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Pt.2: By JOHN CLARKE AC millivoltmeter measures down to one microvolt If you need to measure wideband audio signals up to 200kHz and down to around 1µV, this project is for you. Last month we presented the operating features and circuitry. This month we present the construction and setting up details and give some practical tips of how to use the unit when testing audio equipment. O UR NEW AC Millivoltmeter has been designed for ease of construction and a minimum of internal wiring. To facilitate this, we have produced a front panel PC board onto which mount the switches, potentiometer and input sockets. This board is soldered to the main PC board to complete all the connections between them. 66  Silicon Chip Construction of the AC Milli­ voltmeter involves assembly of the PC boards, drilling out the front and rear panels of case, mounting the hardware and a small amount of wiring. Most of the components for the AC Millivoltmeter are mount­ed on the two PC boards. The main PC board is coded 01510981 and measures 212 x 142mm while the front panel PC board is coded 01510982 and measures 202 x 73mm. The two PC boards are soldered together at right angles and they fit in a plastic instrument case measuring 260 x 190 x 80mm. You can begin construction by checking the PC boards for any shorted or broken tracks and for undrilled holes. The holes for the rotary switches on the front panel PC board and the Attenuator resistors The resistors for the attenuator switch are mounted on both sides of the PC board. Insert the 100kΩ, 10kΩ, 1kΩ, 100Ω, 10Ω and 2.2Ω resistors into the allocated positions from the front of the PC board and solder in position. Then cut the leads at the back of the board. The 8.2MΩ, 820kΩ, 82kΩ, 8.2kΩ, 820Ω and 2.2Ω resis­tors mount on the copper side of the PC board directly behind the previously mounted values. You will need to insert PC stakes at all wiring positions on the main PC board. There are four more mounted near IC1 to support the PC board shields. On the front panel PC board, stakes should be inserted at the input and oscilloscope output termi­ nals, for switch S4, for pot VR4 and at the wiring points for IC8. Mount the PC stakes associated with IC8 from the rear of the PC board so that they do not protrude too far on the component side. This must be done to provide clearance for the panel meter. Also there are three PC stakes on the front panel board for securing the shields which we’ll talk about later. Next, insert the ICs making sure that you place them in their correct positions with the orientation as shown. All ICs on the main PC board are oriented in the same direction. Diodes D1-D8 and ZD1 can then be mounted, paying attention to their orien­tation. REF1 can also be mounted next, as well as the two regula­tors REG1 & REG2. Note that the 7815 (REG1) is located closest to the edge of the PC board. Fig.1: the component layout for the front panel PC board. Note that six resistors associated with the attenuator switch S1 are mounted on the copper side of the PC board. Note the shield for the attenuator switch. fuse clips (F1) on the main PC board should be drilled out to 1.5mm (1/16"). Also, there should be 3mm (1/8") holes for the corner mounting positions on the main PC board. The holes for the PC stakes should be a tight fit before they are soldered, so that they are not likely to loosen when wires are subsequently sol­dered to them. Start assembly of the PC boards by inserting all the links and resistors. The component overlay for the front panel board is shown in Fig.1, while Fig.2 has all the details for the main board. Table 1 shows the resistor colour codes, to help you select the correct value. Alternatively, you can use a digital multimeter to measure each resistor before it is inserted. You can insert the capacitors next. Table 2 shows the IEC and EIA codes which may be on the MKT and ceramic types. The electrolytic types must be inserted with the correct polarity although that is not important with the bipolar (BP) or non-polarised (NP) types. Next, insert all the trimpots. Make sure you insert each one in its correct place. Often these will be marked with EIA codes rather than the resistance value. Table 3 shows the codes. The fuse clips are best inserted with November 1998  67 Fig.2: this is the component layout for the main PC board. Note that those capacitors marked “NP” are non-polarised and may be installed without regard for polarity; ie, they can go in either way around. A shield must be installed, as shown in the righthand bottom corner, to prevent hum and noise pickup by op amp IC1. 68  Silicon Chip Resistor Colour Codes   No.    1    1    1    1    1    1    7    1    1    3    1    1    2    1    4    1  16    1    2    2    1    2    5    1    3    1    1    2    3    1    2 Value 8.2MΩ 2.2MΩ 1MΩ 910kΩ 820kΩ 560kΩ 100kΩ 82kΩ 62kΩ 51kΩ 47kΩ 36kΩ 22kΩ 20kΩ 18kΩ 15kΩ 10kΩ 8.2kΩ 6.8kΩ 5.6kΩ 4.7kΩ 3.9kΩ 3.3kΩ 2.2kΩ 1kΩ 820Ω 470Ω 100Ω 47Ω 10Ω 2.2Ω the 2AG fuse clipped between them first. Push the clips into the holes in the PC board as far as they can go before soldering them into place. Rotary switches You will need to cut the shafts for the three rotary switches to a length of 10mm while the potentiometer (VR4) shaft should be cut to a length of 15mm. Before the switches are installed, you need to set them for the number of positions required; ie 6-position, 3-position or whatever. This is done by first removing the nuts for each rotary switch and taking out the locking pin washer. Rotate each switch shaft fully anticlockwise. With switch S1, insert the locking pin washer in the 4-Band Code (1%) grey red green brown red red green brown brown black green brown white brown yellow brown grey red yellow brown green blue yellow brown brown black yellow brown grey red orange brown blue red orange brown green brown orange brown yellow violet orange brown orange blue orange brown red red orange brown red black orange brown brown grey orange brown brown green orange brown brown black orange brown grey red red brown blue grey red brown green blue red brown yellow violet red brown orange white red brown orange orange red brown red red red brown brown black red brown grey red brown brown yellow violet brown brown brown black brown brown yellow violet black brown brown black black brown red red gold brown 5-Band Code (1%) grey red black yellow brown red red black yellow brown brown black black yellow brown white brown black orange brown grey red black orange brown green blue black orange brown brown black black orange brown grey red black red brown blue red black red brown green brown black red brown yellow violet black red brown orange blue black red brown red red black red brown red black black red brown brown grey black red brown brown green black red brown brown black black red brown grey red black brown brown blue grey black brown brown green blue black brown brown yellow violet black brown brown orange white black brown brown orange orange black brown brown red red black brown brown brown black black brown brown grey red black black brown yellow violet black black brown brown black black black brown yellow violet black gold brown brown black black gold brown red red black silver brown Table 3: EIA Trimpot Codes Table 2: Capacitor Codes  Value IEC EIA  0.22µF 220n 224  0.15µF 150n 154  0.1µF 100n 104  0.047µF   47n 473  .0027µF   2n7 272  .001µF   1n0 102  47pF   47p   47  39pF   39p   39  6.8pF   6p8  6.8 “6” position and replace the nut. Then check that this switch only rotates to six positions. Similarly, switch S2 has  Trimpot Resistance EIA Code   VR1   100Ω   101    VR2   100kΩ   104   VR3   10kΩ   103   VR5    5kΩ   502 its locking tab washer inserted in the “3” position so that it can be rotated to three positions. Switch S3 has its locking tab washer inserted in the “2” position for 2-position operation. When you insert the rotary switches into the holes on the PC board, make sure that you do not stress the pins. If the switch is difficult to insert, check November 1998  69 Fig.4: the wiring details for the AC Millivoltmeter. The various Earth connections are most important if minimum noise pickup is to be obtained. The front and rear panels must also be securely earthed to ensure electrical safety. 70  Silicon Chip Fig.3: these diagrams show the connections to the two different LCD panel meters that can be used with the AC Millivoltmeter. that the holes are large enough and that the switch body is rotated so that the contact wiper pins are aligned correctly with the holes on the PC board. The terminals of potentiometer VR4 are soldered to three PC stakes and these secure it in position. You make the installation more rigid by using a drop of super glue between its case and the PC board. Switch S4 can be mounted in one of two ways. If you have a PC-mount version you can solder its terminals directly to the PC board. Otherwise, you will need to solder it to three PC stakes. Cut these down almost flush with the PC board so that the switch sits as low as possible. Connecting the PC boards As mentioned above, the front panel PC board is soldered at right angles to the main PC board. To do this, place the main PC board in position in the base of the case. Check that none of the integral standoff pillars are preventing the PC board from sit­ting on the four corner pillars. The unused pillars can be cut down with a large drill to prevent them fouling the underside of the PC board. Now place the front panel PC board at right angles to the main PC board so that its lower edge sits on the base of the case. Check that the edge is not sitting on a raised rib section; some cases have ribs and others do not. If a rib is in the way, you can remove it with a sharp chisel. Mark each end of the front panel PC board where it meets the main PC board. Then remove both PC boards and turn the main PC board upside down. Align the two PC boards so that the copper patterns for each match up and the markings are in the correct position. The alignment will mean that the front panel PC board overhangs the main PC board by about 20mm at one end. You can see this in the photos. Temporarily tack solder the two boards together at right angles in a couple of positions on the large copper areas and check that the positioning is correct when placed Below: this photo shows the front panel board before the shields are installed around the attenuator switch on the lefthand side. Note that some of the attenuator resistors are installed on the copper side of the front panel board. November 1998  71 The two boards are soldered together at right angles and the shields installed around IC1 before the whole assembly is in­stalled in the case. Note the earth leads to the shield on the top of the main PC board and to the ground plane underneath this board. in the case. Then solder the remaining connections. Make sure that all connec­tions are soldered to ensure circuit continuity. Copper shields We’ve have mentioned the copper shields previously but only in passing. Because the AC Millivoltmeter is built into a plastic case, it has no inherent shielding against hum and other inter­ fering signals. Therefore we have found it necessary to mount a shield underneath the main board and also around the input op amp, IC1. Smaller shields are also required for the front panel PC board. We made our shields from copper laminate but you could also use flat tinplate if that is more convenient. We made the top shield from two pieces of copper laminate (ie, blank PC board) measuring 25 x 50mm. They are soldered at right angles and to the PC stakes on top of the main PC board. 72  Silicon Chip The shield for the underside of the main PC board measures 80 x 60mm and is located directly under IC1 and fuse F1. It has two 8mm holes drilled near one edge and these fit over the inte­gral corner pillars on the input side of the main PC board. The shield should be placed copper side down to avoid shorting the underside of the PC board. Front panel shields The two shields for the front panel PC board are mounted near the input socket, as shown in Fig.1. Only solder the side shield (30 x 15mm) in place at this stage. The other shield, measuring 65 x 15mm, is soldered in place after the input socket is connected. Front and rear panels The aluminium front and rear panels can now be drilled out. The rear panel requires holes for the transformer, insulated terminal block, Earth terminal and the cutout for the fused IEC mains socket. This cutout can be made by drilling a series of holes around the cutout border and removing the inside piece. The hole can then be filed to shape. Two holes are required for the mounting screws. Install all the hardware in place with screws and nuts. The front panel requires holes for the switches, potentiom­eter, LED bezel, input sockets and the Earth solder lug, plus a rectangular cutout for the panel meter. Use the front panel artwork as a guide to drill the holes. Once the panel is drilled out you can attach the front panel label. The Jaycar panel meter is supplied with a front bezel which secures it in place. If you are using the Altronics meter, it is designed to be fitted in the panel without a bezel. It can be secured with a screw and nut on each side of the meter or by using a dab of contact adhesive. The input sockets must be isolated from the panel using an insulating kit. This can consist of two fibre washers and a short length of plastic tubing. Secure these in place and do not forget Parts List For AC Millivoltmeter 1 PC board, code 01510981, 212 x 142mm 1 PC board, code 01510982, 202 x 73mm 1 shield PC board, 80 x 60mm 2 shield PC boards, 25 x 50mm 1 shield PC board, 65 x 15mm 1 shield PC board, 30 x 15mm 1 front panel label, 249 x 76mm 1 plastic instrument case, 260 x 190 x 80mm 2 aluminium front and rear panels to suit case 1 31/2-digit LCD panel meter (see text) 1 2855 30V centre-tapped mains transformer (T1) 1 IEC 240V fused panel-mount male socket 1 IEC 7.5A mains power lead 1 2-way insulated terminal block 1 SPST mains rocker switch with Neon indicator (S5) 1 PC-mount 2-pole 6-position rotary switch (S1) 1 PC-mount 1-pole 12-position rotary switch (S2) 1 PC-mount 3-pole 4-position rotary switch (S3) 1 PC-mount SPDT toggle switch (S4) 1 16mm 50kΩ linear (B) potentiometer 4 knobs to suit 2 insulated panel-mount BNC sockets 1 5mm LED bezel 5 M3 x 12mm screws 4 M3 star washers 5 M3 nuts 2 M4 x 12mm screws 2 M4 star washers 2 M4 nuts 4 self-tapping screws 4 crimp or solder lugs 2 2AG fuse clips 1 2AG 1A fast-blow fuse (F2) 1 2AG 630mA fast-blow fuse (F1) 1 500mm length of brown 240VAC 7.5A mains wire 1 250mm length of blue 240VAC 7.5A mains wire 1 250mm length of green/yellow 240VAC 7.5A mains wire 1 250mm length of green hookup wire 1 400mm length of 0.8mm diameter tinned copper wire 1 200mm length of 5-way rainbow cable 31 PC stakes to place the solder lugs beneath the retaining nuts. Make sure that the nuts are tight otherwise the sockets will inevitably twist and break their connections when cables are being connected or disconnected. to the panel meter for the decimal point, BP outputs, battery supply and the IN + and IN- connections. These connection details are shown in Fig.4. Note that the Jaycar meter will require a wire bridge to select the automatic polarity indicator (the minus sign). Also the Jaycar meter has both backplane signals (labelled on and off) Interconnecting wiring Rainbow cable should be attached Semiconductors 1 OP27, LM627 low noise op amp (IC1) 3 LM833 dual op amps (IC2, IC3, IC5) 1 SSM2018 Analog Devices voltage controlled amplifier (IC4) 1 LF347, TL074 quad op amp (IC6) 1 TL072 dual op amp (IC7) 1 4053 3-pole 2-position CMOS analog switch (IC8) 1 LM336-2.5 2.490V reference diode (REF1) 1 7815 15V 3-terminal regulator (REG1) 1 7915 -15V 3-terminal regulator (REG2) 2 1N4936 1A fast diodes (D1,D2) 2 1N4148, 1N914 signal diodes (D3,D4) 4 1N4004 1A rectifier diodes (D5-D8) 1 9.1V 1W zener diode (ZD1) 1 5mm red LED (LED1) Capacitors 2 1000µF 25VW PC electrolytic 1 100µF 16VW PC electrolytic 1 22µF non-polarised PC electrolytic 6 10µF 35VW PC electrolytic 3 10µF 16VW PC electrolytic 1 10µF non-polarised PC electrolytic 1 6.8µF non-polarised PC electrolytic 1 1µF 200VDC metallised polyester (19mm max height) 2 0.22µF MKT polyester 2 0.15µF MKT polyester 5 0.1µF MKT polyester 1 .047µF MKT polyester 1 .0027µF MKT polyester 3 .001µF MKT polyester 1 47pF ceramic 2 39pF ceramic 2 6.8pF ceramic Resistors (0.25W 1%) 1 8.2MΩ 16 10kΩ 1 2.2MΩ 1 8.2kΩ 1 1MΩ 2 6.8kΩ 1 910kΩ 2 5.6kΩ 1 820kΩ 1 4.7kΩ 1 560kΩ 2 3.9kΩ 7 100kΩ 5 3.3kΩ 1 82kΩ 1 2.2kΩ 1 62kΩ 3 1kΩ 3 51kΩ 1 820Ω 1 47kΩ 1 470Ω 0.5W 1 36kΩ 2 100Ω 2 22kΩ 3 47Ω 1 20kΩ 1 10Ω 4 18kΩ 2 2.2Ω 1 15kΩ Trimpots 1 100kΩ horizontal trimpot (VR2) 1 10kΩ horizontal trimpot (VR3) 1 5kΩ horizontal trimpot (VR5) 1 100Ω horizontal trimpot (VR1) Miscellaneous Heatshrink tubing, cable ties, solder, etc. which must be wired. The Altronics meter does not require a bridge for the minus sign or the backplane signal to turn off a decimal point. Place the front panel over the front panel PC board and wire the input socket and oscilloscope socket to the PC pins on the board using short lengths of tinned copper wire. November 1998  73 The rear panel of the AC Millivoltmeter is bare except for the IEC mains socket. Use cable ties to keep the mains wiring neat and tidy and be sure to insulate the mains switch and IEC socket connections using heatshrink tubing. The 65mm long shield can now be attached by soldering it to the side shield and PC stakes. Fig.3 shows the details of all the remaining wiring. The mains wires must be 240VAC-rated. Heatshrink sleeving should be placed over the terminals of the IEC socket and mains switch (S1). The Earth wires must be the standard green/yellow striped wire and are terminated to solder or crimp lugs. These lugs are secured to the panels with a screw, nut and star washers. Tie the mains wires together with cable ties at the switch and the IEC socket. A tie should also be placed around the wires entering the terminal block. Testing When you have completed construction and wiring, check your work carefully for mistakes. In particular, 74  Silicon Chip be sure that the ICs are oriented correctly and that each regulator is in its correct position and orientation. Now apply power and check that the Neon glows in the power switch and that the display is on. Check voltages on the circuit using a multimeter. Clip the negative lead of your multimeter to the metal tab of REG1 and measure the supply pins for each IC. IC1 should have +15V at pin 7 and -15V at pin 4. IC2, IC3, IC5 and IC7 should have +15V at pin 8 and -15V at pin 4. IC4 should have +15V at pin 2 and -15V at pin 16. IC6 should have +15V at pin 4 and -15V at pin 11. Check that the voltage at the cathode of ZD1 is about 9V. Now check that the display is operating correctly. Set the dB/V switch to read Volts and rotate the attenuator to check that the decimal points change. The righthand decimal point should be on when the dB/V switch is set to dB. Check your wiring if this is not correct. Calibration Set the attenuator switch to 200V and the dB/V switch to Volts. Adjust trimpot VR5 so that display shows 00.0V. This is the offset adjustment to zero the display. You will require an AC signal source to calibrate the millivoltmeter. Apply a 1V RMS sinewave to the input. This can initially be checked for level using a multimeter set to read AC volts. Set the attenuator to 2V and adjust VR1 so that the panel meter shows 1.000V. If you have an oscilloscope you can check that the waveform at the CRO socket is about 280mV p-p. If your AC signal source has other output ranges you can check that the Millivoltmeter reads accurately at other attenua­tor settings. The dB linearity needs to be adjusted so that these read­ings are accurate. Fig.5: this is the actual size artwork for the main PC board. Check the board carefully for etching defects before installing any of the parts. November 1998  75 Fig.6: these are the actual size artworks for the front panel PC board and the main shield which is installed underneath the main board. The BNC inputs are wired directly to the front panel PC board. It is important that the retaining nuts are tight other­wise the sockets will tend to rotate when cables are being con­nected or disconnected. It can be done with a 1V RMS source or via a signal generator which has calibrated attenuation ranges. Select the 2V range on the attenuator and apply a 1V signal. Now select the dB measurement and adjust the set level pot so that the reading is 00.0dB. Now move the attenuator to the 20V position and then the 200V 76  Silicon Chip position. The readings should be -20dB and -40dB, respectively. Adjust trimpot VR3 slightly clockwise if the readings are low and anticlockwise if the readings are high. Return to the 2V position and readjust the reading for 00.0dB. Check the calibra­tion again on the 20V and 200V positions. Continue the adjustment on VR3 until the readings are accurate. If you have a signal generator with a calibrated attenua­tor, then you can check the calibration by successively attenuat­ing the signal and checking that the readings are correct. Note that an attenuation factor of 3.16 is a 10dB step. A step in attenuation by a factor of 10 is 20dB. You will need to select the 20Hz to 20kHz filter when measuring below about 50dB on any attenuation range. Note also that VR2 must be adjusted correctly before checking any measurement below 50dB. Trimpot VR2 is the last adjustment. It adjusts the refer­ence voltage applied to the IC6a error amplifier. As we know, the circuit operates in a feedback arrangement whereby the voltage controlled amplifier is controlled with the error amplifier so that the signal output after rectifying Fig.7: this full-size artwork can be using as a drilling template for the front panel. AC MILLIVOLTMETER INPUT FLOAT 2mV ATTENUATOR 20mV 200mV 2V EARTH CRO FILTERS SILICON CHIP dB SET LEVEL MEASURE V dB 20Hz-20kHz ‘A’ WEIGHT FLAT 20V 200V When you are measuring audio equipment with this AC Milli­volt­meter there are a few points to note. Firstly, frequency response checks must be made with the filters set to Flat. Other­wise you could be measuring the frequency response of the filters rather than the equipment under test. Signal-to-noise ratio is always measured with respect to a reference signal. When testing amplifiers, the reference level is usually full power output (ie, just before onset of clipping) or 1W. So typically you will adjust the “dB Set Level” control to produce a reading of 00.0 at full power. For line level equipment it is usual to specify the reference at either 1V or 0.775V which is equivalent to 1mW into 600Ω. When measuring the residual noise from a piece of equipment such as an amplifier, it is important to apply a loading resistor to its input. This simulates the source resistance of the normal audio signal generator which may be CD player, for example. A 1kΩ resistor is normal for most audio equipment. After the reference level has been set for a reading of 00.0dB, the applied signal is removed, the input loading resistor is connected and the AC Millivoltmeter’s attenuator is switched down to the 2mV position. It is necessary to count the number of steps that the attenuator is moved from the initial position down to the 2mV position. To obtain the noise figure, add 20dB per step to the reading on the meter. Finally, the earthing can make a difference to the reading. Check that you do not have an earth loop whereby the signal is connected to ground at two different points. You can have the Millivoltmeter grounded or floating, to either ground the signal or remove the ground loop if the equipment under test is also grounded. Note also that the oscilloscope provides the ground to earth. If you do not use an oscilloscope, you can simulate its earthing by connecting the earth of the scope outlet to the mains earth SC on the front panel. OVERLOAD Making measurements POWER and filtering equals the reference voltage level. Now if we short the input to the Millivoltmeter, the vol­ tage controlled amplifier must provide a large amount of gain so that the noise within the Millivoltmeter circuitry equals the reference. If the reference voltage is too high, then the VCA cannot provide enough gain to match it with the noise signal and so we lose control of the feedback circuit. The way to solve this lack of control is to adjust the reference voltage so that the noise produced with the VCA set for maximum gain can match the reference. VR2 adjusts this reference and is adjusted with the input shorted and the attenuator set to 2mV. The filter must also be set to “A” weighting. Now rotate VR2 fully clockwise. Slowly rotate VR2 anticlockwise and watch the reading increase in value. Continue adjustment until the display suddenly increases rapidly and locks up a -1 on the display. Switch off the power and rotate VR2 slightly more clockwise. Switch on power again and check that the reading sits at a stable value. It should be about -66.0dB to -68.0dB. If the reading stays stable, then leave VR2 as set. If the display continues to go to -1, then you will need to readjust VR2. Check the calibration of the dB ranges again before com­ pleting the setting up procedure. November 1998  77