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Theory of Operation
AM700 Audio Measurement Set Service Manual
3–41
The Tunable Bandpass Filter is designed to attenuate the second and third
harmonic distortion products from the DACs to improve the already low
distortion characteristics of the high resolution generator signals. Filters are
tunable in two bands for improved noise characteristics. The bands of filter
operation are from 20 Hz to 750 Hz and from 750 Hz to 10 kHz.
The filters are constant-Q filters with a Q of 2.7. A constant-Q filter maintains a
bandwidth that is a constant percentage of the center frequency of the filter.
Second harmonics of the fundamental frequency are attenuated by 10 or more
dB. Any third harmonic products are attenuated by 15 dB or more to attain the
lower distortion output signal. Filter tuning tracks the generator fundamental
frequency. When operating in Low Distortion mode, the filter components add
slightly to the noise floor so that small amplitude output signals will become
noise limited sooner than operating in the bypassed mode.
A Channel and B Channel Gain/Attenuators (Generator Output) (diagram 6)
Refer to schematic diagram 6 of the A5 analog generator circuit board for the
following circuit description.
Control signals for setting the state of the A and B Channel step attenuators, and
the gain of the A and B Channel Amplifiers comes from the Serial Control
Register (diagram 2). Those control signals used to drive relays are latched into
two 8-bit registers, U25 and U75. The control signals used to set the attenuators
are applied directly to those devices.
The selection between High Resolution data or High Bandwidth data is made by
data selector U100 in the A Channel and U43 in the B Channel. The data
selectors require a high output impedance to reduce distortion, so the outputs are
buffered by U27B and U28B before being applied to the stepped attenuators. The
buffer outputs are AC coupled to the stepped attenuators to eliminate any DC
components that may have been introduced by various offsets in the preceding
stages.
The attenuation selectors are U26 the A Channel and U31 in the B Channel.
Attenuation settings are in 6 dB steps from 0 (no attenuation of the input signal)
to 36 dB. Control signals for switching are applied from the serial control
register (diagram 2). An additional control signal is applied through the logic
circuitry on the A2 input of the data selectors. That is the overload indication
signal, OVLD. When an overload condition on an output amplifier occurs, the
stepped attenuator for the overload channel is switched to immediately add
16 dB of attenuation to the input signal.
Tunable Bandpass Filter
Relay Control Latches
Data Selector
A and B Channel Step
Attenuators