Application of PIN diodes as damping and switching elements. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment
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An article by Max Perner, DM2AUO, discusses the use of PIN diodes as damping and switching elements, made possible by their high on-resistance and very low on-resistance. So, common PIN diodes VA182, VA282, VA479, VA482, VA679 (SMD) have an open resistance of 0,3 ... 0,5 Ohm and a dissipation power of 1 ... 2.5 W. The simplest application of PIN diodes would be as switches in series and parallel circuits (Figure 27, top and bottom, respectively).
In a series circuit, the diode passes a signal from the source to the load in the open state when an appropriate bias voltage is applied, and in the parallel circuit, in the open state, the diode shunts the signal source, shorting the capacitors Sk to a common wire. Fig. 28 shows a combined diagram of a switch on D1, similar to Fig. 27, and an attenuation element on D2, D3. When a bias is applied to point A, the attenuation between the signal source and the load is minimal. If > 0 ... + 12 V is applied to point B, the current will flow through R1, D2, D3, R2 and, depending on its value, the resistance D2 will change, while part of the input RF signal is closed to ground through C4.
A simple switch of two RF signals is shown in Figure 29. When this circuit switches filters, mixers, etc. it should be taken into account that at the moment only one of the two sources can be turned on, and the other must be grounded.
The resistance R1 is determined by the type of PIN diode. PIN diodes can be used as antenna receive/transmit switches (Fig. 30).
Usually, a quarter-wave strip line or a coaxial cable of the same length is connected between the receiver input and the transmitter output (Fig. 30, bottom), and on the KB - a U-shaped LC analog of a quarter-wave line (Fig. 30, top). The L and C values of this chain are calculated by the formulas:
where Zo is in ohms, f is in MHz, L is in µH, C is in pF. With an operating frequency of 28 MHz, a resistance of 50 ohms, we get L \u0,28d 113,7 μH and both capacitors are XNUMX pF each. The power loss is calculated by the formula:
With an output power of 20 W, the resistance of the diode D1 in direct connection is 0,5 ohms, the power loss will be 0,2 W. If the SWR (S) at the output is noticeably greater than 1, the power loss, taking into account the SWR (S), can be determined by the formula:
For the given example, with KCB=S=3, a power loss of 0,45 W can be expected. Thus, it becomes possible to use widely used PIN diodes in QRP transceivers. The use of switches on PIN diodes significantly reduces the time for switching from reception to transmission, which is especially important when working with digital modes of communication. In receive mode, to eliminate intermodulation distortion on D2, it is important to turn off the PIN diodes well by applying a negative bias to the circuits in Fig. 30
Author: Max Perner, DM2AUO; Publication: N. Bolshakov, rf.atnn.ru
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