Frequency meter - digital scale. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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The device performs the following functions:
- frequency meter with the output of the measured frequency value in hertz (up to 8 digits);
- digital scale with AFC smooth range generator (GPA) for an amateur radio transceiver;
- electronic clock.
The device is based on the PIC16F84 programmable controller from MICROCHIP. The high speed and wide functionality of this controller allow you to send a signal with a frequency of up to 50 MHz directly to its counting input, i.e. you can do without the pre-divider, usually used in devices of this type.
The main parameters:
- Range of measured frequencies, MHz 0...50
- Range of programmable IF values, MHz 0...16
- Minimum input signal level, mV 200
- Frequency measurement time, s 1
- Measurement error, Hz ±1
- Supply voltage, V 5±0,5
- Current consumption of the device, mA, no more than 30
The presence of an electrically reprogrammable data memory inside the PIC16F84 made it possible to reprogram the value of the intermediate frequency (IF) without special equipment. This makes it possible to quickly embed a digital scale into the transceiver with any (16 ... XNUMX MHz) value of the intermediate frequency.
The LCD module from "PANAPHONE" type telephones is used as an indication device. Information is entered into the module via two lines in a serial code. The built-in electronic clock function turned out to be useful. Low current consumption causes low interference to radio receiving equipment in which this device can be built.
The scheme of the device is shown in Fig.1. On the transistor VT1 and the chip DD1, an input signal driver is made. The DD2 microcircuit performs the functions of a frequency meter controller, a digital scale with an AFC, an LCD module control, and also allows you to quickly change the operating mode of the device. If pin 1 of the DD2 chip has a logical level "1", then the device performs the function of a frequency counter, if the logical level is "0" - a digital scale.
In the digital scale mode, the indicator displays the value of the input signal frequency equal to Рin + Р„h in the presence of a logical level "1" at pin 2 of the DD2 microcircuit; or Fin-Fpch - at the level of logical "0" at pin 2 of DD2. To record the required value of Fpf, in the frequency counter mode, apply a signal with a frequency Fpch to the device input (signal of a reference oscillator or telegraph local oscillator tuned to the center frequency of the IF filter passband), and to pin 8 of the DD2 chip for a time of 1,5 ... 2s set the logic level "0". The Fp value is stored in memory when the power is turned off and can be repeatedly (at least 106 times) reprogrammed using the above method.
(click to enlarge)
The AFC GPA system works as follows. After measuring the frequency of the input signal, an analysis is made of a number equal to hundreds of hertz and, if it is even, a logic level "8" is output to pin 2 of the DD0 chip; if odd, the logic level "8" is output to pin 2 of the DD1 chip. These logical signals, having previously been integrated, can be used to control the capacitance of the varicap in the GPA circuit. As a result, the frequency is stabilized near even values of hundreds of hertz with an accuracy of ±10 Hz.
In the digital scale mode, you can cancel tens and units of hertz by setting the logic level "0" at pin 9 of the DD2 chip.
To switch the device to the electronic clock mode, press the "NK" button. The "S1" and "S2" buttons are used to adjust the hours and minutes. The printed circuit board of the device is shown in Fig.2.
Author: S. Sapotko (EW2AH), Molodechno; Publication: N. Bolshakov, rf.atnn.ru
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