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SiDRADIO: an integrated SDR using a DVB-T dongle . . . incorporating a tuned RF preselector/amplifier, an up-converter & coverage from “DC to daylight” Last month, we introduced our SiDRADIO communications receiver and described the circuit and PCB assembly. This month, we show you how to make and fit the various metal shields and complete the construction by installing it all in a plastic instrument case. Pt.2: By JIM ROWE A S STATED last month, the PCB assembly and its companion DVB-T dongle are housed together in a low-profile ABS instrument case. Since the case itself provides virtually no EMI shielding (apart from new front and rear panels which are made from double-sided PCB laminate), the fairly high sensitivity of the front-end circuitry means that extra shielding must be added to achieve an acceptable level of performance. In fact, three separate shields are necessary: (1) a small vertical shield in the front centre of the PCB (see Fig.6 78  Silicon Chip in Pt.1 and the internal photos), (2) a lower horizontal shield and (3) an upper horizontal shield. Fig.10 shows the details for the small vertical shield. It’s made from 0.8mm-thick sheet aluminium or tinplate which is first cut to size and then bent up in a small bench vise. A 3.5mm hole near the front of the mounting flange allows this end to be secured under the PCB’s front-centre mounting screw, while a second hole (at the rear) lines up with a matching hole near the centre of the PCB. Once this shield has been made, secure it to the PCB via its rear mounting hole using an M3 x 9mm machine screw, lockwasher and nut (feed the screw up from underneath the PCB). This not only secures the shield in place but also ensures that it is connected to the PCB’s earth copper. Fig.11 shows the dimensions and cutting details of the lower horizontal shield but note that this diagram is drawn half size (50%) for space reasons. This shield is cut from 0.25mm copper foil or tinplate and requires a small semicircular cut-out near its front centre, to provide clearance for siliconchip.com.au 196 20 134 26 19 3.5mm HOLE (LINES UP WITH HOLE NEAR CENTRE OF PCB) 56 38 ALL DIMENSIONS IN MILLIMETRES LOWER SHIELD – MATERIAL 0.25mm COPPER FOIL (OR TINPLATE) 70 59 (SCALE: 50%) UPPER SHIELD IS 195 x 150mm RECTANGLE OF BLANK PCB LAMINATE (OR TINPLATE) Fig.11: the lower horizontal shield is cut from 0.25mm-thick copper foil or tinplate and requires a semi-circular cutout as shown so that it clears the tuning knob. Note: diagram drawn to 50% scale. 3.5mm HOLE (LINES UP WITH MOUNTING HOLE IN FRONT CENTRE OF PCB) 6 10 the tuning knob (note: PCB laminate material is too thick for the bottom shield). When it has been cut to shape, this shield can be fitted inside the bottom of the case and secured using doublesided foam tape. Note that, depending on whether the case comes from Jaycar or Altronics, it may be necessary to cut away a number of central pillars using side-cutters or a chisel, so that they don’t foul the shield. Note also that you must leave a small area of exposed copper (or tinplate) near the lower lefthand (front) corner so that you can solder a short length of hookup wire to it. The other end of this wire is then later connected to the GND terminal pin (TPG2) in that corner of the main PCB. The upper horizontal shield is simp­ly a rectangle measuring 195 x 150mm and is cut from either blank PCB laminate or tinplate. This is secured inside the top of the case using double-sided foam tape. As with the bottom shield, you need to solder a short length of hook-up wire to it, this time at the left rear (ie, roughly above siliconchip.com.au Fig.10 (left): here’s how to bend and drill the vertical shield that’s mounted on the main PCB. It’s bent up from 0.8mmthick sheet aluminium or tinplate. CON3). This wire is subsequently used to connect the top shield to the PCB earth copper via TPG3. Front and rear panels Although the case is supplied with ABS front and rear panels, they cannot be used here as they don’t provide any shielding. Instead, these panels are replaced with custom panels made from doubled-sided PCB material. These PCB front and rear panels (code 06109132 and 06109133) are available from SILICON CHIP (see our website) and are supplied pre-drilled with red solder masking and silk-screened lettering for a professional finish (see photos). Both panels also have a solder pad at one end (on the inside) so that they can be connected via short lengths of hook-up wire to the adjacent earth (TPG) pin on the main PCB. This, together with the added shields, ensures adequate RF shielding for the sensitive front-end circuitry. Preparing the case The upper half of the case needs no preparation at all, apart from fitting the upper shield as described earlier. However, as stated above, it may be necessary to cut away some central pillars on the lower half of the case. In addition, it’s necessary to remove a 30.5mm long section of the ribs on either side of the front-panel mounting slot, to provide clearance for the tuning knob. This can done using a hand-held rotary tool after first marking the section to be removed, using the front panel as a guide. Just remove enough material from the ribs to bring them down to the same level as the inside bottom of the case. Adjusting the rotary switch Before fitting rotary switch S2 to the front panel, you first need to trim its shaft to about 9mm. It then needs to be converted from a 6-position switch to a 5-position switch. That’s done by first rotating the switch fully anticlockwise and removing the mounting nut and lock-washer. The indexing plate is then lifted up and replaced with its pin going into the hole between the ‘5’ and ‘6’ digits November 2013  79 Silicon Chip Binders REAL VALUE AT $14.95 * PLUS P &P Are your copies of SILICON CHIP getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? Keep your copies safe, secure and always available with these handy binders These binders will protect your copies of S ILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Complete with wire rods to hold 12 issues. Order online from www. siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number or mail the handy order form in this issue. *See website for overseas prices. 80  Silicon Chip moulded into the switch body. Check that the switch now has five positions, then fit a flat washer over the indexing plate, followed by the lock-washer. The switch can then be fitted to the front panel and secured with its mounting nut. Once the switch is in position, cut 12 x 40mm lengths of light-duty hook-up wire (eg, from a short length of multi-colour ribbon cable). Remove about 5mm of insulation from the ends of each wire, lightly tin the bared ends and solder two of these wires to the centre (rotor) lugs on the rear of the switch. The other 10 wires should then be soldered to outer lugs 1-5 and 7-11 – see Fig.6 in Pt.1. Note that these digits are moulded into the rear of the switch and you must solder each one to its corresponding number on the PCB (the switch diagram in Fig.6 is representative only). The rotor connection lugs are identified as ‘A’ and ‘C’ (the latter going to the “rotor B” pad on the PCB). The rear panel carries the two antenna sockets and has a cutout to provide access to the USB socket. Note the two earth leads running from TPG2 to the rear of the front panel and to the bottom horizontal shield. Note also that the mounting screw used to secure the central shield near transformer T1 is fed up from the underside of the PCB. Final assembly Now for the final assembly. The first step here is to mount the PAL/Belling Lee socket (CON4) on the rear panel. This must be fitted with its earth lugwasher, lock-washer and nut on the inside and orientated so that the earth lug is at the same level as the rear centre pin. The earth lug must also be to the right of the centre pin, as viewed from inside the case (see Fig.6 in Pt.1). Next, remove the mounting nut from HF input socket CON3, leaving the lock-washer in place, then fit the rear panel over CON3. That done, refit the mounting nut but don’t tighten it up fully at this stage. Adjust the panel so that CON4’s centre pin rests on its rectangular connection pad on the top of the PCB just to the rear of RLY1. The front panel can now be fitted to the main PCB assembly. First, remove VR1’s mounting nut but leave the lock-washer in place, then attach the panel with VR1’s shaft, LED1 and toggle switch S1 all passing through their matching holes. VC1’s tuning knob should also be protruding through its clearance slot, while the body of rotary switch S2 should be just resting on the top of the PCB. Once it’s in place and lined up correctly, fit a flat washer to VR1’s threaded ferrule and then refit its mounting nut to hold it all together. The completed assembly can then be lowered into the case, with the front and rear panels slipping down into the matching slots on either side. During this procedure, make sure that the end of the earthing wire for the lower shield is accessible, down at the front lefthand corner. Once it’s in place, check that the main board is seated properly, then fit the 10 4-gauge x 6mm self-tapping screws to secure the PCB inside the case. These screws all mate with the small mounting pillars that are moulded into the bottom of the case. Note that the screw in the front centre position on the PCB also passes through the front hole of the vertical shield plate. The next step is to solder all the wires from the rear of rotary switch S2 to their correct terminal pins on the PCB. It’s just a matter of matching the pin numbers and letters on the switch to those on the PCB. Earthing wires Next, solder the end of a short piece of hook-up wire to the earth pad on the back of the front panel (near S2), then solder the other end of this wire to the siliconchip.com.au adjacent PCB earth pin (TPG2). The wire from the lower shield should then also be connected to TPG2. This connects both the front panel and lower shield to the main PCB’s earth copper. Similarly, connect TPG3 to the earth pad just to the right of CON3 on the rear panel. That done, connect CON4’s earth lug to its PCB pad using a short length of tinned copper wire. The assembly can now be completed by fitting the control knobs to band switch S2 and to RF gain pot (VR1) and then plugging in the DVB-T dongle. Fitting the DVB-T dongle As shown in Fig.6 in Pt.1, the DVBT dongle fits into the cut-out at the righthand end of the PCB, with its USB plug mating with CON2 at the rear. If necessary, it can be further secured using hook and loop material (eg, Velcro) attached to the underside of the dongle and to the bottom of the case. The DVB-T dongle’s RF input is connected to the PCB via a 100-120mm length of 75Ω coaxial cable fitted with a PAL/Belling Lee plug (or whatever plug is needed for your dongle) at one end. The other end is simply stripped siliconchip.com.au and the centre conductor and screening braid soldered to the appropriate PCB pins. By the way, if your dongle came with one of those el-cheapo baby whip antennas, you can make use of its antenna cable to avoid having to make up a new one. Simply cut the cable about 120mm from the dongle plug end and connect this end to the terminal pins on the PCB. In fact, this is the best way to go if your dongle uses a small MCX connector for its RF input. Initial checkout Your completed SiDRADIO is now ready for initial testing. All that’s necessary to do this is to move toggle switch S1 to its upper position and then connect CON1 to your PC via a standard USB cable (ie, with a USB type A plug at the PC end and a USB type B plug at the SiDRADIO end). Provided you have already installed the RTL-SDR driver and an SDR application like SDR# (see the May 2013 article), Windows should recognise the dongle as soon as the USB cable is plugged in. Assuming that’s the case, connect a suitable VHF/UHF antenna to CON4 of the SiDRADIO and fire up SDR#. You should now be able to see any VHF-UHF signals that are being picked up by the dongle in the usual way, ie, just as if the dongle were plugged directly into the PC’s USB port. If all goes well, click on SDR#’s STOP button and switch on the SiDRADIO using power switch S1. Check that LED1 turns on, then check the output from the DC-DC converter (IC2) by measuring the voltage between ‘TP 12V’ and ‘TPG4’ on the PCB (these two test points are just to the right of the vertical shield plate). You should get a reading of close to 12.5V when gain control VR1 is turned fully anticlockwise, dropping to around 12.0V when VR1 is turned fully clockwise. You should also check the voltages at the input and output of REG1, located November 2013  81 Terminal pin TPG3 (to the right of CON3) is connected to the earth pad on the rear panel using insulated hook-up wire, while a second lead (shown here as the blue wire floating at one end) must be connected from TPG3 to the top horizontal shield. just to the rear of T2. The voltage on its input pin (on the right) should be very close to 5.0V, while the output pin (on the left) should be very close to 3.3V. If these voltages all measure OK, the SiDRADIO’s front-end LF-HF circuitry is probably working correctly. If so, connect a suitable LF-HF antenna to CON3, use rotary switch S2 to select a suitable band (say Band 3, 1-3.4MHz), and set RF gain pot VR1 to mid-way. That done, click on the small box just to the left of the ‘Shift’ label in SDR# on your PC and set it to make allowance for the 125MHz up-conversion. Finally, click on SDR#’s ‘Play’ button again to resume operation. You should now see a spectrum display of LF-HF signals and if you set SDR# to scan in that part of the spectrum centred on about 1.0MHz, you should see a number of signal peaks corresponding to various AM radio stations. Then if you select one of these peaks, you should be able to tune it for maximum signal by nudging the SiDRADIO’s tuning knob one way or the other. Note that the tuning is fairly broad and not at all critical. Note also that if the signal you wish to tune is near the top of the band, you may need to adjust the small trimmer capacitors on VC1 to their minimum settings (ie, fully unmeshed). They’re easily accessed through small holes in the top of VC1 and can be adjusted using a small screwdriver or alignment tool. You should now find that advancing RF gain control VR1 produces an obvious effect on SDR#’s display. In fact, if you turn VR1 up to ‘full bore’, this may well cause the signal peaks to rise above the overload level. In most situations, you’ll rarely need to turn the RF gain up that far. Finishing up There are no further adjustments and the operation should now be quite intuitive. All that remains is to solder the end of the wire from the upper shield plate copper to terminal pin TPG3 on the main PCB (near CON3), after which you can fit the top half of the case into position and secure it using the four supplied screws. You May Need To Install .NET Framwork 2.0 SOFTWARE DEFINED RADIO, May 2013: a reader has discovered that the SDR software combination (Zadig + RTL-SDR + SDR#), as described in the May 2013 article, would not run on a PC with Windows XP (SP2) but would run on another Windows XP/SP2 machine. The solution was to install Microsoft .NET Framework 2.0 and reboot. Newer operating systems may come with this preinstalled. If you do need to install Microsoft .NET Framework 2.0, it can be downloaded from the internet. Note that this note also applies to the SiDRADIO. You should also fit four adhesive rubber feet to the bottom of the case, so that it won’t scratch any surface it’s placed on. With the unit completed, you’re now free to explore the LF, MF & HF bands in the same way that you’ve been exploring the VHF and UHF bands. And of course, you can return to exploring the VHF and UHF bands at any time simply by switching the SiDRADIO off and clicking again on the box just to the left of SDR#’s ‘Shift’ label to de-activate the 125MHz frequency offset. Finally, note that an article on using the SiDRADIO to receive DRM (Digital Radio Mondiale) broadcasts in the shortwave bands is featured elsewhere SC in this issue. Australia’s Lowest Priced DSOs Shop On-Line at emona.com.au Now you’ve got no excuse ... update your old analogue scopes! Whether you’re a hobbyist, TAFE/University, workshop or service technician, the Rigol DS-1000E guarantee Australia’s best price. 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