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By JIM ROWE Input attenuator for the Digital Audio Millivoltmeter If you’d like to be able to use our Digital Audio Millivoltmeter to measure AC voltages up to 140V RMS, this add-on project is the answer. It’s a simple switched input divider which lets you add 40dB, 20dB or 0dB of attenuation ahead of the meter at the touch of a knob. T HE DIGITAL AUDIO Millivolt­ meter described in the March 2009 issue of SILICON CHIP can measure signals over a 79dB range, from about 160μV (-76dBV) up to 1.41V RMS (+3dBV). This is fine for lowlevel measurements but does make the meter unsuitable for measuring higher level signals. With the benefit of hindsight, wecould have built a switched input divider right into the meter itself. However, this would have involved a tight squeeze to fit the additional switch and components into the PC board and box and the front panel would have been very crowded as well. Anyway, to increase the measured voltage range, we have designed this 88  Silicon Chip little “outboard” switched input attenuator. It’s designed to be connected ahead of the Digital Audio Millivoltmeter’s unbalanced input via a short BNC cable. There’s no need for cable swapping to remove it when you are measuring small signals, either – because it incorporates a “straight through” (0dB) switch position as well. So once it’s built and connected to the input of the meter, its own input connector effectively becomes the meter’s unbalanced input. Simple circuit There’s very little in the attenuator, as you can see from the circuit schematic (Fig.1). A 2-pole switch is used to switch the input signal either straight through to the output (0dB) or via one of two taps on the resistive divider. The upper tap gives a 10:1 division (-20dB), while the lower tap gives a 100:1 division (-40dB). As a result the first position of the switch leaves the millivoltmeter’s own ranges unchanged, while the next position effectively subtracts 20dB from the meter readings and extends its “full scale” reading to 14.1V RMS or +23dBV. Similarly, the third switch position subtracts 40dB from the meter readings and extends its measurement capability out to 141V RMS (+43dBV). Note that the meter readings don’t take this added attenuation into acsiliconchip.com.au INPUT Rin = 100k S1a 0dB –20dB –20dB –40dB –40dB S1b OUTPUT RL = 100k 47k 43k 10k 100 1k 22 count, because there’s no way for the PIC micro inside the meter to know how much extra attenuation is being applied. So you have to add the 20dB or 40dB to the readings yourself and/ or multiply the millivolt readings by either 10 or 100 as appropriate. That doesn’t involve a great deal of mental maths though. Now before you ask, we’ll clear up a few points about the resistor values used in the divider. Do they make allowance for the shunting effect of the meter’s own input resistance? Yes, they do. If you care to work it out, you’ll find that the division ratios of 10:1 and 100:1 are only correct when the output of the attenuator is loaded with 100kΩ (ie, the input resistance of the millivoltmeter). The ratios are then within 0.1% of their nominal 10:1 and 100:1 values – which is close enough P SC 2009 ALL RESISTORS 1% TOLERANCE METAL FILM AUDIO INPUT DIVIDER 0/-20/-40 B Fig.1: the circuit is simply a switched resistive attenuator network, with double-pole switch S1 selecting between the 0dB, -20dB (10:1 division) and -40dB (100:1 division) positions. considering we are using 1% tolerance resistors. The input resistance of the input divider/millivoltmeter combination also remains very close to the nominal 100kΩ figure for the meter itself. Clearly, it’s exactly the same in the 0dB switch position but even in the other two positions it is still within 2%. Construction Most of the parts are mounted on a small PC board measuring 76 x 53.5mm and coded 04205091. This is designed to fit inside a standard small diecast aluminium box measuring 111 x 59 x 30mm. This box provides shielding and physical protection. It also matches the larger diecast box used for the millivoltmeter itself. There’s plenty of space inside the box for the BNC input and output connectors, which are both insulated single-hole mounting types. As shown in one of the photos, the board assembly itself mounts centrally in the bottom of the box and is secured via four M3 x 15mm tapped spacers. Note that metal spacers and screws must be used to secure the board because one of the spacers is used to connect the box to the PC board earth copper. Note also that the other three spacers and screws make no contact with the board earth copper. This is done to prevent the formation of earth loops. Bd 0 4/ 0 2/ 0 REDIVID OIDUA RETE MTL OVILLI M GID R OF S1 OUTPUT 43k 1k 22 9002 © 19050240 47k INPUT 10k 100 Fig.2 (left): position the parts on the PC board as shown here and install PC stakes on the copper side at the external wiring points. At right is the completed board – it should only take a few minutes to assemble. siliconchip.com.au May 2009  89 Parts List A 1 PC board, code 04205091, 76 x 53.5mm 1 diecast aluminium box, 111 x 59 x 30mm 1 2-pole 6-position rotary switch 1 small instrument knob 2 BNC connectors, insulated single hole mounting 4 M3 x 15mm tapped metal spacers 4 M3 x 6mm screws, pan head 4 M3 x 6mm screws, countersink head 4 1mm PC board terminal pins 4 stick-on rubber feet Light-duty hook-up wire 22.75 A B 22.75 CL A A 34.25 34.25 CL BOTTOM OF BOX (USED INVERTED) Resistors (0.25W 1%) 1 47kΩ 1 1kΩ 1 43kΩ 1 100Ω 1 10kΩ 1 22Ω ALL DIMENSIONS IN MILLIMETRES C HOLES A: 3.0mm DIAMETER, CSK HOLE B: 9.0mm DIAMETER HOLES C: 9.5mm DIAMETER 12 BOTH ENDS IDENTICAL Fig.2 shows the parts layout on the PC board. There’s just the six resistors and the switch, so assembly will only take a few minutes. Cut the switch shaft to a length to suit the knob before fitting it to the PC board. The connections between the input and output BNC connectors and the PC board are made via short lengths of hookup wire. We fitted PC board pins to the board (from the copper side) to make these connections a little easier. Wiring up the attenuator should be very easy – see Figs.2 & 5. The whole job should take you no more than an hour or so, including the time to drill and ream the holes in the box. Fig.3 shows the drilling details for the case. Note that the case is used inverted, so that the base becomes the front panel (do NOT drill the lid). Use a small pilot drill to start the larger holes, then carefully enlarge them CL Fig.3: this diagram shows the drilling details for the metal case. Note that the bottom of the box is used as the front panel, so be sure to drill holes “A” & “B” in the base (NOT the lid). ATTENUATION –20dB –40dB INPUT OUTPUT 0dB SILICON CHIP SWITCHED AUDIO ATTENUATOR Fig.4: the front-panel artwork is attached to the base of the case. It can either be photocopied or you can download it from our website and print it out. Table 1: Resistor Colour Codes o o o o o o o No.   1   1   1   1   1   1 90  Silicon Chip Value 47kΩ 43kΩ 10kΩ 1kΩ 100Ω 22Ω 4-Band Code (1%) yellow violet orange brown yellow orange orange brown brown black orange brown brown black red brown brown black brown brown red red black brown 5-Band Code (1%) yellow violet black red brown yellow orange black red brown brown black black red brown brown black black brown brown brown black black black brown red red black gold brown siliconchip.com.au © 2009 04205091 OUTPUT TO MILLIVOLTMETER INPUT AUDIO DIVIDER 0/20/40dB FOR DIG MILLIVOLTMETER Fig.5: the PC board is fitted with M3 x 15mm tapped spacers and mounted upside down in the base of the case. The BNC sockets are then wired by running leads to the PC stakes on the board. to the correct size using a tapered reamer. Fig.4 shows a full-size artwork for the attenuator’s front panel. It can either be photocopied onto an adhesive-backed label or you can download the artwork from our website and print it out. This can then be covered with self-adhesive clear plastic film (to resist discolouration due to finger grease) and attached to the base (use a thin smear of silicone sealant to attach the label if it doesn’t have an adhesive backing). Cut out the holes for the switch and the PC board mounting screws using a sharp hobby knife. Next, move the end-stop washer on the rotary switch to the correct location for three switch positions. Do not initially refit the nut after doing this, though – just refit the lockwasher for the time being (the combined height of the switch and lockwasher above the PC board is very close to 15mm, so they match the four M3 x 15mm tapped mounting spacers). Countersink-head M3 x 6mm screws should now be used to secure the four M3 x 15mm tapped spacers to the base. The PC board can then be fitted in position and secured using M3 x 6mm pan-head screws. That done, fit The unit can be switched to give -0dB, -20dB or -40dB of attenuation. Fit the base (ie, the lid) with rubber feet to prevent scratches to desk-tops. the nut to the switch and attach the knob to the switch shaft. Finally, secure the lid in position (this now becomes the base of the case). It’s also a good idea to fit four small adhesive-backed plastic or rubber feet to the box lid, to prevent Issues Getting Dog-Eared? scratches to bench-tops or any other surface the device is placed on. That’s it. Once your add-on attenuator is finished, your Digital Millivoltmeter will be able to make measurements over a 96dB range: from SC 160μV to just over 140V RMS. Keep your copies safe with our handy binders Available Aust, only. Price: $A13.95 plus $7 p&p per order (includes GST). Just fill in and mail the handy order form in this issue or ring (02) 9939 3295 and quote your credit card number. siliconchip.com.au May 2009  91