HP 3580A Operating And Service Manual page 59

Section IV
VTO frequency. The frequency range of the log sweep is
from 20 Hz to 43 kHz. During log sweeps, the SWEEP
TIME control is disabled and the Linear Sweep Generator is
automatically set for a 5 second sweep time. The Log
Sweep Generator is synchronized by the (L)Reset 2
instruction from the Linear Sweep Generator.
A
(Ll RESET 2
(FROM LINEAR
SWEEP
GENERATOR)
B
,-----'>Nv- - -
----,+FEEDBACK
:
~;t
v
I I
_ __,.--___, I
I
+3.6mV TO +7.75V
·>----<>----EXPONENTIAL RAMP
l[
GAINATPOINT@
.-~.-R 77
•-I
JI
R1N R78
GAIN AT+ PORT• I+
~:N
•
I+~~~
•+2
- +7.75V
(43kHz)
- +3.6mV
I..--""'
5 SEC ( 20 Hz)
(TO VTOl
3580A-B· 3568
Figure 4-17. Basic Log Sweep Generator.
4-109. Figure 4-l 7A shows the basic circuit configuration
for the Log Sweep Generator. The major circuit element is
a high input-impedance operational amplifier. The gain of
the amplifier with respect to Point A is - 1 and the gain at
the non-inverting port is
+
2. At the beginning of the log
sweep the following conditions exist:
a. The (L) Reset 2 line is low.
b. FET switch Q32 is closed.
c. The non-inverting port of the amplifier is grounded
through Q32.
d. Capacitor C 14 is fully discharged.
e. The output voltage is
+
3.6 mV de due to the
- 3.6 mV de reference applied to Point A. This sets the
analyzer frequency to 20 Hz which is the starting point for
the log sweep.
When the (L)Reset 2 instruction is cleared, switch Q32
opens and Cl4 charges toward the output voltage through
feedback resistor R66. As C14 charges, the output voltage
becomes increasingly positive. Due to the bootstrapping
effect of the positive feedback through R66, the charge rate
of C14 increases exponentially. The exponential ramp at
the output is as shown in Figure 4-17B.
4-110. Auto Zero Circuit. An Auto Zero Circuit is in-
cluded in the Log Sweep Generator to null out any de
offset introduced by the operational amplifier. The overall
circuit configuration is shown in Figure 4-18.
4-16
(L)RESET 2
(FROM
LINEAR
SWEEP
GENERATOR)
- 3.6mV
Reference
R66
I.BM
r------...vv------ -,
Model 3580A
I I
1 : +FEEDBACK
:
: /
' I
R78
93.1 k
I
I
I
·>---.-+3.6mV TO +7.75V
036
I
I
EXPONENTIAL RAMP
(TO VTO)
I -FEEDBACK
- --WV-- - - - -
_J____..,-
Q35
~--~
3580A-B-3566
Figure 4-18. Overall Log Sweep Generator.
4-111. When the (L)Reset 2 instruction is initially given,
the output of the Monostable Multivibrator goes high for
approximately 0.4 seconds. This closes FET switches Q35
and Q36. With switch Q32 also closed, the offset voltage is
present at the output of the amplifier and capacitor C16
charges to the offset voltage through R74. At the end of
the auto zero period, Q35 and Q36 open and the charge on
Cl 6, in series with the input reference voltage, cancels the
amplifier offset voltage.
4-112. Digital Storage And Display Sections.
4-113. Introduction To Digital Storage.
Low frequency
spectrum analyzers require narrow bandwidths and conse-
quently, slow sweep rates. Because of these slow sweep
rates, the video cannot be displayed directly on a standard
CRT. If, for example, the X and Y axis outputs of the
3580A were applied to a standard CRT, the display would
be merely a dot fluctuating up and down while moving
slowly across the CRT face. Even with the SWEEP TIME
control set to 0.01 SEC/DIV (fastest sweep time), a
satisfactory display could not be obtained.
4-114. To retain the slowly scanned video information of
the 3580A, some form of display storage is required. As
indicated in the Simplified Block Diagram Description
(Paragraph 4-22), a storage CRT having long persistance
could be used. Recent advances in large-scale integrated
circuits and the innovative design efforts of -hp- engineers,
however, have made it possible to use a digital storage
technique in the 3580A. The major advantages of digital
storage are:
a. Digital storage permits the use of a standard oscillo-
scope CRT. Standard CRT's are rugged ( a must for
portable operation) and relatively inexpensive to replace.
b. A digitally stored trace can be retained indefi-
nitely ... as long as the instrument is turned on. If a single