transmission line input impedance increases, the open ended
line input impedance decreases. This produces proportionate
changes to the output signal from the diodes. Signal output
from diode D376 becomes more negative, and signal output
from D373 becomes less negative. This push-pull drive is
applied to the comparator Q260 and converted to a single
ended output signal for the sweep comparator.
Thermal balance is achieved by balancing the current
differential through both sections of the transistor. The com-
mon emitters are connected to a constant current source
Q270. Current (approximately 3 mA) is established by the
voltage drop across the emitter resistance R274.
The IF CENTER FREQ (R256) and the FINE (R259) controls
sum in a DC voltage with the differential signal from the
comparator to allow positioning of the IF center frequency
(200 MHz or a frequency close to 200 MHz) to the center of
the horizontal sweep.
The amplitude of the ramp signal to the sweep comparator
is a function of the DISPERSION RANGE switch SW210 and
the DISPERSION selector SW365 setting. This amplitude
determines the frequency deviation swing of the sweep
oscillator band.
cuit from the swept oscillator to the push-pull amplifier Q340-Q350.
so the dirve signal to the amplifiers is a balanced push-
pull signal.
Transformers T343 and T354 in the collector circuit of
Q340 ond Q350 provide a 4:1 impedance transformation
from the collectors of the transistors to the output transformer
T347.
Transformer T347 converts the push-pull signal to a single-
ended output signal. Push-pull amplification, plus filtering
through the low pass filter circuit of L358-C358 and L348-
C348 reduces the harmonic content in the swept frequency
output signal.
Diode D334 in the base voltage divider circuit provides
the temperature compensation for transistors Q340-Q350.
The single-ended output signal is coupled through a 2:1
impedance transformer T363, to the mixer in the Wide Band
IF. The output signal is also applied, through two feedback
loops, to frequency ond amplitude control circuits.
Frequency Discriminator. Two frequency discriminators
for each position of the DISPERSION RANGE selector SW365
nator comparator Q260. The output voltage from the com-
parator is a ramp voltage that is proportional to the sweep
oscillator frequency. It is applied to one side of the com-
parator Q230-Q220.
The MHz/Div discriminator consists of two matched diodes,
D373 and D376, at the input ends of two transmission lines.
frequency (275 MHz). One line is open ended and appears
capacitive, the other line is shorted and appears inductive,
at the center frequency. As the input frequency to the dis-
criminator increases, the transmission line input impedance
nears the characteristics of a wavelength line. The shorted
cuit.
3-7
