Tektronix 1L30, Instruction Manual

Page: 34 / 129

Circuit Description— Type 1L30
plished by the RF amplitude comparator circuit, Q 290 and
Q280. The RF output signal is detected by diode D361 and
applied through diode D362 to the base, of Q280. This
rectified signal on the base of Q280 is compared against
a reference voltage set by the RF Ampl adjustment R290.
The differential output signal is fed back as a correction
voltage to control the forward bias of Q320. Q 320 is the
current source for the oscillator circuit. Amplitude changes
in the oscillator output are fed back as a corrective signal
to the current regulator to regulate oscillator current or
output power.
To summarize the sequence of Operation for the sweeper
circuit, assume the output from the sweep comparator Q240-
Q 230 is a positive-going ramp. This voltage ramp increases
the bias on the capacitance diode and decreases the cir­
cuit capacitance so the oscillator output frequency will in­
crease. This increase in output frequency is fed back to the
discriminator and detected as an increasing negative volt­
age output from D376 (assuming the DISPERSION RANGE
switch is in the position shown in the schematic diagram)
and a decreasing negative voltage output from D373. The
differential output signal from Q 260 is a positive-going
ramp to the base of Q240, where it is compared against
the input ramp on the base of Q230. The differential sig­
nal . output from the sweep- comparator synchronizes the
sweep oscillator frequency to the horizontal sweep gen­
erator sawtooth signal and the dispersion of the display
becomes a function of the DISPERSION RANGE (SW230)
and DISPERSION (SW220) selector positions.
DISPERSION RANGE BAL adjustment R234 provides IF
center frequency balance adjustment between the M H z /C M
and kH z/C M dispersion positions. Center Freq Range a d ­
justment (R253) and CAL (R252) calibrate the IF center fre­
quency range of the IF CENTER FREQ control.
W ide Band (1 5 0 -2 5 0 MHz) Am plifier and
Second Mixer
The wide band amplifier contains an input 150-250 MHz
bandpass filter, two amplification stages and a mixer am­
plifier with its output tuned to 75 M Hz. Gain through the
amplifier is approximately 20 dB.
The wide* band response from the RF section is applied
through a 150-250 M Hz bandpass filter to the input am pli­
fier Q120. The bandpass filter is a constant-k type, .modi­
fied with m-derived input and output sections to provide a
constant 50 Q input and output impedance through the
pass band. Series-tuned circuit LI 01-C l01 and L I0 7 - 0 0 7
are tuned to the low end of the band; LI 0 2 - 0 0 2 and LI 08-
0 0 8 primarily control the high frequency response char­
acteristic of the filter. All of the adjustments interact and
are adjusted for optimum response flatness over the pass
band.
Toroid transformer T120, T124 and T134 provide the wide
band characteristics for the input and output coupling. LI 24-
0 24 form a 75 MHz trap to provide additional attenua­
tion (approximately 60 dB) to any 75 M Hz signal that may
push through the filters.
0 3 7 at the emitter and L134 at the collector of Q130
are peaking adjustments which are adjusted for optimum
flatness of the IF response. 0 3 7 compensates for the tran­
sistor rolloff toward the high end of the band; however,
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because of the low Q in the collector circuit due to R134
and circuit loading, the overall effect of both adjustments
(1.134 and 0 3 7 ) is seen as. a bandpass response adjustment.
The output from Q 130 is applied through transformer
T134 to the base of mixer amplifier Q140. The swept os­
cillator output is coupled to the emitter of Q140. The col­
lector output load (LI 44 and 0 4 3 ) is tuned to 75 M Hz,
so the difference frequency of 75 M Hz is coupled through
the 65 M Hz trap to the attenuator circuit as the 2nd IF.
The 65 M Hz trap (LI4 7 -0 4 7 ) attenuates or rejects any 65
M Hz signal component from feeding through to mix with
the 70 MHz oscillator. A 65 M Hz signal mixing with 70 M H z
would generate a 5 M H z difference signal and pass through
the narrow band IF amplifier to appear as an undesirable
spurious response on the display.
IF Attenuator
The IF attenuator is a six section network that provides
a total signal attenuation of 51 dB. The input and output
impedances to the attenuator are maintained at a constant
50 H, regardless of the IF ATTEN switch settings. Input and
output filter sections (Cl 51 -LI 51 -Cl 52 and C187-L188-C188)
at the input and output of- the attenuator form a low pass’
filter to prevent high frequency signals from feeding into
the 75 MHz amplifier.
Narrow Band IF Am plifier
This circuit contains two stages of 75 M Hz IF am plifica­
tion, a stable 70 M H z oscillator, a mixer amplifier with its
output tuned to 5 M Hz and one stage of amplification for the
5 MHz IF frequency.
Input to the ..amplifier is AC coupled from the IF atten­
uator to the base of Q420. The 75 M Hz IF amplifiers are
Q420 and Q430. The IF transformers are tuned to the IF
by adjusting the capacitance of C425 and C435. Gain of
the amplifier is varied by changing the forward bias of
Q420, which then sets the bias of Q430 through the DC
return of its base to the emitter of Q420. A feedback wind­
ing of T424 to the base of Q420, provides the neutraliza­
tion for the collector to base capacitance.
The 75 M Hz IF and the output from a crystal controlled
70 M Hz oscillator-Q440 are applied to the mixer amplifier
Q450. The collector load of Q450 is T454, which is tuned
to 5 M Hz and couples the signal to the 5 M Hz IF am pli­
fier Q460. Diode D454 in the collector load of Q450 im­
proves the overload characteristics of the amplifier. Output
of the 5 M Hz IF signal is applied through an insulated con­
nector J470 to the input of the variable resolution amplifier.
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Variable Resolution Amplifier
The variable resolution amplifier is designed to vary the
bandwidth of the 5 M Hz IF from over 100 kHz to less than
1 kHz. Bandwidth of the circuit is a function of the. output
load for a crystal filter network. By varying the output
load a variable resolution bandwidth is obtained.
The signal input to the variable filter circuit is insulated
from chassis ground and connects across R501-R502 as shown
in Fig. 3-7'A. Crystal Y501 is a 5 M Hz crystal, connected
3 -7