Tektronix 491, Maintenance Manual

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The discriminator for the kHz/DIV position of the DISPER-
SION RANGE switch consists of tuned circuits which operate
much like the tuned transmission lines for the MHz/div dis-
criminator. The parallel circuit L384-C384 is tuned slightly
below the center frequency, and the circuit L385-C385 is
tuned above the center of the sweep oscillator frequency.
The output of the detectors is shown in Fig. 3-7. When the
detector output is applied to the comparator, a voltage versus
frequency curve similar to Fig. 3-7C is the resultant output.
The circuit operates over the linear portion of the curve.
The kHz/div Cal adjustment R368 calibrates this range.
Diodes D277 and D279 isolate the narrow band discrimi-
nator tuned circuit when the Type 491 is operating in the
MHz/DIV dispersion range. They prevent parasitic oscillation
due to circuit coupling between the wide band and narrow
band discriminators. The diodes are forward biased when
the DISPERSION RANGE switch is in the MHz/DIV position
and the current through the diodes lowers or spoils the Q
of the tuned discriminator circuit. When the DISPERSION
RANGE switch is in the kHz/DIV position, the diodes are
backbiased and disconnected from the low dispersion dis-
criminator circuit.
Ampliude Comparator.
Uniform sensitivity and linearity
over the dispersion range is maintained by controlling or
regulating the oscillator output amplitude. This is accom-
plished by the RF amplitude comparator circuit, Q290 and
Q280. The RF output signal is detected by diode D361 and
a p p l i e d t h r o u g h d i o d e D 3 6 2 t o t h e b a s e o f Q 2 8 0 . T h i s
rectified RF 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 voltage to
control the forward bias of Q320. Q320 is the current source
for the oscillator circuit. Amplitude changes in the oscillator
output are fed back as correction signals 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 Q220-
Q230 is a positive-going ramp. This voltage ramp increases
the bias on the capacitance diode and decreases the circuit
capacitance so the socillator output frequency will increase.
This increase in output frequency is fed back to the discrimi-
nator, and detected as an increasing negative voltage output
from D376 (assuming the DISPERSION RANGE switch is in
the position shown in the schematic diagram) and a decreasing
negative voltage from D373. The differential output from
comparator Q260 is a positive-going ramp to the base of
Q230, where it is compared against the input ramp on the
base of Q220. The differential signal output from the sweep
comparator,
s y n c h r o n i z e s t h e s w e e p o s c i l l a t o r w i t h t h e
horizontal sweep generator sawtooth signal and the dis-
persion is a function of the DISPERSION RANGE and DIS-
PERSION selector positions.
D I S P E R S I O N R A N G E B A L a d j u s t m e n t R 2 3 4 p r o v i d e s I F
center frequency balance adjustment between the MHz/Div
and kHz/Div dispersion positions. Center Freq Range adjust-
ment (R251) and CAL (R250) calibrate the frequency range
of the IF CENTER FREQ control over the IF center frequency.
Wide Band (1 50-250 MHz) Amplifier and Second
Mixer
The wide band amplifier contains
band-pass filter, two amplification
3 - 8
an input 150-250 MHz
s t a g e s a n d a m i x e r
ampilifer with its output tuned to 75 MHz, Gain through the
amplifier is approximately 20 dB.
The wide band response from the RF section is applied
through a 150-250 MHz band-pass filter to the input amplifier
Q 1 2 0 . T h e b a n d - p a s s f i l t e r i s a c o m b i n a t i o n c o n s t a n t - k
type filter, modified with m-derived input and output sections
to provide a constant 500 input and output impedance
through the pass band. Series-tuned circuits L101-C101 and
L107-C107, are tuned to the low end of the band; L102-C102
and L108-C108 primarily control the high frequency response
characteristic of the filter. All of the adjustments interact and
are adjusted for optimum response flatness over the pass
band.
T o r o i d t r a n s f o r m e r s T 1 2 0 , T 1 2 5 a n d T 1 3 4 p r o v i d e t h e
wide band characteristics for the input and output coupling.
L124-C124 form a 75 MHz trap to provide additional at-
tenuation (approximately 60 dB) to any 75 MHz signal that
may push through the filters.
C137 in the emitter and L134 in the collector of Q130
are peaking adjustments and adjusted for optimum flatness
of the IF response. C137 compensates for the transistor
rolloff toward the high end of the band; however, because
of the low Q in the collector circuit; due to R134 and circuit
loading, the overall effect of both adjustments (L134 and
C137) is seen as a bandpass response adjustment.
T h e o u t p u t f r o m Q 1 3 0 i s a p p l i e d t h r o u g h t r a n s f o r m e r
T134 to the base of mixer amplifier Q140. The swept oscil-
lator output is coupled to the emitter of Q140. The collector
output load (L144 and C143) is tuned to 75 MHz so the dif-
ference frequency of 75 MHz is coupled through the 65 MHz
trap to the attenuator circuit as the 2nd IF frequency, The
65 MHz trap (L147-C147) attenuates or rejects 65 MHz signal
component from feeding through to mix with the 70 MHz
oscillator. Any 65 MHz signal would mix with 70 MHz to
generate a 5 MHz signal for the narrow band IF amplifier
and would appear as an undesirable spurious response on
the output 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 regardless of the IF ATTENUATOR switch setting, Input
and output filter sections (C151-L151-C152 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 Amplifier
This circuit contains two stages of 75 MHz IF amplification,
a stable 70 MHz oscillator, a mixer amplifier with its output
tuned to 5 MHz and a stage of amplification for the 5 MHz
IF frequency,
Input to the amplifier is AC coupled from the IF attenuator
to the base of Q420, The 75 MHz IF amplifiers are Q420 and
Q430. The IF transformers are tuned to the IF frequency 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