Conex Club Magazine - no.4 - 2004

Conex Club Magazine - no. 4 - 2004 - Radio frequency detector for millivoltmeters

Reading time: 3 minute

The main topics presented

1. Up-down counter

The article presents a kit made by Velleman that offers the functions of counter and stopwatch, the display being on 4 digits in decimal or modulo 60 (time) format.

In industrial automation or laboratory applications, where a counting operation or a precision timer with programmable output is required, the use of the kit is recommended. "Advanced UP / DOWN counter", reference K8035, made by Velleman in the disassembled version and to which you can order, optionally, the box (mask) code B8035.

The operation as a precision timer (chronometer) is due to the possibility to set the working mode (from a dip-switch on the wiring) on ​​the internal oscillator, without the need to use an external clock pulse (accurate and stable!).

The assembly is controlled by a microcontroller PIC16C57 with quartz oscillator on 4.194304MHz (figure 1 of the article).

2. Radio frequency detector for millivoltmeter

An application with the new circuits produced by Analog Devices for capturing and rectifying alternative signals of low effective value or high frequency.

The measurement of low radio frequency voltages over a wide range of frequencies is usually done using millivoltmeters with diode detectors or with detectors based on the thermal effect.

Diode millivoltmeters are based either on peak detection and conversion of the result into effective values, or by comparing the RF signal with a low frequency signal., which can be measured accurately and with good linearity. and

In the first case, the scale is nonlinear, and in the second, two well-paired detectors are needed. At millivoltmeters based on the thermal effect, detectors are expensive and hard to find components.

The dynamic range is not very wide, and the frequency range depends on the input circuits. Thermal detectors are sensitive to overloads, the input resistor can be destroyed by a stronger signal.

3. Design of the frequency synthesizer with uC - VHF (II)

It presents elements of design, programming and practical realization of a frequency synthesizer in the VHF band.

For starters, choose the specifications that the final assembly must meet. The following are the considerations based on which the specialized integrated circuit used for frequency synthesis was chosen, its characteristics and block diagram.

The design of each functional block follows, up to the component level (choice of values) and the general electrical diagram.

At the end of the article are presented some considerations that were taken into account when choosing the microcontroller that controls the operation of frequency synthesis.

4. Digital frequency meter 0-1000MHz

A frequency is made with a microcontroller PIC16F84 </strong> Microcontroller. which can measure the frequency of signals up to 1GHz. The display is made on a 2 x 4 digit display.

The frequency meter presented is a modified version of the one published by Conex Club magazine about two years ago, representing a cheaper version, but without giving up the qualities necessary for such a measuring device.

The accuracy of the measurements is given by the precision with which the frequency (through the variable capacitor) of the microcontroller oscillator is adjusted.

5. Transmitter for 1-channel remote control

The company Conex Electronic presents a remote control transmitter with a channel that, together with the remote control receiver (CNX 204 - 1 channel) can make commands up to 50m.

The applications are multiple: actuation of doors and / or anti-theft device on vehicles, remote opening of porches, doors (yard, garage, etc.), lighting control.

The encoder is made with the integrated circuit MC145026 which produces a train of pulses modulated in duration, depending on the electrical state of the coding inputs A1… A9 (see figure 1 - electrical diagram).

Depending on how these inputs are connected at + 12V, to ground or are left unconnected, 19683 possible codes can be made (for the version with a channel).

It is mentioned that the two variants of remote control transmitters are assembled in different housings: with 1 button or 2 control buttons. The electrical diagram in figure 1 (from the article) represents the variant for 1 channel. The wiring is the same for the one-channel and two-channel variants.

6. 1-channel remote control receiver

It has a 1-channel remote control receiver, which is well protected by choosing a security code from those 19683 possible.

The receiver has been made with reliable components and has a high degree of safety in operation. Together with the 1-channel remote control transmitter (CNX 202) it allows the realization of commands at a distance of up to 30-50m in open space (with external antenna).

When only the built-in receiver antenna (L1) is used, the range is max. 10-15m.

To increase the range at 30-50m, an external antenna consisting of a section of solid copper conductor with a diameter of 1,2… 2mm and the length of 170 mm which will connect to the ANT terminal.

The spatial positioning of the antenna will be vertical to ensure a uniform directivity in the horizontal plane. Changing the antenna length by more than 20mm reduces the coverage radius.


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