Realization
of a DIGITAL Frequency meter to 2 digits |
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Created it, 06/10/19
Update it, 06/11/02
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9. - SIXTH EXPERIMENT : REALIZATION OF A FREQUENCY METER DIGITAL A 1 DIGIT
The digital measurement of a frequency is generally more precise than analogical measurement.
To take a digital measurement of frequency, it is enough to hope the number of periods to the signal during a given time.
The frequency being the number of periods a second, it is thus enough to count the number of cycles during one second, as indicated figure 23.
For that, one uses an ordinary meter on the entry CLOCK whose is applied the rectangular signal. A control signal makes it possible to start counting and to stop it at the end of one second. If the frequency is worth 4 Hz (figure 23), the meter will record four impulses.
9. 1. - REALIZATION OF THE CIRCUIT
a) Remove matrix all the components and the connections relating to the preceding experiment.
b) Carry out the assembly indicated figure 24-a.
c) Prepare the generator of Digilab on the frequency of 10 Hz.
The electric diagram of the circuit carried out is deferred figure 24-b.
The integrated circuit LM 555 is assembled into monostable and the impulse which it generates lasts approximately 1 second.
It makes it possible to validate the meter MM 74C193 during 1 second thanks to its entry CLEAR.
The signal which one wants to measure the frequency is applied to entry COUNT UP of the meter.
The result of counting is posted on DIS0 which is connected to exits QA, QB, QC and QD of the meter.
9. 2. - OPERATIONAL TEST
a) Put Digilab under tension.
b) Support on P0 and slacken it immediately.
You see ravelling the figures from 0 to 9 on the bill-poster, then this last stops on figure A, which corresponds to 10.
That indicates that the frequency of the signal applied is worth 10 Hz.
It is possible that the bill-poster indicates a value rather different from 10 Hz. First of all, the duration of the impulse of monostable can be slightly different from 1 second and in addition, the precision of the frequency of the clock signal of Digilab is not very high.
You can also carry out one of the assemblies astables practice 7 and measure the frequency of the output signal.
Nevertheless, you will be limited to 15 Hz.
c) This experiment being finished, put Digilab not under tension.
The synoptic diagram of figure 25 enables you to include/understand the operation of the frequency meter.
While supporting on P0, the monostable one is started and delivers an impulse of 1 second which validates the meter, which enters the impulses which reach him on its entry COUNT UP.
The impulse resulting from the LM 555 also orders the entry of locking LATCH of bill-poster DIS0.
The chronogram of figure 26 represents the evolution of the tensions in various points of the assembly.
The output signal of the LM 555 is reversed and applied to the entry of locking LATCH (LE0) of bill-poster DIS0. This signal which arrives on the entry LATCH is then reversed four times of continuation, which makes it possible to have an identical signal on entries LATCH of the bill-poster and CLEAR of the meter. That arriving on entry CLEAR has a certain delay caused by the 4 inverseuses doors compared to the signal applied to the entry LATCH of the bill-poster. Indeed, it is necessary that the restoring of the meter is carried out after the locking of the bill-poster.
When signal LE0 passes from state 0 to state 1, bill-poster DIS0 memorizes the contents of the meter, this last being given at zero only one certain time after this memorizing of the contents of the meter.
10. - SEVENTH EXPERIMENT : REALIZATION OF A FREQUENCY
METER DIGITAL A TWO
DIGITS
The circuit that you will carry out is very similar to the precedent. Nevertheless, it has better performances on the level of the measurement of frequency. First of all, measurement is automatic and repetitive, then the result is posted on two digits. However, this assembly does not claim to be a didactic laboratory apparatus but only one circuit.
10. 1. - REALIZATION OF THE CIRCUIT
a) Remove matrix all the connections and the components relating to the preceding experiment.
b) Insert on the matrix integrated circuits CD4040, MM74C74, MM74C08, MM74C04, LM 555 and carry out the connections indicated figure 27.
c) Prepare the rectangular generator of signal of Digilab on the frequency of 1 Hz.
Figure 28 represents the electric diagram and the synoptic diagram of the circuit carried out.
The principle of operation is the same one as previously; it is always a question of counting the pulse repetition frequency arriving on the entry of a meter during a given time.
In the experiment present, the measurement of frequency is made automatic and repetitive. For that, the monostable one is replaced by an oscillator which generates a signal either of 1 Hz, or of 100 Hz and by a divider by four.
As you will see it, in this way one takes a measurement every four seconds in the first case (oscillating to 1 Hz), and twenty-five measurements per seconds in the second case (oscillating to 100 Hz).
The meter is consisted the integrated circuit CD 4040 on 12 floors. It is an asynchronous binary counter having an entry of restoring asynchronous.
Here, eight exits of the CD 4040 are used and connected to two bill-posters DIS0 and DIS1.
The Q10 exit of the meter is connected to the detector of rising face installed on Digilab. Thus, when there is a capacity overshooting of the frequency meter, the LED L0 ignites.
The synchronizing circuit is formed by three doors AND of the circuit MM 74C08 and by three reversers of the MM 74C04.
This synchronizing circuit provides the various signals necessary to the operation of the meter, the locking of the bill-poster and the restoring of the indicator of going beyond.
10. 2. - OPERATIONAL TEST
a) Put Digilab under tension and observe the bill-posters. They must indicate 01 since the frequency of the signal applied to the entry is worth 1 Hz.
Nevertheless, the precision of the base of time (signal resulting from the LM 555), like that of the clock signal not being very high, it is possible that the bill-poster indicates 00 or 02.
b) Lay out the generator of clock of Digilab on 10 Hz. You must read 0A on the bill-posters, which corresponds to 10 in decimal base. For the reasons evoked previously, you can read 09, 0B, 0C,…
c) Lay out the generator of clock on 100 Hz. The bill-poster must indicate 6416, that is to say 10010. However, the actual value can deviate notably from this theoretical value.
Until now, the base of time of the frequency meter was worth 1 second, that is to say a frequency of 1 Hz. However, with two hexadecimal bill-posters, it is possible to post to the maximum FF16, that is to say 25510. Thus the capacity of the frequency meter with this base of times is worth 255 Hz.
To increase this capacity, it is necessary to reduce the base of time, therefore to increase the frequency of the signal of the base of time.
To this end, proceed as indicated hereafter.
d) Put Digilab not under tension.
e) Remove the connection connecting pin 6 of the LM 555 to the positive pin of the C2 condenser and connect this pin 6 at the loose lead of the C3 condenser as indicated by the tear line figure 27.
f) Lay out the generator of clock of Digilab on the frequency of 1 kHz.
g) Withdraw contact CP1 the connection coming from pin 9 of the integrated circuit MM 74C08 and introduce it into contact CP2 as indicated by figure 27 tear line.
h) Put Digilab under tension and observe the bill-posters. You must read a value close to 0A16, that is to say 1010. The base of times is worth 0,01 second is 1 / 100e of that of the preceding test, it is thus necessary to multiply by 100 the value posted to obtain the actual value.
In this case, one obtains 10 x 100 = 1 000 Hz, that is to say 1 kHz.
i) Lay out the generator of clock on 10 kHz. The bill-posters must indicate a number close to 6416 or 10010. The actual value is well of 100 x 100 = 10 000 Hz, that is to say 10 kHz.
j) Lay out the generator of clock on 100 kHz. You note that the LED L0 ignites. Indeed, the maximum capacity of the frequency meter is worth : 255 x 100 = 25 500 Hz, are 25,5 kHz.
k) The tests being finished, put Digilab not under tension.
In order to analyze the operation of this assembly, you defer to the figure 28-a as on the figure 29 where the chronogram relating to the operation of the frequency meter is deferred.
The oscillator part consisted the LM 555 can generate two rectangular signals, one of 1 Hz with C2, the other of 100 Hz with C3.
This signal, resulting from the LM 555, arrives on the rocker A which divides by two the frequency of this signal. One thus obtains a signal of frequency F01 = 0,5 Hz (or 50 Hz) perfectly symmetrical.
This second signal of F01
frequency is sent on the second rocker B
which divides again by two. The signal at exit 
(
02)
of this rocker B constitutes the base of
time (0,25 Hz or 25
Hz) of the frequency meter. This signal arrives on one of the two
entries of the door AND n°1, while the
signal which one wants to measure the frequency arrives on the other entry.
This door AND
makes it possible to validate the input signal, either during 2
seconds (02
= 0,25 Hz), or during 2 / 100e s (02
= 25 Hz).
The signal resulting from this door is applied to the entry of clock f of the meter CD 4040.
This duration of counting is noted T2 on the chronogram.
The synchronizing circuit consisted
the three doors AND (2,
3 and 4) and the three
reversers (1, 2 and 3)
generates three control signals a, b and c.
It is a combinative network which use the signals of frequency F0,
F01, 01
and F02.
Signal a is applied to the entries LATCH LE0 and LE1 of the two bill-posters. When this signal a is on the level L (time T3), the result of counting, present on the exits of the meter, is memorized and transferred to the bill-posters.
The signal b allows the restoring of the meter when it is on the level H (time T1).
The signal c allows the restoring of the rocker D being used to detect a face going up (indicating of going beyond of the frequency meter). This restoring is also carried out during time T1.
The measurement of frequency is carried out as follows :
For length of time T1,
the meter and the rocker of going beyond are given to zero.
During T2 time,
the measurement of frequency is carried out, the meter enters the impulses which
forward through the door AND n° 1.
During moment T3,
the result of counting is memorized by the circuit of posting.
Then, a new cycle of measurement starts again.
So during measurement counting exceeds the 8 bits, the Q10 exit of the meter passes on the level H and starts the detector of rising face. In this case, the LED L0 ignites.
You can notice that the Q1 exit of the meter is not used. This is explained by the fact why the base of time is worth 2 seconds (time during which the door AND N° 1 are validated) and not 1 second. Therefore, by using the Q2 exits in Q9 of the meter, one enters only one impulse out of two, but as the base of time is worth 2 seconds and not 1 second, one enters well the pulse repetition frequency real during one second.
This frequency meter remains rather simple and its precision is based primarily on that of the oscillator astable integrated circuit LM 555.
Figure 30 shows an example relating to this problem of precision.
In this example, if the base of time is worth really 1 second, the posted result will be 10 Hz, but if it finishes at moment TA because of a lack of stability, the result will be 9 Hz ; if it finishes at the moment TC, the result will be 11 Hz.
For this reason, it is necessary that the oscillator of the base of time is very stable and precise.
In the laboratoryes apparatus, the oscillators are controlled by a quartz and are stabilized in temperature. Thus, the precision and stability are very high and the result obtained is very close to reality.
Thus this practice finishes. The following one will be devoted to the examination of the converters digital / analogical and analogical / digital.
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