HP 400F, Руководство по эксплуатации и обслуживанию, Страница: 17 / 54

ModeI400F/FL
SECTION IV
Section IV
Paragraphs 4-1 to 4-16 and Figure 4-2
THEORY OF OPERATION
"-1. G E N E R A L .
4-2. The 400F/FL is a801id state. average responding.
rms calibrated ac voltmeter. It may also be used
as
a wide band ac amplifier, with switchable gain and
swttchable bandwidth. Refer to Figure 4-1 for a
functional circuit diagram of the instrument.
"-3. B L OCK DIAGRAM D E SCRI P T I O N .
4-4. The voltage to be measured is applied to the
input attenuator. where it is either attenuated b y 60
db. or coupled directly to the preamplifier. The pre­
amplifier provides 10 db of gain for the input signal
and applies it to the post attenuator. The signal
goes from the post attenuator to the 100 KHz LOW
PASS filter. which may be switched in to limit the
bandwidth to signals from 20 Hz to 100 KHz. The
meter amplUier then ampUfies the signal. couples it
to the meter bridge, and supplies a signal to the AC
OtrrPtrr terminal. The meter bridge rectifies the
ac signal and applies it to meter Ml. which indicates
the rms value of the input voltage. The meter bridge
also provtdes the
ac
feedback to the meter amplifier.
"-S.
S C H E M A T I C THEORY.
4-6. Refer to Figure 6-1 for the following discussion.
4-7. INPUT ATTENUATOR.
4-8. The input attenuator consists of an rc voltage
divider and two reed relays.
On
the . 1 mv through
.3 v ranges. reed relay A1Kl is energized by -26 v
from wafer (A) of the RANGE switch. SI. routing the
input signal directly to the preampllfier. On all other
ranges. the -26 v is applied to relay AIK2. When
A1K2 is closed. the input signal is attenuated 60 db by
the rc divider and coupled to the preamplUler.
4-9. PREAMPlJFlER.
4-10. The preamplifier is a three stage ac amplifier
that amplifies the signalfrom the input attenuator by
10 db.
It
also functions as an impedance matcher to
match the high impedance of the input attenuator to
the much lower impedance of the post attenuator.
4-11. Capacitor A2C5 blocks dc transients and
couples the ac Signal to the preampllIler. The input
signal is Umited to 5.4 volts peak-ta-peak by diodes
A2CR2 and A2CR4. which are biased at +2.7 v and-2. 7
v respectively. by zener diodes A2CR1 and A2CRS.
A field effect transIstor. A2Q1. Is used as the Input
stage of the preampllIier because of Us low noise
characteristics and high Input impedance. The
signal Is taken Crom the drain of AX/:l and is Curther
amplified by A2Q2 and A2Q3.
01795-1
4-12. Feedback from the emitter of A2Q2 bootstraps
thevalueofA2R9. the drain load of A2Q1. Feedback
from the source of A2Q1 bootstraps the input impedance
of the preamplUier and keeps it at a high level over
all
ranges of inputs. Gain stability and linearity of
the preamplUier are maintained by feedback from the
collector of A2Q2 and the emitter of A2Q3. A2R6
provides a bias adjustment Cor the fteldeffect transis­
tor. A2Q1.
4-13. POST ATTENUATOR.
4-14. The post attenuator IS a precision resistive
voltage divider that operates as a function of the
RANGE switch. On the two lowest voltage ranges, the
signal from the preampliller is appUed through two
resistors (SlR1 and 51R1S) to the 100 KHz UP FILTER
and receives
no
attenuation.
Six
precision resistive
divider circuttsprovtde signal attenuation in progres­
sive steps of 10 db for the twelve higher ranges.
4-15. 100 KHz LOW PASS FILTER.
4-16. The 100
KHz LP
FILTER is a 0.01 Jjf capaci­
tor (S2C1) which may be switched into or out of the
circuit by switch 52. When the filter is In the circuit.
the bandwidth of the instrument is from 20 Hz to 100
KHz. U the filter is switched out of the circuit, the
bandwidth is increased to 4
MHz.
Refer to Figure
4-2 for a graph of the filter attenuation character­
istics.
...
-
.
-I.
-15
-20
IOKHl
I
I
100KHZ
I
, 4567811/
Figure 4-2. Filter Attenuation CharacteriBtlcs
4-1