Tektronix 1L20, Инструкция по эксплуатации, Страница: 20 / 117

Circuit Description— Type 1L20
Fig. 3 -2 . Frequency vs. Voltage curves for k H i/C M Discriminator
circuit.
(A .) Output from D383;
(B.) Output from D386;
(C.) Output from Discriminator Comparator Q 2 6 0 .
center of the operating range (275 MHz). This type o f dis­
criminator is used because of the wide (100 MHz) frequency
coverage needed. The kHz/CM Discriminator has better
stability than the line type, but covers only 10 MHz.
In the Discriminator Comparator circuit of Q260, the
detected outputs of the Frequency Discriminator are compared
and the difference signal is sensed, amplified, and sent on to
the Sweep Comparator as has already been described. IF
CENTER FREQ controls R270 and R274 provide a means of
controlling the center frequency of the display by adjusting
the DC level of the feedback signal going to Q240.
proper mixing action in the Wide-Band Mixer Am plifier Q140.
This has been accomplished by the feedback loop through
D361, Q280, and Q290. The amplitude of the swept-fre-
quency signal is detected by D361. This voltage amplitude is
compared to the fixed DC voltage a t the IF Center Freq
Range control R290. The resulting difference signal from the
Amplitude Comparator circuit of Q290-Q280 is used to bias
the base of Q320 to control the current supplied to the Swept-
Frequency Oscillator Q310.
The swept frequency signal is amplified by the push-pull
am plifier circuit o f Q340-Q350, converted to a single-ended
signal by T347, filtered, and coupled to the 200 MHz W ide-
Band Mixer through T363 and J363.
IF Attenuator <Q>
The IF Attenuator is a six-section pi attenuator giving a
total attenuation of 51 dB (355:1 voltage attenuation) in
1 dB steps. The attenuator maintains a constant 50 Q input
and output impedance regardless of the IF ATTENUATOR
switch settings. Low pass filters (Cl 51-LI51-C l52 and Cl 87-
L188-C188) have been added to the input and output circuits
of the attenuator to prevent harmonics from the following
70 MHz Oscillator from feeding back to mix with the Swept-
Frequency (Sweeper) Oscillator signal to generate spurious
signals.
Narrow-Band IF Amplifier
The Narrow-Band IF Am plifier circuit includes a two-stage
75 MHz Amplifier, a crystal controlled 70 MHz Local Oscil­
lator, and a two-stage 5 MHz Amplifier.
The 75 MHz IF Am plifier Q420 and Q430 is a conventional
two-stage common - emitter transformer - coupled amplifier.
Both stages are peaked to 75 MHz— by C425 for the first
stage, and by C435 for the second stage. G A IN control
R411A varies the gain of both stages by changing the DC
bias. Negative feedback through C422 neutralizes the first
stage to keep it from oscillating.
The outputs of the 75 MHz IF Am plifier and the 70 MHz
Oscillator are applied to the base o f the mixer-amplifier
Q450, which amplifies the difference frequency (5 MHz) of
the two input signals. T454 o f the mixer stage is tuned to
5 MHz, and couples the narrow-band signal to the base of
Q460. This amplifier is peaked at 5 MHz by T464. The signal
is coupled through T464 to the Variable Resolution Amplifier,
Variable Resolution Am plifier
<(^ >
Crystal Filter Circuit
The filter circuit of C358-L358 adjusts the linearity of the
M H z/C M display by attenuating some of the high-frequency
components of the Swept-Frequency signal. R368 adjusts the
impedance of D365 to set the slope of the kH z/C M Dis­
criminator at twenty times the slope of the M H z/C M
Discriminator.
The output amplitude of the Sweeper circuit must be kept
constant for proper action of the discriminator circuit and for
The signal from the secondary of T464 is coupled to the
5 MHz crystal filter Y501. To understand how this circuit
operates, consider the impedance characteristic of the crystal.
Fig. 3-3 shows a typical impedance versus frequency curve
of a crystal.
In examining the curve from left to right, we first encounter
a very low impedance point (a) a t the series-resonant fre­
quency. As the frequency increases, the impedance increases
3 -4