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State of the art AC MILLIVOLTME Just how do you measure the extremely low noise output voltages of modern audio equipment, particularly power amplifiers and compact disc players? Our AC millivoltmeter has been designed for those specialist tasks and can measure audio signals down into the microvolt region. By LEO SIMPSON & BOB FLYNN Today's topline high fidelity equipment really does stretch the measuring limits of even the best audio test equipment. The run-ofthe-mill AC millivoltmeter with a bottom range of 1 or 3 millivolts full scale is nowhere in the race. Just consider a typical CD player today. It will have a signal to noise ratio of - 96dB as a minimum and it might be as low as - 106dB. Take the - 96dB figure for a moment, which is with respect to the maximum 2 volts RMS output. To confirm that noise figure, the AC millivoltmeter must be able to measure accurately down to below 30 microvolts. To confirm a noise figure of -106dB below 2V, it must be able to measure accurately to below lOµV. Modern stereo preamplifiers and power amplifiers present a similar challenge. Consider the Sony TA-N77ES stereo power amplifier reviewed in the February 1988 issue of SILICON CHIP. It has a signal to noise ratio of - 120dB 'A' weighted with respect to its rated power output (200 watts into 80). To be able to confirm that, the millivoltmeter must be able to be accurately measure below 40µV. · Or consider the Studio 200 Stereo Control Unit presented in our June and July issues. It has a signal-to-noise ratio of - 107dB 'A' weighted with respect to its rated output of 1.25V. To confirm that, you need an instrument capable of measuring signals down to less than 5µ V! In fact, if the measuring instrument's own internal 'noise floor' is not to intrude on the measurement, it must be able to measure down to around 1µ V. Now you see why conventional millivoltmeters are just not in the race. In addition, modern hifi equipment may be measured with wide frequency bandwidth, say to lOOkHz or beyond, band-limited (20Hz to 20kHz) or, as already mentioned, with 'A' weighting. As far as we know, there is no commercial Most of the parts, with the exception of the resistors on the main attenuator switch, are mounted on this PCB. TER Designed specially for audio measurements, this instrument has a noise floor of less than one microvolt. equipment available today which is up to the task. OK, so we've demonstrated the problem. Now we present the solution, our new state-of-the-art AC millivoltmeter. Its performance is summarised in the accompanying panel. Incidentally, our use of the term "state of the art" may lead some people to jump to conclusions. They may associate SOA equipment with programmable microprocessorcontrolled digital measuring equipment costing tens of thousands of dollars. But while such equipment is available, their absolute measuring limits are often pretty ordinary. As you can see from the specification panel, the new AC millivoltmeter is designed especially for measuring modern high performance audio equipment. It does not have the very high input impedance of 10 megohms, typical of digital voltmeters and some older designs of AC millivoltmeters. Such high input impedances are not required, for two reasons. First, most audio measurements are made at the output of equipment which has very low impedance. For example, most CD players, tuners, cassette decks and preamplifiers have output impedances considerably less than 1k0. For measurements on power amplifiers we are looking at outputs with source impedances of a few milliohms! Second, even where measurements are being made between stages of audio equipment, they still involve low impedances. Accordingly, we have adopted the standard input impedance used by commercial noise and distortion meters; ie, lOOkO unbalanced. The frequency response of the instrument is - 3dB at 5Hz and 160kHz, on the 3V range. We quote the specific range because the ultimate bandwidth does vary slightly, depending on the input range selected. Mind you, some power amplifiers have a small signal bandwidth far in excess of 160kHz, sometimes to as high as 1MHz or more. Where such figures have to be confirmed, the only practical way is to use an oscilloscope. For the vast majority of audio measurements though, the 160kHz bandwidth of the AC millivoltmeter will be more than adequate. More typically, where noise measurements are to be made, bandwidth limiting is required. For unweighted noise measurements (ie, with a flat frequency response), it is usual to measure with a frequency response of 20Hz to 20kHz, at the - 3dB points. The alternative is to make an 'A' weighted measurement, with a frequency response specified by the IHF-A-202 (or EIA RS-490 ) specification, and defined in ANSI S1.4 (specification for a sound level meter) as shown in Fig .1. This filter characteristic is designed to more Specifications Input impedance 1 00k0 (unbalanced) Frequency response 5Hz-160kHz at - 3d8 points (on 3V range) 5Hz-130kHz at -3d8 points (on -30dB range) Measuring ranges 1mV to 1 00V RMS f.s.d. in 11 ranges Noise (ratio) ranges 0dB to -60d8 f.s.d. in six ranges Noise floor -64d8 below 1mV (630nV) with 20Hz to 20kHz filter; -68 .5d8 below 1mV (375nV) with 'A' weighting . AUGUST 1988 19 ~ /,r .... "'"'" ~r-,....._ " -10 / V -2 I) II' II 0 I/ / -4 0 I -50 I j -6 0 -7 0 The circuitry / I I 100 10 HERTZ 1k 10k 20 Fig.1: this is the A-weighting characteristic used for measuring most audio equipment today. It tends to approximate the response of the human ear to very low noise but neglects the more audible effects of mains ripple in power amplifiers. or less approximate the sensitivity of the human ear to low level sounds. Therefore, both the high frequency response and low frequency response are rolled off, as illustrated. No provision has been made for DC measurements. The average digital multimeter is more than adequate for this task, even where voltage measurements down to a fraction of a millivolt have to be made (as for example, when measuring the DC offset voltage at the output of an op amp). Features Our prototype is housed in a folded metal case with aluminium base and blue Marviplate (steel) cover. It measures 235mm wide, 210mm deep and 120mm high (including rubber feet). It is mains powered and is switched on at the rear of the unit. On the front panel it has a large meter movement (nominally 100mm wide) with scales 0-lV, 0-3.16V, and decibels, with 0dB referenced to 0.775 on the 0-lV scale. There are four rotary switches and one potentiometer: the Input range switch, Noise range switch, Mode switch and Filter selector. The input selector has 11 ranges measuring from lmV fsd (full scale deflection on the meter) to 100 volts 20 SILICON CHIP the instrument and when very low noise measurements are being made; eg, lower than - 60dB with respect to lmV. Just to show how good this instrument is, it can measure signal noise ratios of better than - 120dB with respect to 1V RMS (or - 126dB below 2V). Alternatively, for an input reference of 30V RMS (typical for a 100W amplifier), it can measure SIN ratios better than -150dB. In other words, our new AC millivoltmeter is several orders of magnitude better than even the best audio equipment. fsd. The input divider runs with the standard 3.16 ratio between ranges. This odd figure is used because it is equivalent to lOdB steps when switching ranges. The Noise range switch has six positions, giving settings of 0dB to - 60dB. It is used in conjunction with the Set Level potentiometer which sets the meter's pointer to the 0dB mark on the scale before taking a signal-to-noise ratio measurement. The Mode switch has three positions: Volts, Set Level and Noise. These will be explained later in this article. Finally, the Filter switch has three positions: Flat (giving the widest freqency response), 20Hz 20kHz, and A Wt ('A' weighted) which has already been mentioned above. There are two insulated BNC sockets on the front panel, one for the input signal and one for the output signal to an oscilloscope or a frequency meter. The output level from this socket is around 140mV RMS, for a full scale deflection of the meter. There are two more switches to be mentioned. One is a toggle switch which is used to connect the CRO signal earth to the case of the instrument or to the mains earth. The other is a pushbutton switch used to check the "noise floor " of The circuitry relies for its performance on a number of carefully selected op amps. The most important op amp is the input device. Contrary to what a number of readers have expected, we have not used the low noise LM833 in this application. Instead, we have used a quieter and more tightly specced device, the ultra-low noise OP27. This was first produced by Precision Monolithics, Inc, USA and has since been second-sourced by Harris Corporation and Motorola Inc. Not only is the OP27 one of the quietest op amps currently available, it also has the advantage of a relatively high input resistance which is a minimum of 700k0. Also specified are three LM318 op amps. These have been selected for their wide bandwidth. The LM833 dual op amp is featured too, in the precision rectifier and meter driver. Apart from those, there are two LF351 Fet-input op amps and one LF353 dual Fet-input op amp. Let's now have a look at how the circuit works. The easiest way to understand it is to look at it in the "Volts" mode first and then look at the other functions. In the "Volts" mode, the Mode selector (S3) is set to Volts, the Noise (ratio) switch (S4) is set to its 0dB setting and the Filter selector switch (S5) is set to Flat. This is depicted on the complete circuit Fig.2 (right): of the eight op amps ► specified in the circuit, IC1 (OP27) is the key to the high performance of the unit. It has very low noise and high input resistance. I N ... I (0 0:) 0:) .... ~ en c::: ~ > c::: E 240VAC - L--S&. (Ut//30k) . ALL RUISTOIIS 1% 111111() 5.54k -10d8 PRESS FOR Mlt NOISE +15V +15V 1-~ 6 F'' f NOISE All'l.flER NOISE .,. 6 f NOIS S-4' 6 f +15V 10tlf GND II -~~~M .,. S5a SC04-1-11888-1 AC MILUVOLTMETER -15V +15V ______·_·:~~! VOLTS 1 100k: FlA 3.3k 3.3k 20k .002 (.0011/.001) +1_5V +1JV 20Hz IIGH•PASS FI.TER +15V 25.5k * SELECT FOR MIIIIUII OFFSET 18k 'A' WElll1111116 FI.TER 2k CAL VR2 50 . • MAINS EAIITN rJ~, ~~ steps (ie, 3.16 times) by switch sections S4a and S4b. S4a switches the feedback resistors for IC3 while S4b switches the feedback resistors for IC4. For the OdB setting, both op amps have a gain of unity. For the - lOdB setting, IC3 has a gain of + lOdB. For the - 30 to - 60dB settings, IC3 has a gain of + 30dB (set by the 12k0 and 3900 resistors). The gain of IC4 varies from unity at the - 30dB setting to + 30dB at the - 60dB setting. The output from IC4 is then fed via S3b to IC2 and then passes through to IC7 as before. We'll explain just how noise and ratio measurements are made in practice, later in the article. In the Set Level mode, the gain of IC2 is varied by the Set Level control, VRL The gain can be varied between 6 and 2L6 times. Filter stages Here's what the completed unit looks like inside the chassis. Note that the power supply is mounted near fhe rear panel, to keep mains hum away from the meter circuitry. Full constructional details will be published next month. diagram, Fig.2. The signal to be measured is fed in via the BNC socket to the stepped input attenuator SL The total resistance of the attenuator is very close to l lOkO and the resistor values are arranged to give the 1 to 3.16 ratio between ranges mentioned above. If set to its highest setting (ie, lOOV) and with an input signal of lOOV, the signal at the wiper of Sl will be lmV. In fact, for each range, when the maximum input signal is fed in, the signal at the wiper will be lmV. For example, on the 300mV range, if you feed in 300mV, you will get lmV at the wiper of SL This lmV signal is then fed via the 4.7k0 resistor to the non-inverting (+)input of !Cl, the OP27. This is arranged to have a voltage gain of 34, as set by the 3.3k0 and 1000 feedback resistors. So for an input of lmV from Sl, the output at pin 6 of !Cl will be 34mV. From there the signal is fed via switch S3a and S3b to IC2. In the Volts mode, as set by S3c, the gain of IC2 is 6, as set by the 6.Bkn 22 SILICON CHIP and 3300 resistors, and the lkn pot, VRL For the same lmV input to Sl, the output from IC2 will now be 204mV (6 x 34). This signal is fed via switch S5a, trimpot VR2 and S5b and then via a 50µF capacitor to the stages consisting of IC7a and IC7b. IC7a and IC7b form a precision full wave rectifier and filter circuit to drive the meter. The lµF capacitor across the 160k0 resistor, in the feedback loop of IC7b, provides a DC-averaged output to drive the meter movement. VR4 is provided for calibration. ICB provides a buffered version of the signal from S5 for viewing on an oscilloscope. After calibration has been performed on the instrument, a lmV RMS sinewave fed into the input of ICl will be displayed on the oscilloscope with an amplitude of about 400mV peak-to-peak. Noise & ratio measurements In the Noise mode, the output signal from ICl is fed via S3a to variable gain amplifiers IC3 and IC4 which are LM318s. The gain of these op amps is varied in lOdB Now let's have a look at the filter stages, as selected by S5. As mentioned before, there are two filters, for 'A' weighting and for the 20Hz to 20kHz bandpass. The 'A' weighted characteristic (shown in Fig.l) is provided by a 4-stage passive filter, consisting of four capacitors and four resistors. This is buffered by IC6 which is connected as a unity gain voltage follower. The 20Hz to 20kHz filter is provided by IC5a, connected as a 20kHz low pass filter, followed by IC5b which is connected as a 20Hz high pass filter. In other words, IC5a effectively passes all frequencies below 20kHz and IC5b passes frequencies above 20Hz. Between the two of them, they provide the 20Hz to 20kHz bandpass. The 'A' weighting filter has a loss of about - 3dB and to ensure that there is no jump in gain when the 20 to 20kHz bandpass or Flat filter conditions are selected, trimpots VR2 and VR3 are provided to equalise the signal levels, at lkHz, for all three settings of switch S5. Power supply The millivoltmeter is powered from a 30V centre-tapped transformer feeding a full-wave rectifier, two lO00µF capacitors and PARTS LIST 1 aluminium case with Marviplate lid, 235 x 21 0 x 117mm (W x D x H) 1 Scotchcal front panel label, 228 x 113mm 1 30V 1 50mA centre-tapped transformer (Altronics Cat. No. M-2855) 1 meter PCB, code 04108881, 193 x 98mm 1 power supply PCB, code 04106881, 71 x 52mm 1 MU65 1 OOµA panel meter, 100mm x 82mm (Altronics Cat. No. Q-0550 or equivalent) 1 set of metal shields (see metalwork diagrams, Part 2) 2 insulated panel-mount BNC sockets (Belling Lee LX04-0503-ZZOO5N or equivalent) 2 miniature SPST toggle switches 1 single pole 12-position switch (make before break contacts) 1 2-pole 3-position switch (make before break contacts) 1 2-pole ?-position switch (2 wafers, make before break contacts) 1 3-pole 3-position switch (3 wafers, make before break contacts) 1 momentary contact, miniature pushbutton switch 4 23mm fluted plastic pointer knobs (Altronics Cat. No. H-6050 or equivalent) 1 15mm knob 8 6mm PC standoffs 28 PC pins two 3-terminal regulators to provide balanced outputs of ± 15 volts DC. The regulator outputs are further filtered by 100µF and 220µF capacitors. There are also 16 0.lµF capacitors dotted around the circuit to provide extra power supply rail bypassing. Three switches remain to be mentioned. S2 is the momentary contact pushbutton switch. It shorts out the 4. 7k0 resistor in series with the input to IC1. It is used when making extremely low noise measurements or when confirming the "noise floor" of the instrument. 1 3-core mains flex with moulded 3-pin plug 1 cord-grip grommet 1 2-way insulated terminal block 1 3-way tagstrip 2 solder lugs 4 rubber feet Semiconductors 1 OP2 7 ultra low noise op amp 3 LM318 op amps 2 LF351, TL071 FET-input op amps 1 LF353, TL072 dual FETinput op amp 1 LM833 dual low noise op amp 1 7815 3-terminal +15V regulator 1 7915 3-terminal -15V regulator 4 1N4002 1A silicon diodes 2 1N4148, 1N914 small signal diodes Capacitors 2 1OOOµF 25VW PC electrolytics 2 220µF 16VW PC electrolytics 2 100µF 16VW PC electrolytics 3 4 7 µF 50VW bipolar electrolytics 1 1µF 200VW metallised polyester (greencap) 1 1µF tantalum or low leakage electrolytic 2 0.22µF 63VW miniature metallised polyester 2 0.15µF 63VW miniature metallised polyester S6 is a toggle switch which connects the CRO signal earth to the case of the instrument. This is used if the equipment being measured gives an erroneous display on the CRO, which is likely to occur with double-insulated audio gear. S7 is the power switch and is mounted on the rear panel so that mains wiring is kept as remote as possible from the sensitive front panel circuitry. Using the millivoltmeter As a further help to understanding how the circuit works, let's consider a typical signal-to-noise 16 0 .1µF miniature ceramics or greencaps 1 .047µF 63VW miniature metallised polyester 1 .0022µF metallised polyester (greencap) 3 .001 µF metallised polyester (greencap) 1 39pF ceramic 1 22pF ceramic 1 12pF ceramic 1 1OpF ceramic Potentiometers 1 1 kO linear pot, 16mm diameter, PC mount 1 1OkO trimpot, horizontal mount 2 2k0 trimpots, horizontal mount Resistors (0.25W, 1 %) 1 X 910k0, 1 X 160k0, 3 X 100k0, 1 x 91 kO, 1 x 75k0, 1 x 68k0, 1 x 56k0, 3 x 51 kO, 1 X 30k0, 2 X 22k0, 2 x 20k0, 2 x 12k0, 3 x 1 OkO, 1 x 7 .5k0, 3 x 6.8k0, 1 X 6 .2k0, 2 X 5.6k0, 1 X 4.7k0, 2 X 3.9k0, 4 X 3.3k0, 1 X 2. 7k0, 1 X 2.2k0, 2 X 2k0, 1 x 1.8k0, 1 X 1.5k0, 1 X 7500, 1 X 3900, 1 X 3300, 1 X 2200, 1 X 1800, 4 X 1000, 1 X 750, 1 X 220, 1 X 200, 1 X 180, 1 X 7.50, 1 X 2.70, 1 X 1.80, 1 X 1.10 Miscellaneous Insulated hook-up wire, tinned copper wire, shielded cable, heatshrink tubing, copper or brass shim, screws, nuts, lockwashers, solder, Presspahn insulating material ( 1 50 x 75mm). ratio measurement. Say we're measuring a run-of-the-mill 60 watt power amplifier. For 60 watts into an 80 load, it will deliver 21. 9 volts RMS. This would be confirmed on the 30V range. Note that the Filter switch must be in the 'Flat' position for voltage measurements. Then the mode switch would be moved to the 'Set Level' position and the Set Level control adjusted to bring the meter's pointer to 0dB. The input signal would then be removed from the power amplifier and its inputs loaded with a 4. 7k0 continued on page 71 AUGUST 1988 23 ,------------------~I I STOP PARITY START I I m -~- m __ I TO DATA BUS MICROPROCESSOR DATA BUS ..,.__ _,,. BUFFERS I I ...,__.__...__ _._~ SHIFT REGISTER I I I SERIAL 1----,,-DATA OUTPUT TRANSMIT CIRCUITS RS-232 INTERFACE ClRCUITRY I PARITY CHECK I _____________ UART_j CLOCK (SETS BAUD RATE) ADDRESS DECODE - . . , _ _ . J Fig.9: block diagram of a UART. It is capable of full duplex operation. specifies all of those characteristics. They are summarised briefly in Fig.8. The UART The main logic functions of the serial interface are usually taken care of by a special LSI serial interface chip called a UART, or universal asynchronous receiver transmitter. A simplified block diagram of a UART integrated-circuit chip is shown in Fig.9. Bi-directional data-buffers connect the CPU data bus to the UART. Inside the UART, there are two separate sections: one for transmitting, the other for receiving. The heart of each section is a shift register that performs the parallel-to-serial or serial-toparallel conversion as required. Other logic circuits add the stop, start and parity bits in the transmit mode, or extract and respond to them in the receive mode. Most UARTs can operate full duplex, meaning AC millivoltmeter - SERIAL --DATA INPUT RECEIVE CIRCUITS cs L_ _ _ - SHIFT REGISTER CONTROL --..;..._- CONTROL LINES LOGIC send and receive operations can take place simultaneously. The UART chip is set up and controlled by the host microprocessor. Special data words transmitted to the UART specify things like baud rate; 1 or 2 stop bits; odd, even or no parity; and data word length from 5 to 8-bits. A short initialising subroutine in the main program sets up the UART prior to its use. Another way to create a serial interface is to do it with software. A short program can be written to do the parallel/serial or serial/parallel conversions, deal with the start, stop and parity bits, and provide the timing for the desired baud rate. We will show you how that is done in the next and final instalment of this series devoted to microprocessor programming. lltl Reproduced from Hands-On Electronics by arrangement. (c) Gernsback Publications, USA. ctd from page 23 resistor (or shorted, according to the manufacturer's specs). The amplifier's output voltage will then drop to a very low value. The next step is to move the Mode switch to the Noise setting. The Noise range switch should be at the 0dB setting. Now we switch down the input attenuator until a reading above 1/3 of meter deflection is obtained. If the amplifier is any good (ie, reasonably quiet), very little pointer deflection will be obtained even on the lmV range. At this point, we are measuring a signal which is better than - 90dB with respect to the amplifier's rated output voltage of 21.9 volts. (Remember each change of range on the input attenuator corresponds to todB). To increase the gain of the measurement, we start rotating the Noise range switch until the meter's pointer moves up the scale. That may be obtained with the Noise switch on the - 20dB range. If the pointer is indicating - 4dB, the overall signal-to-noise ratio of the amplifier is - 90 + - 20 + - 4dB = -114dB. This is a measurement of the wideband residual noise. For most hifi equipment this measurement would be taken with the 20Hz to 20kHz filter selected which will normally improve the measurement slightly, to say, - 116dB. If an 'A' weighted measurement is taken instead, the reading may improve slightly again, particularly if there is hum in the residual noise. The procedure is similar when measuring separation between channels of a stereo amplifier, except that the Flat filter condition would be selected. Next month, we will conclude the description with the info on construction and calibration. lltl AUGUST 1988 71