Electronic switching with diodes. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment

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The reason for writing this article was my first acquaintance with HF equipment from "far abroad".

When the cover of the ICOM-735 transceiver was opened and its circuitry was examined, it turned out that there were no relays in it that switched electrical circuits. All switching in the transceiver is carried out using special pin diodes. This is done in order to reduce the cost of the transceiver and reduce its size.

Of course, no one is going to dispute the advantage of relays over diodes used as switches, especially in high frequency circuits.

However, in amateur radio practice, it sometimes becomes necessary to switch electrical circuits using diodes. Analyzing the list of diodes produced in the CIS countries, KD409 diodes were chosen for use as the closest in parameters to pin diodes. But the best results will undoubtedly be obtained using imported pin diodes, such as BAR 12-3 from Siemens. The pin diode does not behave like a switch, but like a resistance whose value is linear with the current through the diode. The resistance of imported pin diodes at a current of 25 mA is 0,1 ohm, and in the absence of bias - about 1,5 MΩ, while the dynamic capacitance of the diode is 0,1 pF. Such characteristics of diodes allow them to compete with ordinary mechanical switches.

The following are circuits in which the switching of electrical circuits is carried out using diodes, although the use of diodes for switching electrical circuits is, of course, not limited to these circuits.

Fig.1 (click to enlarge)

1. Electronic switch of bandpass filters (Fig. 1).

The main disadvantages of diode switching in preselector circuits are a decrease in their selectivity due to interference on the diode supply circuits, a narrowing of the dynamic range and an increased noise figure due to signal attenuation due to the finite value of the diode resistance in the forward direction.

Foreign manufacturers of transceivers use special pin diodes as diodes, which minimize the above disadvantages, and this fully justifies itself in the manufacture of low and middle class transceivers.

If you apply a voltage of +12 V to the 1,8 MHz output, then a current of about 1 mA will flow through the diodes VD10 and VD10 (determined by the resistance of 680 ohms in the dividers). The signal from the antenna through the VD1 diode enters the coupling coil, passes a 1,8 MHz band-pass filter and enters the mixer. The remaining diodes of the electronic switch VD2 ... VD9 and VD11 ... VD18 are closed by a reverse voltage (about +6 V) that occurs on voltage divider resistors. This circuit allows you to get by with only positive voltage.

Such a switch provides more than 50 dB isolation between the individual bandpass filters. Signal loss when using coils with a diameter of 5...8 mm is 8...10 dB. Losses can be somewhat reduced by increasing the forward current through the diodes, using larger diameter coils, or introducing a high-frequency amplifier to compensate for the losses.

The antenna switch is made on the VD19 diode. When receiving, the voltage across the diode is 0 V.

It is closed, and the signal from the input passes through one of the bandpass filters. When transmitting a voltage of +12 V, the VD19 diode opens and thereby shunts the input. In this case, the signal does not pass to the receiving path.

2. Crystal oscillator switch (Fig. 2).

Fig.2 (click to enlarge)

Using the specified electronic switch allows you to change the frequency of the reference crystal oscillator. When +12 V is applied to the LSB input, the VD1 diode opens (the VD2 diode is closed), and an inductor is connected in series with the quartz, as a result of which the frequency of the reference oscillator decreases. When +12 V is applied to the USB input, the VD2 diode opens (the VD1 diode is closed), and capacitor C5 is turned on in series with the quartz. The frequency of the reference oscillator is increased.

3. Crystal filter band switch

Fig.3 (click to enlarge)

To switch the bandwidth of the transceiver's crystal filter to SSB or CW, you can switch the capacitances included in the crystal filter using diodes. So, when +12 V is applied to the quartz filter, the diodes VD1, VD2, VD3 open and shunt the series capacitances C4, C5, C6, as a result of which the band of the quartz filter narrows from 2,5 kHz to 0,8 kHz.

4. Reversing cascade (Fig. 4).

Ris.4

This scheme of the reversing stage of the diplexer (which is an active load for the mixer in a wide frequency band) on the KP903 transistor is familiar to many. When receiving, it operates as a common-gate amplifier. In this case, the diode VD1 is open, and the diode VD2 is closed. When transmitting, the transistor acts as a source follower.

Literature

1. Bashkatov V. Improvements to the single-board universal path. - HF and VHF radio amateur, 1997, No. 6.
2. Bunin S.G., Yaylenko L.P. Shortwave Ham Radio Handbook. - K .: Tekhnika, 1978.
3. Gorshkov B.I. Elements of radio electronic devices. Directory. - M.: Radio and communication, 1988.
4. QST, N5, 1991.

Author: V.Bashkatov (US0IZ), Donetsk region, Gorlovka; Publication: N. Bolshakov, rf.atnn.ru

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