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By GARRY CHATT, VK2YBX Build this simple converter and listen to the 2-metre band on a shortwave radio Here's a ·VHF converter that's really easy to build. By combining it with a standard shortwave receiver, you can monitor activity on the 2-metre band. Most converter designs comprise an RF amplifier, oscillator and multiplier stages where necessary, and a mixer. The theory of operation is as follows: by amplifying the incoming VHF signal, and then mixing it with a fixed frequency (normally a crystal oscillator), the following outputs will be produced: fc + fO and fc - f0 where fc is the carrier input signal frequency and f0 is the local oscillator frequency. If we use 146MHz as the desired input signal and 128MHz as our local oscillator, the resultant output frequencies will be 2 74MHz and 18MHz. The latter is a very convenient output frequency as it allows a shortwave receiver is to be used as the "tunable IF" stage. We deliberately chose 128MHz as the local oscillator frequency so that the "image" (ie, the local oscillator frequency minus the IF) fell outside the commercial FM broadcasting band. If this had not been done, high power FM signals would interfere strongly with the operation of this converter. By carefully selecting the local oscillator frequency, we are able to eliminate additional stages of filtering from the converter front end. As it stands, the range of "images" is from 109MHz to 112MHz, a band where no high power signals should appear. Most shortwave receivers these days are equipped for SSB and FM reception. These are the two most popular modes of operation on VHF, so this converter can be quite useful for monitoring the local repeater, or for listening to some of the more exotic SSB signals. In addition, this converter can also be ANTENNA LOCAL OSCILLATOR SBL-1 MIXER 18MHz - - - O UTPUT Block diagram The design presented here is a good compromise between complexity and performance. Fig.1 shows a block diagram of the converter. We settled on a GaAsFet front end (Ql} to amplify the incoming signal. This stage is similar to the GaaAsFet preamplifier design published in August last year. The local oscillator stage is based on FET Q2 (configured as a Clapp oscillator) and this drives a buffer stage based on FET Q3. This configuration has been used before in VHF weather fax receivers by John Day, VK3ZJF. It provides OdBm output which is just sufficient to drive the mixer, an SBL-1 hot carrier dou- : 1 ~ 01 GaAsFET 146MHz PRE AMPLIFIER used to listen to the geostationary and polar orbiting weather satellites. This is easily done by realigning the converter to the 13 7MHz band and tuning the receiver to 9MHz or so. Note that the IF output of the converter is broadbanded, so that the exact frequency received is determined by tuning the shortwave radio (or antenna tuner) used. II II II II '"' II ------+----' 02 FET 128MHz OSCILLATOR 03 FET BUFFER Fig.1: block diagram of the converter. The incoming signal is amplified using Ql and mixed with the output of a 128MHz local oscillator. 98 SILICON CHIP IF OUTPUT IN;~T~ Fig.2: internal wiring of the SBL-1 double-balanced mixer module. 100 + .,. 16VWr 100k 18MHz SBL-1 -- -MIXER ~3,4 ... ANTENNA ~ .r 2 5 6 7 .001 'J:' ~-,,PUT 8 VC1 2-20pF 33k 22 2-20pF VC3 t ,. 16VWr 03 2N4859 .001J .001 10pF +10.6V .,. + 100!l G I L4 ~~~q 10pF 10pFj 100k """! ' J"" .,. 10pF+ .,. BLUE PIN G14=D s VIEWED FROM ABOVE VIEWED FROM BELOW 144MHz CONVERTER Fig.3: Q2 & X1 form a crystal oscillator whose output is buffered by source follower Q3 and mixed with the amplified signal from Qt. The broadband output from the mixer is then tuned using a shortwave receiver. ble balanced mixer module. Actually, best performance is obtained when an injection level of ± 4dBm is used but this would have involved adding another stage to the converter. Our choice of the SBL-1 mixer module was made to overcome the problems commonly encountered with active mixers; eg, noise, desensitisation, and insufficient local oscillator isolation between input and output ports. The advantages of using the SBL-1 are simplicity, outstanding strong signal performance, and high port isolation. Fig.2 shows the internal wiring of the SBL-1 mixer module. This particular model can be used at frequencies up to 500MHz. It is a passive device and hence has a 6dB insertion loss, but the preamplifier stage gain (Ql in Fig.3) has been set to overcome this and to provide some usable conversion gain. Circuit details Fig.3 shows the complete circuit diagram for our converter. As can be seen, Ql is a 3SK121 GaAsFet and is biased via a 100kn/33k0 divider network for about l0dB of gain. The output of this preamplifier stage is fed via a tuned circuit (VC2, 12, .001µ,F) to the 50-ohm input port of the mixer module. Q2 and Q3 respectively form the crystal oscillator and buffer stages. Crystal Xl is a seventh overtone crystal, so that the output of the oscillator is 128MHz. VC3, L3 and the associated .001µ,F capacitor form the tuned drain load for Q2. Note that the oscillator uses a U310 FET which is a rather special transistor commonly used in television tuners. FET Q2 is wired as a source follower to buffer the oscillator signal prior to injection into the mixer. The output from the buffer appears at Q2's source (S) and is fed to the low impedance LO (local oscillator) port of the mixer module via a low-pass filter stage consisting of L4 and two 10pF capacitors. To ensure stability, the preamplifier stage is run from a zenered 5.6 volt supply, while the crystal oscillator is operated from a 12 volt supply derived from a 3-terminal regulator (7812). This arrangement ensures that no damage can occur if the unit is inadvertently connected to a higher voltage. Construction The entire circuit is built on a double-sided circuit board, the top of which forms a groundplane to enSEPTEMBER 1990 99 Fig.3: the PC board should be assembled and tested one stage at a time as described in the text. When installing the parts, solder the leads on both sides of the board if the groundplane comes right up to the edge of the hole. !jlUT NO N 1001) ~ · sure stability. Fig.4 shows the wiring diagram. The main point to remember is that any component lead that goes to earth must be soldered on both sides of the board. In practice, this involves soldering the lead to the groundplane if the copper pattern comes right up to the edge of the hole. If the copper has been etched away from around the hole, no connection is made to the groundplane. Take care when soldering to the groundplane side of the board to enKeep all component leads short when installing the parts on the PCB and take care with the orientation of the SBL-1 mixer module. The blue pin is pin 1. 100 SILICON CHIP . ,O 'I !I r- · uf RFC1 sure that the component is not damaged. Use a soldering iron with a conical tip for best results. The most difficult components to mount were the lOOµF electrolytic capacitor and the small Murata trimmers which have only a very small lead area exposed on the top side of the board. This lead must be soldered to the groundplane. If difficulty is encountered soldering the trimmer leads, they can be bent outwards at right angles and soldered directly to the top of the board, without passing through the board. Construction requires no special techniques, although all component leads must be kept as short as possible. This is why most components are mounted horizontally Q on the PCB. The two shields ensure good isolation between the local oscillator and the RF input and should be mounted last. Note that these shields can be made from PCB material, or copper or bronze foil, then soldered directly to the top of the board. Winding the coils We deliberately designed the circuit so that constructors could wind their own coils (Ll-14), rather than relying on hard to get pre-wound types. Fortunately, there is a very easy way to wind the coils and that is to use a threaded 5mm-diameter bolt (obtainable from most hardware stores) as the former. It is quite an easy task to wind the wire PARTS LIST TABLE 1: COIL W INDING DET AILS L 1: 9T 25B&S tinned copper wire on 5mm thread, tapped 2.5T from cold end. L2: 6.5T 25B&S tinned copper wire on 5mm thread, tapped 2T from hot end. L3: 3T 25B&S tinned copper wire on 5mm thread. L4: 7T 25B&S tinned copper wire on 5mm thread. RFC1: 2T 25B&S enamelled copper wire on F29 ferrite bead . 2 2 1 1 2 PC board with groundplane, code SC 06109901, 168 x 70mm BNC sockets 50 x 5mm double-sided PC strips, or copper or bronze foil (for metal shields) SBL-1 double balanced mixer module 128MHz 7th overtone crystal, Hy-O code GE03S F29 ferrite beads (DSE Cat. L-1433) Semiconductors 1 3SK121 GaAsFet (01) 1 U310 FET (02) 1 2N4859 FET (03) 1 7 81 2 3-terminal regulator 1 5 .6V 400mW zener diode (ZD1) Capacitors 1 100µF 1 6VW electrolytic 2 22µF 16VW tantalum 10 .001 µF ceramic 1 27pF ceramic 4 1 0pF ceramic 3 2-20pF trimmers Resistors (0.25W, 5%) 2 100kD 1 1 800 1 33kD 2 1000 3 2700 Although not shown on the overlay, metal shields should be installed between the preamplifier, mixer and oscillator stages. These can be made from blank PCB material. into the thread of the bolt (see photo) and the pitch of one turn per millimetre is ideal. Table 1 shows the winding details for each of the coils. After winding the correct number of turns onto the bolt, cut the start and finish leads to a manageable length (about 10mm), then slowly wind the entire coil off the "former" by rotating the bolt. Mounting the parts We built the converter one stage at a time, to ensure that there were no errors. Start with the GaAsFet preamplifier stage (Ql}. After construction, it can be checked for correct operation using a known VHF signal and a scanning receiver, or by connecting the output at the .OOlµF capacitor via a CRO probe A 5mm-diameter bolt makes a very convenient former for winding the four coils (L1-L4) - see Table 1. to the input of a frequency counter. Normally there is some RF radiation in a typical amateur "shack" and the counter should show increased sensitivity. The next stage to build is the oscillator (Q2}. The output of this Where To Get The Parts The 3SK 1 21 GaAsFet is available from Dick Smith Electronics (Cat. Z-1845), the SBL-1 mixer and U31 0 FET from Stewart Electronic Components (phone 03 543 3733), and the 2N4859 FET from RS Components (stock number 649 021; phone 02 669 3666 or 03 330 3666). The 128MHz seventh overtone crystal is available from Hy-O Crystals (phone 03 783 9611 ), while the stage can be checked on a frequency counter or a shortwave receiver. The adjustment of the trimmer (VC3} is critical but by monitoring the DC current drawn by the stage, it's quite easy to determine when the oscillator is operating. Once the oscillator is running, the double balanced mixer and the buffer stage can be wired and the shields installed. SEPTEMBER1990 101 SC06109901 0 0 00 cO 0 Here are the actual size patterns for the double-sided PC board. Alignment Once the unit is built, alignment is easy. First check that the total current drain is in the order of 40mA or so. Next check that the DC voltages shown on the circuit are correct. There are only three adjustments to be made: VCl, VCZ and VC3. First, adjust VC3 so that the oscillator is running at the correct frequency. This can be done by connecting a frequency counter to pin 8 of the SBL-1. Alternatively, adjust VC3 until the input frequency can be heard on the correct IF. To do this, select your local repeater or beacon and 102 SILICON CHIP subtract 128MHz from the repeater output frequency to give the desired IF to which the shortwave receiver should be tuned. For example, a 146MHz input gives an IF of 18MHz, while 146.725MHz gives 18.725MHz, etc. Having aligned the converter to the correct frequency, peak both VCl and VCZ for maximum sensitivity or maximum quieting. Use the receiver's S-meter if one is available. A good time to align the converter if repeater use is spasmodic is during one of the WIA broadcasts which are normally transmitted on Sunday mornings . Transmission times vary from state to state, but many repeaters are used to re-transmit these broadcasts and transmission duration is about one hour. Our prototype performed remarkably well, giving good reception of all Sydney 2-metre repeaters plus many operating along the northern and southern coasts. Measured sensitivity was 0.2µ,V for 1 ZdB SINAD when using the converter with an FRG-7700. The unit showed a conversion gain of 3dB. Finally, the unit can be mounted in a metal box for best results. If you do this, use shielded RF cable between the board and the BNC input and output connectors. ~