Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
Table 5-13
ATTENUATION SELECTION CODES
U2015
Pin 15
(Q67)
Pin 16
(Q7)
Attenuation Factor
low
low
X1 (K3065)
high
low
X 0.1 (K3075)
low
high
X 0.01 (K4072)
Decade Attenuator
Since accuracy of the digital-to-analog converter is speci
fied as a percentage of full scale, the accuracy decreases as
the attenuation is increased. To maintain accuracy at 1%, it
is never used at an attenuation factor of more than ten. If
more attenuation is required, the decade attenuator, con
sisting of K4072, K3075, K3065 and the connected divider
network, provides further sweep attenuation of X0.01, X0.1,
and X1. See Fig. 5-26 for a simplified circuit diagram.
The “2” side of U4025 is controlled by data on the Q
6
and Q7 lines from U2015. The “2Y” outputs of U4025 are
applied through buffer amplifiers in U4042 to select the ap
propriate attenuation factor for the output sweep. Table
5-13 lists the states required to energize the attenuation
relays. A diode across each relay coil protects the driving
circuit from inductive feedback transits.
Fig. 5-26. Simplified span decade attenuator.
FIRST LOCAL OSCILLATOR DRIVER
<§>
Refer to the block diagram adjacent to Diagram 36.
The 1 st LO Driver performs the following functions: Buff
ers the TUNE VOLTS signal and applies it to the dot marker
circuit; combines the SPAN VOLTS and TUNE VOLTS sig
nals, and applies the combination to the Preselector Driver.
The combined signal is also applied to the Oscillator Driver
circuits, which drive the 1st Local Oscillator coils; selects
and applies the appropriate tune bias voltage to the 1 st Mix
er; controls the oscillator filter switch in the 1 st LO Driver;
produces a stable and precise — 10 V reference for both the
1 st LO and the Preselector Driver circuits.
The major circuits and their function are as follows:
1
) the digital control section buffers the incoming data from
the data bus, decodes the address data, selects the re
quired mixer bias, connects or disconnects the TUNE
VOLTS and SPAN VOLTS signals for the summing amplifi
er, energizes the filter switch for the 1st LO, and controls
the drive and filtering of the oscillator driver stage;
2) the tune volts buffer buffers the COARSE TUNE VOLTS
signal from the Center Frequency Control circuits, and re
duces the signal amplitude to drive the dot marker circuits;
3) the oscillator filter driver furnishes drive current to the
capacitor switching relay in the 1st LO;
4) the input switching circuits combine the SPAN VOLTS
and COARSE TUNE VOLTS signals, and applies these sig
nals to the summing amplifier;
5) the summing amplifier furnishes the drive signal to the
oscillator driver. The summing amplifier sums the SPAN
VOLTS ramp signal from the span attenuator with the
coarse tune voltage from the Center Frequency Control cir
cuit. In less than maximum span, a sweep voltage of ± 10 V
sweeps the oscillator at a rate of 333 MHz/division. As the
TUNE VOLTS signal varies from —10 to +10 V, the oscilla
tor's center frequency is moved over its full range of 2.072
to 6.35 GHz, plus about 50 MHz overtune at each end;
6
) the oscillator driver furnishes swept current drive to the
1st LO coil;
7) the reference supply, which produces a precise — 10 V
reference for the Preselector Driver and 1 st LO Driver;
8
) the mixer bias driver produces the required bias voltages
for the 1st Mixer in the IF stages.
Digital Control
The digital control circuit sets the oscillator span volts,
the 1st Mixer bias and programmable bias for the 492P.
Decoder U4034 output Y1 (pin 14) goes low when input
address is 72 and output Y7 goes low for address 7E. When
either of these outputs go high, data is clocked or latched
into U4017, U4024, and U4022. Data for U4017 consists of
control codes for the oscillator drive circuits and switch
U1016 which selects 1st Mixer bias for the 3.0 to 21 GHz
bands, and the bias set by the front panel PEAKING control.
The codes are described where each applies to the descrip
tion and in Table 5-14. Data for DAC U3022 is converted to
an analog signal which provides the Programmable Bias for
