Section IV Model 4342A
Paragraphs 4-14 to 4-30
verter Q3-Q4 consists of a pair of emitter followers 4-21. RF AMPLIFIER AND DETECTOR(A5)
connected in series which provides ahigher input im-
4-22. The RF Amplifier and Detector assembly in-
pedance and lower output impedance. cludes the Impedance Converter, the RF Amplifier,
4-14. ALC AMPLIFIER(P/O A81 and the Detector circuits. The impedance converter
Ql-Q4is a?unity?gain buffer stage amplifier between
4-15. The ALC Amplifier circuit Q9-Q13 provides the Tuning Capacitor assembly A2 and the RFAmpli-
the appropriate correction signal to the Oscillator fier Q5-Q9. It provides a high input impedance and a
assembly(AlA1) in order to control the oscillator out- low output impedance similar to what has been de-
put in accordance with the fixed reference dc level scribed in paragraph 4-13.
set by the OX LEVEL control.
4-23. The RF Amplifier circuit Q5-Q9 is a highgain
4-16. Q,?nQ RANGE ATTENUATOR(A3) and broad band amplifier. The frequency response of
the amplifier is flat and covers the entire spectrum
4-17. The Q RANGE Attenuator consists of four range given in the specifications, while broad band
switches which provide a total attenuation of 30.4dB.
An additional switch is used for the AQ measurement. RF transistors supply power gain. The approximate
gain is about 34dB. The amplified signal is detected
The Meter Scale Indicator (Al 1) ganged with Q RANGE by diodes CR2-CR5 and coupled to the DC Amplifier
switches, utilizes four lamps, two of these lamps are assembly A6.
used for the Q scale display and the other two for the
n Q scale. The attenuator output is fed to an Imped-
4-24. DC AMPLIFIER(A6)
ance Converter(A4) which consists of transistors Ql
and Q2 and which is similar in operation to the one 4-25. The DC Amplifier Ql-Q5 provides a gain from
described in paragraph 4-13. 0 to 20dB. It is used to drive linearly the meter.
Various gain adjustment, balance control, aQ COARSE
4-18. TUNING CAPACITOR AND INJECTION AND FINE adjustments, METER ZERO ADJUST, and
TRANSFORMER(A2) A Q function are provided for in this assembly. A Q
ANALOG OUTPUT is also supplied which can be inter-
4-19. The Tuning Capacitor sometimes referred to
as the Q Capacitor is an important part of the QMeter. faced with other instruments. Frequency signals
down to and including dc can be handled by theampli-
It is the reactance standard in the Q measurement. fier. By combining direct coupling with a resistive
Because the Q Capacitor can be calibrated precisely, feedback circuit, good stability is obtained.
the Q Meter provides direct reading of inductance in
addition to Q. To achieve this high accuracy, the
4-26. Q LIMIT SELECTOR(A7)
capacitor is designed with low loss and low residual
inductance. Minimum capacitance is low to maintain 4-27. The Q Limit Selector assembly includes acom-
accuracy at high frequencies. The Q Capacitor covers parator circuit Ql-Q3, a Schmitt trigger Q4-Q5, a
a range of 2OpF to 475pF. Residual inductance is less monostable multivibrator Q6-Q7 and a driver Q8-Q9.
than 10nH. The comparator compares the output of the detected
RF signal with the Q LIMIT setting. Thecomparator
4-20. The Model4342A uses a new method of inject- output is then coupled via an emitter follower to the
ing a constant voltage through a transformer as shown Schmitt trigger which generates a fast rise pulse out-
in Figure 4-3, which has very low output impedance. put. This signal is coupled to the monostable multi-
The transformer has a toroidal core and nearly flat vibrator which has a fixed time constant of 1 second,
frequency reaponse from 1OkHz to 70MHz. The LO and also supplies the necessary drive signal to the
terminal consists of a one-turn secondary winding driver stage. An OVER LIMIT SIGNAL OUTPUT and
which has an output impedance of approximately 1 mil- DISPLAY TIME(l set Oreo) are provided.
liohm. High measurement accuracy is thus achieved.
4-28. CIRCUIT DETAILS
4-29. LC OSCILLATOR(P/O AlAl)
FROM
OSCILLATOR 4-30. FREQUENCY RANGE switches select the ap-
propriate LC circuit, setting the operating frequencies
yy$(Y-zq of the oscillator &l-&2. In the Hartley configuration,
when an RF current flows in the tuned circuit, there
is a voltage drop across L. The tap on the Lcoil will
be at an intermediatepotentialwith respect to the two
ends of the coil. The amplified current in the Q2coI-
lector circuit, which flows through the bottom section
of L, is in phase with the current already flowing in
Z-Y
the circuit and thus in the proper relationship for
50:1 positive feedback. The Colpitts arrangement uses the
1twcT10N TRANSFORMER voltage drops across the two capacitors Cl8 and Cl9
in series in the tuned circuit to supply the feedback,
Other than this, the Colpitts operation is the same as
Figure 4-3. Constant Voltage Injection System just described for the Hartley configuration.
4-2 |
ACEC
