Conex Club Magazine - no.5 - 2000

Conex Club Magazine - no.5 - 2000 - DTMF (Dual Tone Multi Frequency) Standard

Reading time: 3 minute

The main topics presented

1. DTMF standard

The name DTMF arose from the need to optimize the transmission of information and which replaces the pulses that characterized the numbers with certain tones or more precisely electrical signals formed by two basic frequencies from which comes the name Dual Tone Multi Frequency.

The base frequencies are between 300 and 3500Hz and were chosen so that they could be easily transmitted over telephone lines.

The signal combination is automatically chosen by the telephone keypad. Thus, the number 1 appears as a combination of 1209Hz and 697Hz. Exemplification with exact frequencies is relevant in the table presented in the article.

At the reception, specialized circuits decode the received tones in the representative figures.

The duration of a transmitted tone is 50ms and so is the pause time between two tones. It should be noted that in some radiocommunication systems the DTMF code is used.

The shape, level and other technical data related to these tones are already standardized and in order to comply with these norms, specialized integrated circuits have been created, such as MT8800 for transmission and MT8870 or CD22202 for reception.

2. Timer

The timer shown, provides programmable operating times with variable durations, from hundreds of milliseconds to several tens of hours.

Assembly, made with integrated circuit CD4541, presents two timing variants - configurable from R and C elements, each with four time ranges.

The main technical data of the assembly are:

  • operating modes: Monostable / Unstable;
  • the output is made on a 6A / 250V relay with a pair of NI / ND contacts;
  • optical signaling, with LED, at the exit from the timing state;
  • manual programming performed from a dip-switch;
  • supply voltage: 12VDC.

3. Automatic fan control

It is known that power semiconductor devices are mounted on the radiator to maintain the capsule temperature at a value lower than the maximum allowable temperature.

Proper sizing of the radiator can lead to large overall dimensions. One solution to reduce the size of the cooling element is use of a fan to force the cooling of the thermal radiator.

The circuit whose diagram is presented in figure 1 (from the article) contains two comparators with independently adjustable drive thresholds. The temperature sensor used is type LM135 or LM335 in capsule T092.

4. SWR - Automatic measurement

By defining the standing wave ratio, measuring this parameter is an operation that involves several maneuvers.

Cross-needle measuring devices have simplified their operations, but… shows how the automatic calculation of SWR can be achieved by transforming the voltages generated by a directional furnace into a pulse train with SWR-dependent duration.

When a radio frequency energy source is used that flows through a power transmission line on a load (we have a transmitter, a line and an antenna) and the impedances of the 3 elements are not equal, appear on the transmission waves where reflected.

By composing these waves reflected with direct waves they appear on the stationary wave line.

Two discontinuity points are created: one at the source and the other at the load. The reflected energy flows back and forth between the transmitter and the antenna, ending up being absorbed by the antenna (for the most part) and dissipated by the losses (always non-zero) in the line.

A good adaptation of the load to the line is required and this can be done in several ways. To check the fit it is necessary to measure the standing wave factor (SWR).

5. Peak detector

The peak detector, used in conjunction with a level indicator, allows determining the level of an audio frequency signal.

It can be particularly useful in laboratories, in measuring instruments, because it performs precision mono-alternating rectification or in conditioning the signals taken from a small current transformer, for example.


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