DRM signal converter for DEGEN 1103. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / radio reception

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The second IF of the "DEGEN 1103" receiver, equal to 450 kHz, is too high for direct input to the computer's audio card, which makes it impossible to use existing programs for receiving CW, SSB, PSK30 and RTTY signals from amateur radio stations and digital radio broadcasting in DRM format. To do this, you need to introduce another frequency converter (converter) into the receiver, which lowers the IF to 12 kHz. A diagram of such a converter, built into the "DEGEN 1103" receiver and powered by it, is shown in fig. 1.

Rice. 1 (click to enlarge)

Since the receiver's second IF filter has a bandwidth of 6 kHz, which is not sufficient to receive DRM, the signal applied to the input of the converter is picked up at the receiver before this filter. The converter has its own 450 kHz IF filter (ZQ1) with a 20 kHz bandwidth. It provides about 30 dB of rejection of the receive image channel that occurs when converting from 450 kHz to 12 kHz. The signal attenuation introduced by this filter is compensated by the RF amplifier in the TA7358AP (DA1) chip.

Fig. 2

The same microcircuit contains a mixer and a local oscillator, the frequency of which is stabilized by a ZQ2 piezoceramic resonator (CRB 465 kHz). Capacitors C5 and C6 lower the local oscillator frequency by 3 kHz relative to the nominal frequency of the resonator and equal to 462 kHz. You can also tune the local oscillator to a frequency of 438 kHz by installing a ZQ2 resonator at 440 kHz in the converter. The deviation of the local oscillator frequency by ± 2 kHz from the specified one (462 or 438 kHz) is permissible.

The receiver's LIN OUT jack is used as the output connector of the XS1 converter, from which the circuits previously connected to it are disconnected. The converter works when the XP1 plug of the connecting cable is inserted into this socket, its circuit is shown in fig. 2. The XP2 plug is connected to the microphone jack of the computer, which has voltage to power the electret microphone. This voltage feeds the phototransistor of the optocoupler U1, which provides electrical isolation of the computer from the converter and receiver, and significantly reduces the level of interference to reception.

Fig. 3

The optocoupler is mounted on a small board, which is included in the break of the shielded wire connecting the plugs. A piece of heat-shrinkable tubing protects the board from moisture and damage. If the computer is well grounded and does not cause interference, the XP1 and XP2 plugs can be connected directly, eliminating the optocoupler.

The printed circuit board of the converter is shown in fig. 3. It can be made one-sided by replacing the six printed conductors located on the installation side of the parts with insulated wire jumpers.

The assembled converter should be checked before installation in the receiver. A supply voltage of 3 ... 9 V is applied to pins 9 (plus) and 5 (minus) of the microcircuit when checking. First of all, you need to make sure that there is no absolutely identical voltage at adjacent pins of the microcircuit and printed conductors. It should be borne in mind that for different instances of the TA7358AP microcircuit, the UHF input can be either the first or the second output. If during the test it turned out that the constant voltage at pin 1 of the microcircuit is higher than at pin 2, the circuits going to these pins must be swapped. The signal is applied to one of them, the constant voltage on which is higher, and the capacitor C2 is connected to the other.

The frequency of the local oscillator is determined by finely tuning the radio receiver with a digital scale to its third harmonic 1386 (1314) kHz. With the help of "DEGEN 1103" it is possible to accept the first harmonic of the local oscillator, but it is possible to determine the frequency from the third more accurately. If necessary, the local oscillator is tuned to the desired frequency by selecting capacitors C5 and Sat.

At the end of the test, it makes sense to connect the converter to a computer and, using a program that has a spectrum analyzer mode (WinRad, HDSDR, Dream), make sure that any signals are received when you touch pins 6, 4, 3, 2 (1) of the microcircuit with a screwdriver.

The "DEGEN 1103" receiver is assembled on two boards attached to the front and back of the case. The converter board is placed on the back. To gain access to it, you need to unscrew all the screws on the back of the receiver housing, with the exception of the one that secures the telescopic antenna. One of the screws is located in the battery compartment. Then, carefully separating the front part of the housing and lifting it up, remove the encoder from the grooves in the rear part.

Next, you should unsolder (remembering the place of soldering) the wire coming from the encoder to the rear receiver board. Do the same with the wires from the dynamic head. The front and back of the receiver remain connected by a ribbon cable plugged into a connector on the rear board. This connection should not be disconnected.

Fig. 4

The receiver is placed on the table with the back board up, and the front part is shifted to the side, as shown in Fig. 4 to free access to the future installation location of the converter board. Swinging from side to side, oxide capacitors C301 (3) and C302 (2), located near the LIN OUT jack (1), break them off the board. By heating the body of the resonator X401 (4) with a soldering iron, in order to soften the glue with which it is fixed on the board, gently bend it in the opposite direction and fix it so that it does not interfere with closing the receiver. Make sure that the resonator leads are not connected to each other. The +5V voltage to power the converter will be taken from pin 6 of IC2 (5). It is necessary to remove the varnish and tin the contact pad near this output.

So that the front part of the receiver does not interfere with the installation of the converter, it is lifted and supported, for example, with a wooden stick, without pulling strongly on the flat cable. Pieces of adhesive tape are glued to the metal screen located on the board installed in this part of the case, in those places where the screen can come into contact with the newly installed converter board when assembling the receiver.

Fig. 5

After soldering the wire connecting pin 6 of IC2 to the +5 V circuit of the converter, install its board in the receiver with the parts down, as shown in fig. 5. The input wire of the converter is soldered to the pins for the removable jumper located on the receiver board between the T201 and T604 circuits. The jumper itself must be removed, and with a soldered wire, connect both pins together. Capacitor C1 is soldered between the corresponding pad of the converter board and the screen of the T604 circuit.

It is advisable to twist the wires connecting the converter to the XS1 jack (formerly LINE OUT). They are soldered to the pads from the remote capacitors C301 and C302 connected to the corresponding socket contacts.

Having soldered the wires from the encoder and the dynamic head into place, we close the receiver. Having screwed two self-tapping screws diagonally on the back of the case, they check the receiver's performance. Then connect it to the computer and check the work with the converter. The procedure for such verification and work with the Dream program is described in my article "The experience of receiving DRM radio stations in Irkutsk" ("Radio", 2008, No. 7, pp. 22-25; No. 8, pp. 14-17).

In operation, the device showed good results. Embedding the converter inside the receiver not only ensured its power supply from the batteries of the latter, but also eliminated pickups on rather long wires that would have had to be connected to the receiver's IF path when the converter was placed in a separate case.

Author: V. Boyko

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