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AMATEUR RADIO By GARRY CHATT, VK2YBX Relative signal strength indicator for amateur gear A relative signal strength indicator can be a very useful gadget but is seldom encountered on modern scanning receivers and 2-metre hand-helds. Here's a simple circuit that you can build yourself for just a few dollars. Being able to monitor the level of received signals can be quite an advantage, particularly if you are evaluating a new antenna or monitoring propagation paths. In addition, a signal strength meter can also be useful as a tuning indicator, particularly in difficult reception conditions. Retrofitting a signal strength meter to an existing piece of equipment is not usually an easy task. On small hand-held transceivers, there's usually not enough room on the front panel and even on larger scanning receivers the job can be a mechanical nightmare. The answer is to build a selfcontained external signal strength meter which can be mounted piggyback fashion on the existing gear, or simply plugged in as required. That's the approach taken here. We opted for a bargraph display and designed the circuit so that only two connections to the receiver were required. How it works Fig.1 shows the circuit of our signal strength indicator. As can be seen, it uses an LM3915 dot/bar display driver from National Semiconductor. This device was +12V C1 INPUT .001 Construction 2x1N914 02 0-11a.--1,,4--+-4---t GND + LM3915 D1 .,. chosen because of its logarithmic response which is similar to a conventional "S" meter. Don't use devices which have a linear response (eg, the LM3914). Cl couples a 455kHz IF (intermediate frequency) output from the receiver to diodes Dl and D2. These rectify the IF signal and apply it to pin 5 which is the input pin for IC1. Capacitor C2 decouples any remaining RF on pin 5 while Rl and VR1 on pin 7 set the LED brightness. VR1 is also used to set the meter sensitivity. Its wiper connects to pin 6 which is the Rm (reference high) input. By varying the setting of VR 1, we can vary the reference voltage on pin 6 and thus the signal level at which the last LED in the series lights. A functional block diagram of the LM3915 IC is shown in Fig.2. Pin 9 of IC1 is the mode select input and controls either bar or dot mode operation. In this application, we want the display to function in bargraph mode and so we have connected pin 9 to pin 3. If you want to operate in dot display mode, just leave pin 9 open circuit. R1 1k .,. VR1 10k Fig.1: the circuit uses an LM3915 IC which drives 10 LEDs in bargraph mode. VR1 sets the sensistivity of the indicator. The circuit is so simple that it should only take about 15 minutes to build it up on a small piece of Veroboard. But be careful - it's very easy to make a mistake with Veroboard. Use an oversize drill to make the cuts in the copper tracks and be sure to install the LEDs, IC and diodes the right way around. If you like, you can use different coloured LEDs at the top end of the JANUARY 1989 71 The LM3915 Dot/Bar Display Driver LED Fig.2 shows a simplified block diagram of the LM3915 . Basically, it consists of a high impedance input buffer which drives a series of 1 0 comparators. Each of these comparators has its non-inverting input biased to a different level by means of a 1 0-step voltage divider which is connected to a reference voltage. As shown here, the top of the resistor string (pin 6, RH1) is connected to an internal 1.25V reference voltage (ie, the chip maintains 1.25V between pins 7 and 8). This means that the output from the buffer amplifier must exceed 1 .25V for the last comparator in the series to switch low and light its LED . The previous LEDs in the series are turned on in 3d8 steps in response to a rising input signal to pin 5 . This pin connects via a 20k0 resistor to the buffer amplifier. In practice, the reference voltage can be easily adjusted by connecting a voltage divider to pins 7 and 8 as shown in the NS Linear Databook. For the signal strength meter, pin 8 (REF ADJ) has been connected to ground to give a 1.25V output from pin 7. This voltage is then sampled by VR1 and applied to pin 6 (RH1). By simply adjusting VR1 , we can set the maximum voltage at which the last LED in the series lights and thus the sensitivity of the meter. A feature not completely shown by the block diagram (Fig.2) is the LED brightness control. This is set by the load resistance connected between pin 7 and ground . The output of each comparator consists of an NPN transistor and an scale. For example, you could use green LEDs in the first seven positions and red LEDs in the last three positions to indicate high signal strengths. It's really up to you. Check your work carefully before applying power. You can either derive a suitable 9-lZV supply from the receiver or you can use a separate 9V battery. The completed 72 SILICON CHIP v' Ult. 4.l!k 121 l.l1k 13 I I I'✓ . I'✓ LED MOGRAM CURRENT ml DUT 7 THIS lOAD DETERMINES LED BRIGHTNESS - REF ADJ I I Ulll -=1 I I v•--, I.SI> J 0.4111: I I •t• I , v• FROM l'IN It I 2• SIG § IN !-'--W,.,.+-t+ 4-1 l-= L_____________ J internal feedback loop regulates the transistor drive which in turn controls the current through the LED. Finally, the chip also contains a mode select amplifier at pin 9 (again not shown completely). This permits multiple LM391 5s to be circuit board could be housed in a small plastic case. Receiver connection Most scanning receivers on the market today either use the industry standard MC3357 (or LM3357) IF chip or an improved version of this - the 3359 or 3361. This circuit can be used with any of cascaded, and controls bar or dot mode operation . For a bargraph display, pin 9 is connected directly to pin 3; for a dot display, pin 9 is left open circuit. For further information on the LM3915 , refer to the National Semiconductor Linear Databook. those integrated circuits. The MC3357 is a dual conversion low power narrow band IF strip. It contains (among other things) a local oscillator for conversion to a second IF of 455kHz, a mixer and limiting amplifiers. There are sufficient gain stages to provide a suitable signal level at pin 3 (the mixer output), which can then be The MC3357 Narrow Band IF Chip If you have a scanner, it will most likely include the MC3357 IF chip or a near equivalent. As shown in Fig .3 , this chip includes an oscillator, mixer, limiting amplifier, quadrature discriminator, active filter , squelch, scan control and muting circuitry - all in a 16-pin package. It is specially designed for use in FM dual conversion communications gear. The mixer-oscillator combination converts the input IF (eg, 10. 7MHz) to 455kHz and makes this signal available on pin 3 for external filtering. After filtering, the 455kHz IF signal is applied to an internal limiter and then demodulated using a conventional FM quadrature detector. To measure the received signal strength, it's simply a matter of connecting the input to the signal strength meter directly to pin 3 (before the signal is filtered and passed to the limiting amplifier) . If you want further information on the MC335 7, refer to the Motorola databook on Linear and Interface Integrated Circuits. used to drive the indicator. The connection to the receiver IF chip should be run using miniature RG178 coaxial cable or similar, Vee making sure that the braid is connected to the ground of the IF strip. A simple detachable connection could be made using a 3.5mm phono plug and socket arrangement, although care should be taken to ensure that the internal shorting contacts in the socket are disabled. li: '~ensational Price Offer" I IC Ill 0 IC 0 _, c :I CONTACT 00027 00372 00390 00387 00245 00260 00100 00282 00392 00350 00012 00297 00342 00040 00359 00528 I Iii0 IC 0 00419 00420 :I 00427 00450 i CONTACT CONTACT COMPUTER PERIPHERALS RETAIL 80.00 MULTI 1/0 CARO + FOC .. 26.95 PRINTER CARD .. TOWER COMPUTER CASE + OISPLA Y 325.00 80.00 XT CASE + LOCK . 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