Generator of harmonic oscillations on logical elements with digital control. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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The generator of harmonic oscillations brought to the attention of readers is intended for use in various radio engineering devices with digital control. The generator is built on an accessible element base and provides the formation of harmonic oscillations with stable parameters. The article provides a method for calculating the generator.
The functional diagram of the generator is shown in fig. one.
Rice. 1 Functional diagram of the generator
The generation of harmonic oscillations requires the fulfillment of two conditions [1]:
- balance of amplitudes Kos*K=1;
- phase balance φк+φaxis\u0,2d XNUMX * π.
As a feedback amplifier Kos, as a rule, an inverter with φaxis = pi. As an amplifier K, it is advisable to use a band-pass filter [2], which provides the phase shift φ necessary in this case at the resonant frequencyk = pi. The high slope of the phase response of the filter in the passband ensures the formation of oscillations with a stable frequency, and its selective properties - the suppression of harmonics.
The filter [2] is used as a band-pass filter in the proposed generator. The generator is excited at the resonant frequency of the bandpass filter, which is determined from the expression:
where R (G) and C are the resistance (conductivity) and capacitance of the timing circuit.
It can be seen from expression (1) that the generation frequency is proportional to the conductivity of the timing circuit, which provides a linear frequency control law.
The scheme of the generator of harmonic oscillations is shown in fig. 2. A band-pass filter is made on the elements DD1.2-DD1.4, on the element DD1.1 - a feedback amplifier and on the elements DD2.1-DD2.4 and RM - a digitally controlled resistance matrix. The structure of the filter makes it possible to obtain antiphase voltages at the outputs, for which it is necessary to use Uout2 additionally. Inputs IN0 and IN 1 are digital inputs for controlling the generation frequency.
(click to enlarge)
Below is a method for calculating the generator.
- Let us set the values fmin - the lower generation frequency, Df - the tuning step, n - the number of digits of the binary number of the digital control signal.
- We accept С1=С2=С and R7=R5=R.
- Let us set the value of С and from the formula (1) we will find the value of R.
- We determine the value of RM using the formula
- Let us determine the value of fmax by the formula
- We define R3 by the formula
- Let us determine R2 from the relation R2= R3/0,7.
On fig. 2 shows the values of the elements obtained as a result of the calculation according to the above method for fmin=1,5 kHz, Δf=1,5 kHz and n=2.
In general, the output frequency is
where i is the input number (bit of the control word); ai - signal value at the control input (log. 1 or log. 0); n - bit width of the control word.
The resistances in the conductance matrix are determined by the formula:
If it is necessary to obtain a continuous law of frequency change, the resistors RM are replaced by a dual potentiometer with a resistance Rn and a resistor Radd connected in series with it. The values of these resistances are found from the formulas:
In the generator, you can use the K561LA7, K561LE5 microcircuits when turned on according to the inverter circuit. With a supply voltage of 3 V, the generator consumes a current of 1-3 mA.
The use of similar foreign microcircuits of the 74AC, 74NS series will significantly increase the operating frequency range, by about 10-15 times.
In the case of a large word width of the control word, a two-channel digital potentiometer, for example, manufactured by Microchip (MCP42XXX series), can be used as a matrix of conductivities. Its undoubted advantages include small size, low consumption and relatively low cost.
Literature:
- Bystry Yu. A., Mironenko I. G. Electronic circuits and devices - M .: Higher School, 1989.
- Onyshko D. Band pass filter on CMOS inverters. - M.: Circuitry, 2001, No. 7.
Author: Dmitry Onyshko, oda78@pisem.net; Publication: radioradar.net
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