HP 3335A - Advanced Operating Features; Attenuator Life and HP-IB Operation; Telecommunications Sweep and Blanking

Section IIT
3-88. Programming Amplitude to Maximize Attenuator
Life. The attenuator assembly of the 3335A introduces an
attenuator pad by use of an electro-mechanical mechanism.
As with any mechanical system, the more it is used, the
greater the wear on the system. A programmer can mini-
mize the mechanical wear on the 3335A attenuator by
carefully selecting amplitudes which require minimum oper-
ation of the attenuator. Table 3-7 indicates the 50 ohm out-
put level, the attenuation introduced by the attenuator and
which pads are actuated to accomplish it. Note that the
attenuator attenuates in 2 dB steps while amplitude changes
from +.01 dB to +1.99 dB are realized electronically by the
leveling loop circuit (A6 Assembly —~ Section VII Service
Group M). Programming amplitudes within the 1.99 dB
capability of the leveling loop and/or selecting amplitudes
that minimize pad actuations will increase attenuator life.
Table 8-N-1 in Section VIH Service Group N provides a
detailed indication of the attenuator pads selected for a
particular programmed amplitude for both 50 ohm and 75
ohm output impedances.
3-89. Unaddressed or Local Operation When Connected To
the HP-IB.
3-90. If the 3335A is connected to the HP-IB and operated
locally or operated in remote but not addressed to listen,
interruption of the display/keyboard scan and the sweep
mode function can occur. The instrument is controlled by
one microprocessor. The processor must service the instru-
ment function and also handle the HP-IB lines. When the
ATN line is true or when the 3335A is addressed to listen,
microprocessor responds to every DAV handshake. As each.
data byte is placed on the bus, DAV is pulled true which
activates the 3335A HP-IB interrupt request line (IRQ) to
the controller section. This causes the controller to inter-
rupt the scan and sweep operation and service the interrupt
request before returning to local operation of the 3335A
functions. If a long data string is sent on the HP-IB, a data
byte rate can be achieved when the 3335A controller has
only time to service the interrupt requests. At this point
the 3335A controller cannot properly service the basic
instrument. The following symptoms can occur; display
completely or partially blanks, a keyboard entry is not
recognized, sweep output is not linear, and the auto, single
10 or 50 second sweep in the sweep mode exceeds the
time interval.
3-91. Telecommunications.
3-92. There is an amplitude blanking feature and a special
sweep feature which make the -hp- 3335A especially suit-
able for applications in the telecommunications industry.
These features are standard and appear on all instruments.
On instruments equipped with Options 002, 003 or 004,
the special sweep is switch selectable. Each feature is de-
scribed separately in the paragraphs that follow.
3-146 Rev. A
Model 3335A
NOTE
No instrument with a serial number below
1604A00376 is equipped with a TELECOM-
MUNICATIONS SWEEP Switch.
3-93. Amplitude Blanking. This feature causes the output
amplitude to be blanked for 20 msec whenever a new fre-
quency is entered or whenever the INCREMENT or DE-
CREMENT controls are actuated. Since the amplitude is
blanked, no signal will appear at the output during the time
that the frequency is changing. This allows the instrument
to be stepped from one channel to another without gener-
ating spurious signals that might interfere with operating
channels. Amplitude Blanking is implemented by modifing
the instrument internally. Only qualified service personnel
can do the modification safely. Hazardous voltages are
exposed when the top cover is removed. The procedure for
implementing this feature is presented on the apron page
of Figure 8-D-1 in Service Group D.
WARNING
Since access to the Amplitude Blanking Switch
requires removal of the top cover, this feature
should be implemented oniy by qualified ser-
vice personnel. Hazardous voltage are exposed
when the top cover is removed.
3-94. Telecommunications Sweep. This sweep is really a
modification of the AUTO sweep. The amplitude is blanked
for 20 msec after each step and the duration of each step is
extended so that the time duration of the step plus the 20
msecs blanking equals 4 seconds. Since the AUTO sweep
consists of 100 steps, the total sweep time is extended to
400 seconds or 6.7 minutes. Blanking the amplitude pre-
vents the -hp- 3335A from outputting spurious signals
while it is changing frequencies. The long time duration
allows certain Selective Level Measuring Instruments to
track the -hp- 3335A output frequency in an open loop test
configuration (e.g. — see see Operating Manual for the -hp-
3745A/B Selective Level Measuring Set). If the output is
displayed on an oscilloscope, a slight overshoot will be
observed at the beginning of each frequency step. This is
normal and will cause no problems in the tracking applica-
tion for which this operating mode was designed. On instru-
ments equipped with Options 002, 003 and 004, the
communications sweep is implemented by moving the
TRACK/ NORM switch, located on the rear panel, to the
TRACK position. On standard instruments, it is implement-
ed by modifying the instrument internally. Only qualified
service personnel can do the modification safely. Hazardous
voltages are exposed when the top cover is removed. The
procedure for implementing this feature is presented on the
apron page of Figure Group D.