The name "metal detector on the beat" is an echo of the terminology adopted in radio ever since the first superheterodyne receivers. The beat is a phenomenon that is most visibly manifested in the addition of two periodic signals with close frequencies and approximately the same amplitudes and which consists in the pulsation amplitude of the total signal. Frequency ripple is equal to the difference frequency of the two folded signals. Conceding like this a pulsating signal through a rectifier (detector), we can distinguish the signal the difference frequency. Such circuitry has long been traditional, however, in currently she is not used neither in radio nor in the metal detectors. And there and there - in place of the amplitude detectors came synchronous detectors, but the term "on the beat" has remained until now.
The principle of operation of the metal detector on the beat is very simple and consists in reception frequency difference between two oscillators, one of which is frequency stable and the other is the sensor comprises an inductor in its castorocauda chain. The device is configured so that in the absence of metal near the sensor, the frequencies of the two generators match or are very close the value of the. The presence of metal near the sensor leads to a change in its parameters and, as a consequence, to change the frequency of the corresponding oscillator. It the change is usually very small, however, the change in the frequency difference between the two generators have significantly and can be easily registered.
The difference frequency may be recorded in many different ways, ranging from simple, when the signal of the difference frequency is listening on head phones or through the loudspeaker, and to digital methods of measuring frequency.
The sensitivity of the metal detector on the beating depends, among other things, conversion options changes in the impedance of the sensor in the frequency.
Usually the transformation is to obtain a stable difference frequency generator and generator coil sensor in castorocauda chain. Therefore, the higher will be the frequency of these generators, the greater the difference frequency in response on the appearance of a metal target near the sensor Registration small deviations frequency is a certain difficulty. Thus, the ear can confident care to register the frequency of the tone signal of at least 10 Hz. Visually, the blinking of the led, you can register the departure of the frequency of at least 1 Hz. Other ways you can achieve reception and a smaller frequency difference, this registration will require significant time, which is unacceptable for metal detectors, always working in real time.
A method of separating small value difference between frequencies of the two generators raises significant technical problem - capture phase. The problem is that two generators that are configured on a very close frequency, tend to parasitic mutual synchronization. This synchronization is that if you attempt to bring any by the difference frequency of the two generators to zero, to achieve a differential frequency a certain threshold occurs abrupt transition to generators when their frequencies match. The oscillators become synchronized. Physically, the phenomenon of the capture phase due to nonlinearities, inevitably present in any generator, and parasitic penetration of one signal generator to another (for power circuits, via parasitic capacitance, etc.). In practice, unless you resort to special tricks type optoelectronic isolation of generators, we really to obtain the difference frequency of the threshold of occurrence of spurious synchronization order 10"4 relative frequency generators. Hence we can obtain an estimate for the frequency at which to operate a metal detector on the beats, to obtain maximum sensitivity 10... 100 kHz and above.
Selectivity for metals at these frequencies, is far from optimal, appears very weak. In addition, the frequency shift generator to determine the phase of the reflected signal is almost impossible. Therefore, the selectivity of the the metal detector on the beat is missing.
The instrument's response to the metallic object is inversely proportional to the sixth power distance. It is almost the same as that of the detectors on the principle "transmit-receive". However, the range of detection devices of this type usually much worse due to the effects of spurious synchronization.
Author: A. I. Shchedrin