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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.
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