Model 8640B
TM 9-4935-601-14-7&P
SERVICE SHEET 18
PRINCIPLES OF OPERATION
General
The RF A8A1 Scaler Assembly processes the RF input to the counter. In INT or EXT 0 - 550 MHz counter
modes, the input frequency is divided by 64. In the EXT 0 - 10 MHz mode, the input frequency is not divided.
Comparator (A8A1)
In the INT counter mode, the RF signal from the RF oscillator Frequency Counter Buffer Amplifier (Service
Sheet 5) passes through relay K1 into Comparator U5 which converts the input signal to EECL compatible
pulses. In the EXT counter mode, the external input couples into U5 through relay K2 and a diode network
(CR2 and CR3) which protects U5 from large voltages.
Dividers (A8A1)
EECL dividers U1 and U2 divide the frequency by 2 and 16 respectively; U4 is an ECL divide-by-two. In the
INT or EXT 0 - 550 MHz counter modes, the divider stages are enabled through the OR input of U1 and set
(S) input of U4. The output of U3A is high, U3D is low, U3B inverts the Q output of U4. Note that for ECL
and EECL, ground is a logical high and an open and a negative, is a logical low. In the EXT 0 - 10 MHz
counter mode the OR input of U1 is disabled (high), and the set (S) input of U4 is high; therefore, the Q output
is low. The output of U3A is low, U3D inverts the RF input and U3B inverts the output from U3D with no
frequency division. Transistors Q1 and Q2 shift the EECL logic levels to ECL logic levels. The output from
the bypass gate U3D is ac coupled into transistor A8A3Q2 which converts the ECL logic levels to TTL logic
levels. The output of A8A3Q2 drives the counter circuits.
TROUBLESHOOTING
It is assumed that a problem has been isolated to the counter RF scaler
circuits as a result of using the troubleshooting block diagrams. Troubleshoot
by using the test equipment listed below, performing the initial test
conditions and control settings, and following the procedures outlined in the
table.
Test Equipment
Digital Voltmeter ................................................................................................................. . HP 3480B/3484A
Oscilloscope ................................................................................................................. HP 180A/1801A/1820C
Frequency Counter............................................................................................................................ HP 5327C
Meter Circuits (A2, A4, A6)
SERVICE SHEET 17
SERVICE SHEET 18 (Cont'd)
Initial Test Conditions
Top cover removed (see Service Sheet F for removal procedure). A8 Counter/Lock Assembly casting cover removed with
access to A8A1 RF Scaler Assembly and A8A3 Time Base Assembly (see Service Sheet B for procedures). Connect RF
OUTPUT to COUNTER INPUT.
Initial Control Settings
COUNTER MODE:
EXPAND ........................................................................................................................ Off
LOCK ............................................................................................................................ Off
Source ................................................................................................................EXT 0 - 10
AM ............................................................................................................................... OFF
FM ............................................................................................................................... OFF
RANGE ................................................................................................................ 4 - 8 MHz
FREQUENCY TUNE ............................................................................................... Full cw
OUTPUT LEVEL ............................................................................................ 100 mVOLTS
RF ON/OFF ................................................................................................................... ON
Counter RF Scaler Troubleshooting
Component
Test Conditions and
Normal Indication
If Indication
or Circuit
Control Settings
is Abnormal
RF SCALER
Initial conditions and
Frequency at COUNTER
Check K2, U5, Q1, U3A,
(A8A1)
settings. Check fre-
INPUT the same as U3B
U3D, and associated
quency at COUNTER
pin 6
circuitry
INPUT jack and at U3B
pin 6.
Set RANGE to 32-64
Frequency at COUNTER
Check K2, U5, U1, U2,
MHz and COUNTER
INPUT 64 times fre-
U3B, U4, Q2, and asso-
MODE to EXT 0-550.
quency at U3B pin 6
ciated circuitry
Check frequency at
COUNTER INPUT
jack and at U3B pin 6.
COMPARATOR
Initial conditions and
Frequency at U3B pin 6
Check U5, U1, K1, and
(A8A1)
settings except set
- 8.58 MHz
associated circuitry
COUNTER MODE
to INT, RANGE to
256-512 MHz, and
FREQUENCY TUNE
to 550 MHz (with
counter at RF OUT-
PUT).
Set COUNTER
Frequency at U3B pin 6
Check CR2, CR3,'K2,
MODE to EXT 0-5508.58 MHz
and associated circuitry
ECL to TTL
Initial conditions and
0-5V square wave at
Check Q2 and associated
TRANSLATOR
settings except set
Q2-C at- 8 MHz
circuitry
(A8A3)
COUNTER MODE
to INT
8-54
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