Tektronix 492 Service Manual page 162

Span Attenuator (refer to diagram 35). Two registers
(75 and 76) control the span attenuator (see Table 4-13).
Table 4-13
SPAN ATTENUATOR REGISTERS (75 AND 76)
Bit
Span Magnitude (75)
Lower 8 bits of 10-bit attenuation code
DB7—DBO
(000 is max attenuation)
Span Magnitude and Attenuator (76)
DB7 Sweep ± (1/0) to match ± mixing
DB5 Sweep decade attenuator
DB6
0 0
1
0
1 0
DB4 DB3 Output select and calibration
0 0 1 st LO main coil
1
0
1 0
DB2 For future use
DB1—DBO Upper two bits of attenuation code
Function
X 1.0
X 0.1
X 0.01
1st LO FM coil
2nd LO
Maintenance—492/492P Service Vol. 1 (SN B030000 & up)
1st LO Driver (refer to diagram 36). One register (72)
controls functions on the 1st LO Driver board. Another reg­
ister (7E) is added for the 492P to make the PEAKING con­
trol programmable (see Table 4-14).
Table 4-14
1st LO DRIVER REGISTERS (72 AND 7E)
Bit
1st LO Driver Functions (72)
DB7 Normal/max span mode (1/0)
DB6 Connect/disconnect sweep voltage to driver
(1/0)
DB5 Driver off/on (1/0)—off for degauss
DB4 Filter on/off at driver output (1/0)—on for
unphase-locked narrow spans
DB3 External mixer disconnect/connect (1/0); con­
nected in bands 1—5 if external mixer selected;
always connected in higher bands
DB1
DB2 DBO
1 1 0
1 1 0
1 1 0
1 0 1
0 1 1
1 1 1 Selects no internal mixer bias
PEAKING Control (7E)
DB7 0 steers DB4—DBO to upper latch
0 steers DB5—DBO to lower latch
DB6
1 sent to DB4 of upper latch disables
DB5—DBO
front-panel PEAKING control; DB3— DBO
of upper latch and DB5—DBO of lower
latch form 10-bit input to DAC for
programmable PEAKING voltage
Function
Internal Mixer Bias for
Band 1
Band 2
Band 3
Band 4
Band 5