ANALYSIS OF DETECTION

Manawatu Times, Volume XLIX, Issue 3491, 29 October 1926, Page 4

ANALYSIS OF DETECTION

The method of detection in valve work commonly used makes use of a grid leak an dcondenser. The operation of the circuit may be roughly outlined as follows:

Xa the absence of incoming waves the potential or the grid is the same as that of the filament. Incoming waves cause the grid to become alternately more positive and more negative than the filament. While the grid is more negative, nothing happens, but while it is more positive it attracts negative electrons. These electrons cannot get oft the grid once they are on it (the grid is not hot like the filament) except via the high resistance, which is called the grid leak. If, for the moment, we suppose there is no grid leak provided, we can see that after a very few waves have come in the electrons drawn to the grid will charge it to a steady negative potential equal to the maximum instantaneous potential of the top of the coil in the antenna circuit. This steady negative potential causes a reduction in the plate current. Even if the waves cease coming in or their amplitude is diminished the grid retains its negative charge, since there is no way for the electrons to get off it- So we put in a very high-resist-ance path, by which they may slowly (compared to the wave frequency) escape so that, if the amplitude of the incoming waves slowly (i.e., at voice frequency) diminishes, electrons will leak off until the grid potential drop to the new maximum value of potential at the upper end of the coil. In this system the greater the strength of incoming waves the loss the plate current. The size of the grid leak is more or less important. If its resistance is too groat it is possible that the grid of the valve will block on strong signals, whereas if the grid leak resistance is too Iow r , the signal strength will bo decreased. For the ordinary 201-A type of valve a grid leak of about four megohms resistance will give the best results. If a soft detector valve is used a somewhat lower resistance leak is generally required, about one-half to two megohms being about right.