signal for the Y- OUTput socket is derived.
The Y-FUNCTIONS blocks of channel 2 and 4 have an INVert function for signal inversion.
The selection of the vertical channel, MTB and DTB trigger source is initiated by the block DISPLAY
MODE + TRIGGER SOURCE CONTROL. The channels and trigger sources to be displayed are
controlled by the microprocessor (µP). In ALTernate display mode the switching between
channels/trigger sources occurs at the end of the MTB sweep. The signal ALT controls this. For the
vertical display mode CHOPpped a 2MHz CHOPPER OSCILLATOR is present. Display blanking
during switching from one channel to another is done via signal CHP.
The selected vertical channel(s) are applied to the DELAY LINE DRIVER. This block is an amplifier
that has the correct output impedance to drive the DELAY LINE. The DELAY LINE itself consists of
a coaxial cable giving sufficient signal delay so that propagation delay in the trigger circuits is
compensated. Because of this leading edges of fast-rising pulses can be made visible. The
Y-OFFSET + TRACE SEPARATION block can influence the offset of the signal applied to the DELAY
LINE DRIVER. This is used as offset compensation and also for TRACE SEParation in ALTernate
Time Base mode.
The PROBE CALIBRATOR is a generator delivering a 2kHz/600mV square- wave signal. This signal
can be used to adjust the square-wave response of attenuator probes.
DESCRIPTIONS