The discriminator for the kHz/DIV position of the DISPER-
ampilifer with its output tuned to 75 MHz, Gain through the
SION RANGE switch consists of tuned circuits which operate
amplifier is approximately 20 dB.
much like the tuned transmission lines for the MHz/div dis-
The wide band response from the RF section is applied
criminator. The parallel circuit L384-C384 is tuned slightly
through a 150-250 MHz band-pass filter to the input amplifier
below the center frequency, and the circuit L385-C385 is
Q120. The band-pass filter is a combination constant-k
tuned above the center of the sweep oscillator frequency.
type filter, modified with m-derived input and output sections
The output of the detectors is shown in Fig. 3-7. When the
to provide a constant 500 input and output impedance
through the pass band. Series-tuned circuits L101-C101 and
frequency curve similar to Fig. 3-7C is the resultant output.
L107-C107, are tuned to the low end of the band; L102-C102
The circuit operates over the linear portion of the curve.
and L108-C108 primarily control the high frequency response
The kHz/div Cal adjustment R368 calibrates this range.
characteristic of the filter. All of the adjustments interact and
Diodes D277 and D279 isolate the narrow band discrimi-
are adjusted for optimum response flatness over the pass
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
wide band characteristics for the input and output coupling.
band discriminators. The diodes are forward biased when
L124-C124 form a 75 MHz trap to provide additional at-
the DISPERSION RANGE switch is in the MHz/DIV position
tenuation (approximately 60 dB) to any 75 MHz signal that
and the current through the diodes lowers or spoils the Q
may push through the filters.
of the tuned discriminator circuit. When the DISPERSION
RANGE switch is in the kHz/DIV position, the diodes are
C137 in the emitter and L134 in the collector of Q130
backbiased and disconnected from the low dispersion dis-
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
Ampliude Comparator. Uniform sensitivity and linearity
of the low Q in the collector circuit; due to R134 and circuit
over the dispersion range is maintained by controlling or
loading, the overall effect of both adjustments (L134 and
regulating the oscillator output amplitude. This is accom-
C137) is seen as a bandpass response adjustment.
plished by the RF amplitude comparator circuit, Q290 and
Q280. The RF output signal is detected by diode D361 and
The output from Q130 is applied through transformer
applied through diode D362 to the base of Q280. This
T134 to the base of mixer amplifier Q140. The swept oscil-
rectified RF signal on the base of Q280 is compared against
lator output is coupled to the emitter of Q140. The collector
a reference voltage set by the RF Ampl adjustment R290.
output load (L144 and C143) is tuned to 75 MHz so the dif-
The differential output signal is fed back as a voltage to
ference frequency of 75 MHz is coupled through the 65 MHz
control the forward bias of Q320. Q320 is the current source
trap to the attenuator circuit as the 2nd IF frequency, The
for the oscillator circuit. Amplitude changes in the oscillator
65 MHz trap (L147-C147) attenuates or rejects 65 MHz signal
output are fed back as correction signals to the current
component from feeding through to mix with the 70 MHz
regulator to regulate oscillator current or output power.
oscillator. Any 65 MHz signal would mix with 70 MHz to
generate a 5 MHz signal for the narrow band IF amplifier
To summarize the sequence of operation for the sweeper
and would appear as an undesirable spurious response on
circuit, assume the output from the sweep comparator Q220-
the output display.
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
The IF attenuator is a six section network that provides
from D376 (assuming the DISPERSION RANGE switch is in
a total signal attenuation of 51 dB. The input and output
the position shown in the schematic diagram) and a decreasing
impedances to the attenuator are maintained at a constant
negative voltage from D373. The differential output from
50 regardless of the IF ATTENUATOR switch setting, Input
comparator Q260 is a positive-going ramp to the base of
and output filter sections (C151-L151-C152 and C187-L188-
Q230, where it is compared against the input ramp on the
C188) at the input and output of the attenuator form a low
base of Q220. The differential signal output from the sweep
pass filter to prevent high frequency signals from feeding
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
into the 75 MHz amplifier.
horizontal sweep generator sawtooth signal and the dis-
persion is a function of the DISPERSION RANGE and DIS-
PERSION selector positions.
Narrow Band IF Amplifier
DISPERSION RANGE BAL adjustment R234 provides IF
This circuit contains two stages of 75 MHz IF amplification,
center frequency balance adjustment between the MHz/Div
a stable 70 MHz oscillator, a mixer amplifier with its output
and kHz/Div dispersion positions. Center Freq Range adjust-
tuned to 5 MHz and a stage of amplification for the 5 MHz
ment (R251) and CAL (R250) calibrate the frequency range
of the IF CENTER FREQ control over the IF center 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
Wide Band (1 50-250 MHz) Amplifier and Second
adjusting the capacitance of C425 and C435. Gain of the
amplifier is varied by changing the forward bias of Q420,
The wide band amplifier contains an input 150-250 MHz
band-pass filter, two amplification s t a g e s a n d a m i x e r
which then sets the bias of Q430 through the DC return of