# 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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