Analog frequency counter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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An analog frequency meter makes it possible to monitor the dynamics of the process during frequency measurements. This is especially important when it is necessary not only to measure the true value of the frequency, but also to follow its changes over time. Frequency counters with digital readings are more accurate, but also more complex in design and do not make it possible to monitor frequency changes over short periods of time.

The frequency meter circuit with digital signal processing, but with an analog indicator, is shown in the figure.

(click to enlarge)

The main parameters

The measured frequency signal is fed to the pulse shaper assembled on the D1 chip. The frequency divider block, made on the D2, D6 and D7 microcircuits, serves to divide either the measured frequency or the reference oscillator frequency (depending on the position of the range switch). This is necessary to ensure that pulses with a frequency of one order of magnitude arrive at the mixer. The reference oscillator is assembled on the elements of the D5 chip, its frequency is stabilized by the Z1 quartz resonator.

The role of the mixer is performed by the trigger (element D4.1). For stable operation, its input must receive pulses with a duty cycle equal to 2. The shaper of such pulses is a trigger on element D3.1. The shaper of rectangular pulses of calibrated amplitude and duration is necessary for the normal operation of the measuring instrument of the frequency meter. Capacitors C4 and C5 set the reference oscillator frequency to 100 kHz, and capacitors C and C7 suppress higher frequency harmonics.

The shaper functions are performed by elements D4.2 and D5.4. The indication circuit of the direct and reverse scale of the pointer indicator is made on the elements D9, D8.1, D8.2, D10.1, D10.2 and D11.1. Direct indication is provided by LEDs V3 and V4. Before starting measurements, check the calibration by pressing the button S1. In this position of the contacts of the button, the trigger D4.1 switches to the frequency division mode by 2, the instrument pointer is set to the last division of the scale with a variable resistor R5. The button S1 is released and the voltage of the measured frequency is applied to the input of the device. By sequentially pressing the buttons x100, x10, x1 and x0,1, first, units of megahertz, then hundreds of kilohertz, tens of kilohertz, units of kilohertz and hundreds of hertz are counted on the pointer device.

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