Electronic circuits > Computers > PC based Frequency Meter
PC based Frequency Meter Here is a simple technique for measuring frequencies over quite a wide
frequency range and with acceptable accuracy limits using a PC. It follows the
basic technique of measuring low frequencies, i.e. at low frequency, period is
measured for a complete wave and frequency is calculated from the measured
time-period. Cascaded binary counters are used for converting the high-frequency
signals into low-frequency signals. The parallel port of a computer is used for
data input from binary counters. This data is used for measuring time and
calculating the frequency of the signal. The block diagram shows the basic
connections of the counters and parallel port pin numbers on 25-pin ‘D’
connector of a PC (control register 379 Hex is used for input). External
hardware is used only for converting the higher frequency signals into low
frequency signals. Thus, the major role in frequency-measurement is played by
the software. The PC generates a time-interrupt at a frequency of 18.21 Hz, i.e.
after every 54.92 millisecond. Software uses this time-interrupt as a
time-reference. The control register of the PC’s parallel port is read and the
data is stored continuously in an array for approximately 54.9 ms using a loop.
This stored data is then analysed bit-wise. Initially, the higher-order bit (MSB
or the seventh-bit) of every array element is scanned for the presence of a
complete square wave. If it is found, its time period is measured and if not
then the second-highest order bit (sixth bit) is scanned. This operation is
performed till the third bit and if no full square wave is still found, an error
message is generated which indicates that either there is an error in reading or
the frequency signal is lower than 19 Hz. Lower three bits of the control
register are not used. When a wave is found, along with its time-period and
frequency components, its measurement precision in percentage is also calculated
and displayed. Number of data taken in 54.9 ms is also displayed. As stated
above, the lower starting range is about 19 Hz. Data is read for approximately
54.9 ms. Thus, the lowest possible frequency that can be measured is 1/.0549 Hz.
Lower range depends only on the sampling time and is practically fixed at 19 Hz
(18.2 Hz, to be precise). Upper range depends on factors such as value of the
MOD counter used and the operating frequency range of the counter IC. If MOD-N
counter is used (where N is an integer), upper limit (UL) of frequency is given
by UL=19xN5 Hz. Thus for MOD 16 counters UL@20 MHz, and for MOD 10 counters
UL@1.9 MHz. Care should be taken to ensure that this upper limit is within the
operating frequency range of counter IC used. Precision of measurement is a
machine-dependent parameter. High-speed machines will have better precision
compared to others. Basically, precision depends directly upon the number of
data read in a standard time. Precision of measurement varies inversely as the
value of MOD counter used. Precision is high when MOD 10 counters are used in
place of MOD 16 counters, but this will restrict the upper limit of frequency
measurement and vice-versa.
Published on: 2004-08-21 (3628 reads)
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