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Theory of Operation
AM700 Audio Measurement Set Service Manual
3–25
Digital signal gain is controlled using a programmable current source to a
differential amplifier circuit formed by Q7 and Q8. The programmable current
source DAC, U107, receives 10-bit wide word that is the digital value of the
output current needed for the gain wanted. The converted signal is the current
source for the differential amplifier. The output of the amplifier if fed through a
20 MHz low-pass filter to operational amplifier U72. This filter provides a
smooth transition (15 ns rise and fall time) that reduces a possible EMI source
and preconditions the signal for better transmission characteristics through an
audio cable. The signal for the Unbalanced output connector is taken from the
output of U72, prior to the cable simulation filter.
The cable simulator filter is a switchable low-pass filter that produces an effect
simulating distortion of the signal that is inherent in cable transmission. It is a
simple RC roll-off filter. When the filter is not in use, it is bypassed around the
filter by U71, a solid-state switch.
The switched output of U71 is applied to a unity-gain amplifier, U106. This
amplifier buffers the signal that is AC-coupled to the front-panel XLR connector
through A2, the XLR board. A second output of the amplifier is the LOOPBK
signal, a selectable input source to the digital circuitry. This loop-back path is
used to directly apply the digital generator signal to the input of the digital audio
board for testing purposes.
Digital Receiver Input (diagram 6)
A simplified block diagram of the Digital Audio Receiver was previously shown
in Figure 3–7. Refer to that illustration and to schematic diagram 6 of the A3
circuit board for the following digital receiver circuit description.
A solid-state FET switch, U109, is switched to select the source of the digital
audio signal. The input signals are applied to the AGC circuit to provide about a
5 V peak-to-peak signal at the output of U111 (TP3) for any of the selected
inputs. With AGC and auto equalization, the AM700 is able to lock on to a wide
dynamic range of input signals that may be applied from external sources.
Programmable Gain
Cable Simulation Filter
Unity Gain Output
Amplifier
Input