Boonton 1121A, Instruction Manual

Page: 48 / 181

Section 4 Theory of Operation
4-2
resident in read-only-memory (ROM). Timing of the mi-
croprocessor operations is controlled by a 5 MHz clock. A
random-access-memory (RAM) provides storage ca-
pability for microprocessor data. To ensure retention of
data in storage, the non-volatile RAM is powered from an
internal 3 volt lithium battery. The main power supply
develops the operating power required by the instrument.
4-5.
The microprocessor communicates with the internal
circuits through a data bus and an address bus.
Command information is entered into the microprocessor
through the front panel keyboard or an IEEE-488
interface. DIP switches are provided for option and test
purposes. Input data selection is displayed by means of a
digital readout and LED indicators. The microprocessor
stores and processes input data, and generates data and
address information to cause execution of command
functions.
4-6.
The audio input signal is first applied to a pair of dif-
ferential attenuators followed by an instrumentation
amplifier. The combination of amplification and
attenuation works to normalize a 300 mV to 300 V input
level range to a range between 1.2 and 3 volts. The DC
component of the input signal is detected after the
amplifier, filtered, and measured with one channel of the
analog-to-digital converter (A/D) for the DC level
measurement mode.
4-7. The AC component of the input signal is AC coupled
and amplified further by factors of either X1, X2.5, X5, or
X10. The rms level of the AC waveform is converted to
DC and measured with another channel of the A/D
converter. The level measurement at this stage is used to
autorange the input attenuators and amplifiers and is also
used in the calculation of the distortion and SINAD
measurements. After the rms detector, connectors are
provided for up to two optional filters which can be se-
lected individually and inserted into the signal path.
4-8.
A programmable notch filter tuned to the fundamental
frequency is inserted into the signal path in the distortion
and SINAD measurement modes. The notch filter is tuned
by the microprocessor circuits based on a manually
selected or measured fundamental frequency. An
amplifier with gain factors of X1 or X10 follows the notch
filter. The notch filter and associated amplifier are
bypassed in the frequency, AC and DC level, and
signal-to-noise (S/N) measurement modes.
4-9.
A programmable gain amplifier follows the notch filter
circuits and provides amplification over a range of X1 to
X10000 in X1, X2, or X5 increments. The amplifier is used
in conjunction with the input amplifier in the AC level and
S/N measurement modes to provide extended range from
300mV down to 0.3 mV full scale. In the distortion and
SINAD measurement modes the amplifier is used in
conjunction with the notch amplifier
to boost harmonic and noise content in the pass band of
the notch filter by up to 80 dB. Low-pass filter selections
are provided before the last stage of the amplifier to
attenuate out-of-band noise and preserve significant
harmonic components. Following the low-pass filters are
three level detectors. The rms, average, and quasi-peak
level of the AC waveform is converted to DC and mea-
sured with another channel of the A/D converter. The
level measurement at this stage is used to autorange the
post-notch detector amplifiers and is also used in the AC
level, distortion, SINAD and S/N measurement modes.
The signal presented to the AC detectors is buffered and
presented at the rear panel MONITOR output for external
analysis.
4-10.
The audio output signal is generated by a low
distortion oscillator design which tunes from 10 Hz to 140
kHz. Microprocessor controlled coarse and fine tuning
precisely sets and maintains the source frequency. A
peak detecting sampler is used in the automatic level
control circuits (ALC) to maintain a constant amplitude at
all frequency settings.
4-11.
The output of the oscillator is applied to a pro-
grammable attenuator which adjusts output level in 1 mV
increments over a 0.001 to 3.000 volt range. The
attenuator output is amplified and attenuated further to
provide a total level range of 0.01 mV to 16.000 volts
open circuit. A class A power amplifier is used to convert
the single-ended source to a differential output with a 50
ohm impedance. The oscillator, output attenuator and
power amplifier are isolated from the chassis by an
optically coupled digital interface and a floating power
supply to enable the source to operate in a floating
configuration. Output impedance selections of 150 and
600 ohms are achieved by inserting 100 and 450 ohm
resistors in series with the 50 ohm output.
4-12. The period counter circuits are shared by the source
and analyzer. The actual source frequency is measured
to enable fine tuning of the oscillator as part of a
frequency lock loop. The analyzer frequency is measured
for the frequency measurement mode and for automatic
tuning of the notch filter.
4-13.
The power supply circuits convert the incoming line
voltage into regulated DC operating voltages to power the
instrument circuitry.