Frequency-Converters

Author: J.B. Hoag

The frequency-converter may use two tubes, one as the oscillator, the other as the first-detector or mixer.

Fig. 32 I. A frequency-mixer circuit

A typical mixer circuit is shown in Fig. 32 I, where both the signal and the local-oscillator voltage are applied to the same control grid of the pentode. The tube acts as a plate-detector. Instead of the capacitive coupling C to the oscillator, inductive coupling may be used. The oscillator should deliver as high a voltage as possible, up to the point where the sum of this voltage and that of the signal are equal to the C-bias on the control grid. Actually this voltage is quite small. The oscillator need not deliver power.

Fig. 32 J. A frequency-mixer circuit using a pentagrid tube

In Fig. 32 J, the signal's voltage and that of the local oscillator are applied to separate grids of a pentagrid-converter tube (penta means " five "). A small amount of power must be supplied by the oscillator. This is an excellent circuit.

Although more stable operation is obtained with a separate oscillator and mixer tube, it is possible to combine these functions into a single tube.

Fig. 32 K. A typical pentagrid converter

A typical circuit is shown in Fig. 32 K, where the cathode and first two grids are used in the oscillator circuit. The first grid acts like the usual control grid of the oscillator, while the second grid acts like the usual plate. The oscillator circuit shown is of the tuned-grid or tickler type discussed in Sec. 14.2. The electrons which pass through the second grid, en route to the plate, increase and decrease in number at the frequency of the local oscillator. They are still further controlled by the signal voltage on the fourth grid. The latter is carefully shielded from the oscillator by the third grid and from plate voltage fluctuations by the fifth grid.

Voltages of signal and oscillator frequency are bypassed to ground in the plate circuit because they are not wanted in the i.f. stages; only the beat-frequency is to be passed on. Hence the plate tank circuit of the mixer tube is tuned to the intermediate frequency.

When the i.f. output voltage is large for a given r.f. input voltage, the conversion efficiency is said to be high. This is desirable.

Sometimes, tuning the grid circuit results in changes in the frequency of the oscillator. This is bad. It is called pulling. It is lessened with careful shielding and with smaller difference between signal and oscillator frequencies.