Frequency and Period Measurement
Frequency and Period Measurement
l
transmitter
l
=
v
2
t
receiver
t
Frequency Counter
Display
Input
Input Signal Processor
Input
Gate
t
Output
Gate
Memory
Strobe
Decade counter
Reset
Time base
N pulses
f
=
N t
Time Base
Crystal Oscillator Circuit (OSC)
Frequency Divider
Time base signal
?Non-compensated OSC ?Temp. compensated OSC ?Oven-type OSC
Quartz crystal
High Q ~ 10000
symbol
Equivalent circuit
Time base
4-bit Binary Counter
10 Hz 100 Hz
1 kHz
A B C D 1 s
0.1 s 0.01 s
Crystal Oscillator Circuit (OSC)
Frequency Divider
Time base signal
Logic Diagram of a time base for a frequency counter
State decoder ABCD
"0000" State 0
ABCD
"0010" 2
ABCD
"1100" 12
ABCD
"1110" 14
Reset
Open gate
Close gate
Store
Input Signal Processing
Input (Analog)
Amplifier
Output (Digital)
Comparator
Input (Analog)
Amplifier
Comparator (Schmitt Trigger)
Output (Digital)
Schmitt Trigger: Comparator with Hysteresis
Vref
+
Vin
Vref
R2
R3
R1
+
Vin
Vo V(1)
Vo
Vo
Vref
V(1)
V(0) Vin
Vo
V(0)
Vin
VTL VTH
Trigger level (Vref) output
VTH VTL
output
Period Measurement
Time base f = 1 MHz
1 ?s
X pulses counted
T
Digital measurement of time period T = (X pulses)(1 ?s)
= X ?s
Time base f = 1 MHz
1 ?s
Y pulses counted
PW
Digital measurement of pulse width PW = Y ?s
Period Measurement (between pulses)
Signal A Signal B
A (start)
start
Input Signal
S
Proccessor
stop
Input Signal Proccessor
R
Control circuit
Gate Q
Count
Counter
1 MHz 100 kHz
CLK 1 kHz
10 kHz
Core system S 0 0 1 1
RQ 0 Not use 11 00 1 No change
B (stop)
Gate
t
CLK
Count
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