Tektronix 491, Руководство по обслуживанию

Страница: 43 / 226

Variable Resolution Amplifier
T h e v a r i a b l e r e s o l u t i o n a m p l i f i e r i s d e s i g n e d t o v a r y
the bandwidth of the 5 MHz IF from approximately 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. Crystal
Y501 is a 5 MHz crystal, connected in series between the in-
put and the parallel resonant circuit L508-C508. Bandwidth
or resolution of the circuit is dependent on the characteristic
response of the crystal at its series resonant frequency and
the Q of the parallel resonant circuit L508-C508.
Video Detector and Vertical Amplifier (Fig. 3-10)
The 5 MHz IF response from the variable resolution am-
plifier is applied to a band-pass filter circuit to shape the
response and attenuate spurious signals. VERTICAL DISPLAY
switch SW600 selects one of three possible displays; LOG,
LIN and SQ LAW.
The LOG position applies the signal directly to the base
of the amplifier Q620. This direct coupling, with no signal
attenuation, provides the full dynamic range required for
the LOG diode circuit at the output of the video amplifier
and a logarithmic display.
Fig. 3-8 illustrates the impedance response versus fre-
quency curve of a quartz crystal. Capacitor C504 neutralizes
the stray shunt capacitance around the crystal so the response
of the crystal is equivalent to a series tuned circuit with a
very narrow band-pass 1 ; see Fig. 3-9.
The bandwidth of the filter network is a function of the
crystal output load, which is primarily the parallel resonant
circuit; therefore, bandwidth becomes a function of the Q
for the resonant circuit. The Q of the output load circuit for
the crystal is varied by changing the bias of diodes D506,
which changes the shunt loading across the parallel-tuned
circuit.
As the forward bias of D506 is increased, the Q of the
parallel resonant circuit decreases and the response char-
acteristic of the crystal becomes the dominant factor in
determining the bandwidth of the filter network. The crystal
response is very narrow, so the display resolution is in-
creased as the diode forward bias increases.
S W 5 5 0 , t h e R E S O L U T I O N s e l e c t o r , i s c o u p l e d t o t h e
DISPERSION selector and when coupled, provides normal
resolution for each position of the DISPERSION selector.
H o w e v e r , b y p u l l i n g t h e c o n t r o l k n o b , t h e R E S O L U T I O N
selector is uncoupled and any desired resolution within the
range of the control
can
be obtained for a given DISPERSION
selector setting.
The 100 MHz Resol Cal adjustment R543, adjusts the re-
solution bandwidth to approximately 100 kHz with the RE-
SOLUTION control at the fully clockwise position, and to
60 kHz at the —6dB point in the next position. The other
R E S O L U T I O N c o n t r o l p o s i t i o n s a r e n o t c a l i b r a t e d . H o w -
ever, the bandwidth at each step provides adequate re-
solution for most displays.
Emitter followers Q510-Q520 isolate the high impedance
of the filter network from the relatively low output imped-
ance, thus minimizing circuit loading on the filter network.
Q530 is a grounded-emitter operational amplifier with a
relatively low output impedance to provide the signal ampli-
tude required to drive the Log and Sq Law circuits.
1
(Ref: F. Langford-Smith RAC Radiotron Designer’s Handbook; 4th
edition.)
F i g . 3 - 1 1 . C h a r a c t e r i s t i c c u r v e s f o r 1 N 6 4 d i o d e s ; ( A ) v o l t a g e v s
current; (B) voltage vs dynamic resistance.
3-11