Theory Of Operation; Introduction; Simplified Block Diagram Description; Amplitude Section - HP 3580A Operating And Service Manual

Model 3580A
Section IV
SECTION IV
THEORY OF OPERATION
4-1. INTRODUCTION.
4-2. This section contains a Simplified Block Diagram
Description and a Functional Description of the 3580A
Spectrum Analyzer.
4-3. SIMPLIFIED BLOCK DIAGRAM DESCRIPTION.
4-4. Refer to the Simplified Block Diagram (Figure 4-1) for
the following discussion.
The 3580A can be divided into four major sections:
1) Amplitude Section
2) Frequency and Sweep Section
3) Digital Storage Section
4) Display
4-5. Amplitude Section.
4-6. The Amplitude Section consists of an Input Circuit, an
Overload Detector, an Input Mixer, an IF Filter, Log and
Linear IF Amplifiers, a Video Detector, a Video Filter and
a Video Output Circuit.
4-7. Input Circuits.
The Input Circuits, controlled by the
front panel INPUT SENSITIVITY switch, provide the gain
or attenuation needed to maintain the proper signal level at
the input of the Mixer. The Input Circuits also contain a
50 kHz low-pass filter which prevents image frequencies
(200 kHz and above) from reaching the Mixer.
4-8. Overload Detector. The Overload Detector at the
input of the Mixer senses when the input level exceeds the
design limits and, in turn, lights the front panel OVER-
LOAD indicator. This is an important function since signals
that overdrive the mixer can produce harmonic and
spurious mixing products which ultimately appear on the
display.
4-9. Input Mixer.
The Input Mixer is a double-balanced
active mixer in which the 0 Hz to 50 kHz input signal is
mixed with a 100 kHz to 150 kHz signal from the
Voltage-Tuned Local Oscillator (VTO). The output of the
Mixer is a composite signal containing the upper and lower
sidebands.
4-10. To select a given frequency component present at the
input of the Mixer, the VTO frequency is tuned so that the
difference between it and the frequency of interest is
100 kHz:
Fvto - Fin = 100 kHz
Where:
Fvto
=
100 kHz to 150 kHz VTO frequency
Fin= 0 Hz to 50 kHz input frequency
The 100 kHz intermediate frequency (IF) is fed through
the IF Filter, detected and displayed on the CRT. Signals
outside the passband of the IF Filter are rejected.
4-11. IF Filter.
The IF Filter contains five cascaded crystal
filter stages. The center frequency of the filter is 100 kHz
and the 3 dB bandwidth varies from 1 Hz to 300 Hz as a
function of the front panel BANDWIDTH setting. Since the
Input Circuits and Input Mixer are broadband through
50 kHz, the selectivity of the instrument is determined
entirely by the bandwidth of the IF Filter.
4-12. Log and Linear Amplifiers. The 100 kHz output of
the IF Filter is applied to the Video Detector through a Log
Amplifier in the Log amplitude mode or through a Linear
Amplifier in the Linear amplitude mode. The Log Amplifier
converts the amplitude of the incoming IF signal to a
logarithmic value, providing an 80 dB display range. The
linear Amplifier is a conventional amplifier circuit in which
the gain is varied to provide the 20 V, 10 V, 2 V, 1 V
ranging sequence used in the linear mode. Also, the Linear
Amplifier contains a variable attenuator which increases the
overall gain. as the AMPLITUDE REF LEVEL switch is
changed from the XI position.
4-13. Video Detector.
The Video Detector is an average-
responding, full-wave detector circuit which produces a de
voltage proportional to the amplitude of the 100 kHz log or
linear input signal.
4-14. Video Filter.
The Video Filter is an R/C filter
network controlled by the BANDWIDTH and DISPLAY
SMOOTHING controls. The purpose of the filter is to
smooth-out the ripple and noise riding on the detected
video signal.
4-15. Video Output Circuit.
The Video Output Circuit
functions as an output buffer in the Linear mode and as a
variable gain amplifier in the Log 10 dB and Log 1 dB
modes. In the Log 10 dB mode, a variable de offset voltage,
controlled by the AMPLITUDE REF LEVEL switch, is
summed with the video input signal. This allows the entire
display to be offset in steps of 10 dB as the AMPLITUDE
REF LEVELsetting is changed from 0 dB to - 70 dB. In the
Log 1 dB mode, the gain of the Video Output Circuit is
increased to provide an expanded scale of 1 dB per division.
Changing the AMPLITUDE REF LEVEL setting then varies
4-1