Sweep frequency generator with frequency response indicator on LCD. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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This device is based on two designs - a function generator and a pocket oscilloscope, the descriptions of which were published earlier in our journal. It can be used to determine the resonant frequency of an oscillatory circuit or a quartz resonator, the shape of the frequency response of the amplifying path or filter in the range from several hertz to ten megahertz.

The device consists of two blocks - the oscillator itself and the indicator.

Technical specifications

• Center frequency at output 1, Hz .......1...107
• Relative frequency deviation at output 1, % ......0.30
• Output frequency deviation 2, kHz .......0.100
• Output signal amplitude, V .......0.2
• Indicator sensitivity, V .......0,1
• Consumed current, mA in the circuit -5 V ....... 10
• +5 V circuit ...... 50

Rice. 1 (click to enlarge)

The scheme of the generator part of the device is shown in fig. 1. It is based on the generator on the MAX038 microcircuit, the scheme and detailed description of which are published in [1]. The details required to obtain triangular and rectangular signals at the output of this microcircuit (DA3) are excluded, only a sinusoidal signal is left. A sawtooth voltage from a generator assembled on transistors VT3, VT3 and VT4 was applied to the frequency tuning input FADJ of the DA6 microcircuit. The "saw" frequency is set by the capacitor C19, and its fine tuning can be performed by selecting the resistor R15, changing its resistance by no more than ±20%. The node on transistors VT8 and VT10 generates a short sync pulse at the beginning of each sawtooth voltage period to start the indicator sweep.

With a variable resistor R22, you can set the relative frequency deviation of the generator on the DA3 chip from 0 to 30% of the average frequency set by the SA1 switch and the variable resistor R10. For smooth tuning, you can enter in series with R10 another variable resistor with a nominal value of 4,7 kOhm.

The tuning range of the generator from 1 Hz to 10 MHz is divided into seven subranges with a tenfold frequency change on each. The overall tuning range can be extended as far as the capabilities of the DA3 chip allow. To do this, it is necessary to increase the number of positions of the SA1 switch and select the capacitors that are connected to pin 5 of the microcircuit in new positions. A sinusoidal signal is fed to the device under test from connector XW1 "Output 1".

To study the frequency response of audio devices, a larger relative frequency deviation is needed (for example, from 20 Hz to 20 kHz). To obtain it, the method of beating the signals of two generators - tunable and exemplary (non-tunable) was used. An exemplary quartz oscillator at a frequency of 1 MHz is assembled on a transistor VT1. The difference frequency of the two generators is formed by a mixer on transistors VT2, VT5 and fed to connector XW2 "Output 2" through an emitter follower on transistors VT7 and VT9.

When using this output, the main oscillator on the DA3 chip must be configured so that the lower limit frequency of its sawtooth tuning is as close as possible to the frequency of the crystal oscillator (1 MHz), and the upper limit is higher by the required frequency deviation at output 2. For example , if you set the upper limit to 1,1 MHz, then the signal frequency at this output will sawtooth change from 0 Hz to 100 kHz.

The signal level at both outputs of the generator is simultaneously regulated by a dual variable resistor R26.

Fig. 2

The generator part (with the exception of the power supply unit on the transformer T1, rectifier bridges VD1, VD2 and integrated stabilizers DA1, DA2) is assembled on a printed circuit board made according to the drawing shown in fig. 2. Frequency-setting capacitors C1, C5, C6, C10, C14, C15, C17 are soldered directly to the terminals of the switch SA1.

Rice. 3 (click to enlarge)

An oscilloscope described in [2] was used as an indicator displaying the frequency response of the device under study. Minor changes have been made to its circuit and microcontroller program. The modified scheme is shown in fig. 3. Buttons for selecting operating modes are excluded from it, and only a sweep with a duration of 10 ms is left in the program, which is slightly longer than the period of the sawtooth voltage of the generator on the unijunction transistor VT3. To start the sweep, clock pulses from the collector of the transistor VT7 are received at the RB10 input of the microcontroller.

Fig. 4

The main part of the indicator parts is located on the printed circuit board shown in fig. 4. However, the detector assembly with connector XW3, diode VD3, capacitors C28, C29 and resistors R30, R31 is designed as a remote probe connected to the microcontroller with a shielded wire. This is convenient for connecting the probe to the device under test. In addition, probes can be made interchangeable and different in design depending on the frequency and amplitude of the signals applied to them.

The device begins to be adjusted from a selection of frequency-setting capacitors of the main generator so as to cover the entire frequency range without gaps. Next, the operation of the quartz oscillator and mixer is checked by setting the frequency of the main oscillator to 1 MHz at zero deviation and controlling it by zero beats at output 2, to which headphones can be connected for control. The establishment of a sawtooth voltage generator is reduced to the selection of the capacitor C19 to obtain an oscillation frequency of at least 80, but not more than 100 Hz (indicator sweep frequency).

The disadvantage of this indicator is that the contrast of the image on the screen as a result of its constant updating is low. You can increase it by temporarily stopping the sweep. To do this, install the one shown in Fig. 1 dashed line switch SA2. When it closes, the receipt of clock pulses at the PB7 input of the DD1 microcontroller will stop, and the last displayed curve will be “frozen” on the HG1 indicator screen with maximum contrast.

The "pocket oscilloscope" made according to the description in [2] can be used without any changes, but in this case the screen will be updated once every two seconds, and after each turning on the device it will be necessary to set the sweep speed.

In order to be able not only to qualitatively evaluate the frequency response of the device under study, but also to determine the exact frequency of its characteristic points, it is recommended to supplement the device with a frequency meter, which can be made according to one of the schemes published in the journal. The frequency should be measured by setting its zero deviation on the device.

PCB files in Sprint Layout 5.0 format and the microcontroller program can be downloaded from ftp://ftp.radio.ru/pub/2014/08/oscg.zip.

Literature

1. Nechaev I. Function generator with a frequency range of 0,1 Hz ... 10 MHz. - Radio, 1997, No. 1, p. 34, 35.
2. Pichugov A. Pocket oscilloscope. - Radio, 2013, No. 10, p. 20, 21.

Author: . Kamenev

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