Dynamic AM Detector

Radio 1991, 6

The circuit shown in Figure 1 is a conventional AM detector that based on the voltage doubler circuit, in this circuit the capacitor is replaced with a varicap VD1. This simple replacement allows to reduce the distortion of the detected signal.

Circuit diagram of dynamic AM detector

Fig. 1
VT1 - KT315B (hfe = 50...350 at Uke=10 V and Ik=1 mA; ft = 250MHz)
VD1 - KV105A (The varicap, Cnom = 400..600 pF at Ureverse = 4 V, Q ≥ 500, Kc = 3.8)
VD2, VD3 - D9V (Germanium diodes)
C1 - 6.8 nF, C2 - 0.01 μF

The detector operates in two phases. In the first phase the capacitance of the varicap is charging to the peak value of detected signal, and in the second phases the capacitance is fully discharged. The voltage across the varicap adds to the voltage of the signal. Because the voltage at the collector of transistor VT1 changes, the varicap capacitance changes relative to its mean value just like the input signal changes. Thus, the capacity of the varicap in the first phase is equal to the average charge capacity Cc, and in the second phase it is equal to the average discharge capacity Cd. It is obvious that the values of capacitances Cc and Cd will depend on the amplitude of the received signal - the greater the amplitude, the smaller Cc and higher Cd. To explain it better it is convenient to consider the case of reception of unmodulated carrier.

When the positive half-wave of the signal is present at the collector of the transistor VT1, the charge of the varicap is q=Cc*Uc. When there is the negative half-wave, the voltage of the signal Uc summed with the voltage across the varicap VD1 (U=q/Cd), if consider the value of q, we get U=(Cc/Cd)*Uc.

In other words, the resulting voltage across the load of the detector would depend on the ratio of Cc/Cd, the higher the amplitude of the input signal, the less this ratio, and vice versa. Therefore, this detector provides immediate automatic adjustment of its gain, this reduces the nonlinear distortion, that is common in quadrature detection. This happens due to the partial alignment of the detector gain, which is different for the minimum and maximum values of the envelope of the input signal when detector works in quadratic mode of detection. Almost all tuned radio frequency receivers works in this mode.

The voltage at the collector of transistor VT1 is in the range of 4..5 V (when there is no input signal). At this voltage the capacity of the varicap VD1 is in the range of 400...600 pF. If two varicaps of the same type (KV105A) are connected in parallel, then the gain of the detector will slightly increase.

This detector can be used in almost any tuned radio frequency receiver with a voltage doubler envelope detector.

A. Rudnev

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