HP 3562A, Руководство по обслуживанию

Страница: 228 / 570

MODEL 3562A C I RCUIT DESCRIPTIONS
SECTION VI
CIRCUIT DESCRIPTION
6 - 1 INTRODUCTION
This section provides the overal l instrument description and the detailed description of
each assembly to a functional block. The overall instrument description describes the
interaction of the H P 3562A's individual assemblies. Use this section to gain an overall
understanding of the HP 3562A's operation. Use the circuit descriptions along with the
schematics in Section IX to understand the operation of an assembly. Refer to the end
esc
0
si
a
.........
' •• �
•••.
6-2 OVERALL . INSTRUMENT DESCRIPTION
To understand the operation of the H P 3562A, the measurement it makes must be
u nderstood. The H P 3562A uses the Fast Fourier Transform (FFT) algorithm to make
frequency domain measurements. The FFT algorithm takes time domain data and operates
on it to prod uce a frequency spectrum. Recall that the Fourier transform, of which the
FFT is a special case, states that all time domain waveforms can be represented as a
summation of sine waves in the frequency domain.
Since the FFT is a special case of the more general Fourier transform, it requires special·
data col lection requirements in order to operate. All the numerical values associated with
the FFT such as sample rate, number of frequency data points, time required for a complete
conversion, and others are all related to powers of two. For example, a time record is
defined to be N consecutive, equally spaced samples of the input where N is a multiple
of 2. This time record is transformed as a complete block into a complete block of
frequency lines. With a Dynamic Signal Analyzer we do not get a val id result until a full
time record has been gathered.
Taking a simple look at an FFT dynamic signal analyzer, the hardware can be characterized
as a digital waveform recorder with a great amount of computation power. This view is
ill ustrated by the waveform recorder diagram in figu re 6-1 .
scaling and then converted to digital data by the ADC (Analog to Digital Converter)
assembly. The input and ADC assemblies receive the signals to control the amplitude scaling
and conversion from the front end interface. The front end interface is programmed directly
from the CPU (Central Processing Unit). The digital output of the ADC is then stored into
RAM and displayed. H P-I B is included for remote control and a keyboard for local control.
In figure 6-2, the instrument is expanded to include two separate input sections and ADCs.
A source is also included to make network measurements. All measurement assemblies
are connected to the CPU through a common bus.
6-1