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Design by ROGER KENT* Audio Lab: a PC-controlled audio test instrument; Pt.2 In this second article on Audio Lab, we include all the circuit and wiring diagrams and give a rundown on the software. The calibration procedure includes 10 multi-turn pots and the software guides you through the full procedure. In last month’s article, the operation of Audio Lab and the functions of the various PC boards were described. Two more PC boards remain to be mentioned. The power supply board is fed by an external AC plugpack capable of delivering 1A. The 25-pin 74  Silicon Chip D connector for the serial port is also mounted on this board, together with the reset switch which is mounted on the rear of the case. There is also an optional board for fault-finding on the Boot board. This consists of eight LEDs with limiting resistors and plugs into port P1 on the Boot board. How this is used will be described later in the construction details but before we move on to that topic we will discuss some of the system software. System software On power up, machine code software is dumped from the PC to Audio Lab via the serial port. To show the code has been successfully loaded, the LEDs on Audio Lab flash and a verifica­tion message is displayed on the PC. The opening menu screen is then accessed and the various +5V CONN 5 TO A-D PCB D0 8 D1 7 D2 6 D3 5 D4 4 28 20 1 D0 A0 D1 A1 D2 A2 D3 12 14 15 D6 D7 11 11 8 12 13 15 16 17 18 19 D0 C3 0.1 OE 22 A14 27 D1 A13 A13 26 A12 2 D2 D3 9 10 IC6d D5 D7 14 C6 0.1 8 D6 IC4 D7 20 CE RAM 1 A14 26 A13 2 A12 RD WR 11 12 13 15 16 17 18 19 22 27 +5V CE 20 12 14 C4 0.1 C8 0.1 D5 24 A9 25 A8 A9 24 A8 25 D6 D4 23 A11 21 A10 A11 23 A10 21 D4 A13 +5V C18 1 D3 4 A6 3 A7 IC3 EPROM +5V C1 0.1 D1 D2 6 A4 5 A5 6 A4 5 A5 4 A6 3 A7 D0 8 A2 7 A3 8 A2 7 A3 IC2 A3 D4 74HC573 A4 16 3 17 D5 A5 2 18 D6 A6 1 19 D7 A7 D5 10 A0 9 A1 10 A0 9 A1 13 C17 0.1 1 A15 5 A14 4 14 IC6c 13 7 74HC00 IC6a 11 J4 3 A000 2 1 3 A000 2 IC6b C000 6 A000 40 C14 10 9 C000 30 39 38 37 36 35 34 33 32 21 22 23 24 25 26 27 28 ALE P0.0 P0.1 P0.2 P0.3 P0.4 P0.5 P0.6 P0.7 P2.0 P2.1 P2.2 P2.3 P2.4 P2.5 P2.6 P2.7 16 P3.6 +5V IC1 80C31 18 X1 11.059MHz P3.0 P3.1 10 11 19 P3.6 P3.5 P3.4 P3.4 14 13 P3.3 12 P3.2 P3.7 P3.5 RESET R1 1k P3.7 31 20 17 P3.3 P3.2 CONN 3 TO A-D PCB 15 +5V C15 33pF CONN 1 TO PSU PCB RST C16 33pF C9 10 2 +5V 16 10 RST OUT 7 IN 8 0V 3 IC7 ADM232 IN 7805 OUT C7 0.1 GND C2 1 TANT 5 +5V C5 1 TANT C13 1 TANT AUDIO LAB BOOT PCB C12 10 6 2 3 C10 10 7 3 C11 10 15 4 1 J1 4 REG3 +DC 1 1 9 16 IC5 74HC257 10 11 5 13 2 14 6 15 8 I G0 Fig.1: The boot board circuit contains the 80C31 micro­processor, the EPROM, RAM and the RS-232 interface chip. measurement options can be select­ ed. The “Lab” screen consists of five main sections: a scope display which graphs the selected input, a display of the Audio Lab front panel which shows which Range, Mode and Mon- itor func­tions have been selected, two digital readouts showing the output frequency of the sinewave generator September 1995  75 +5V +5V 20 19 R12 10k VREF 4 D0 D2 INT 16 D2 15 D3 D3 D4 ZD1 CZM242 2.5V WR D6 CONN 3 FROM BOOT PCB C11 1 C4 0.1 3 R14 10k C7 O.1 C6 0.1 C1 0.1 R13 330 D2 1N914 +5V R10 100k RD 2 CE 8 1 10 C8 0.1 5 7 12 D6 11 D7 D7 C13 0.1 -5V C10 33pF 14 D4 13 D5 D5 C12 1 VIN 6 18 D0 17 D1 D1 9 IC7 ADC0804 C9 100pF R5 1k R11 10k Q1 2N3904 C B IC3 4093 2 14 1 3 5 IC3b 1 4 E D1 1N914 3 2 6 7 IC6a C5 100pF R5 20k +5V 16 LIN GAIN R8 VR3 20k 20k +5V 20 2 19 D0 IC5 DL0 18 3 74HC574 D1 DL1 17 4 D2 DL2 16 5 D3 DL3 15 6 D4 DL4 14 7 D5 DL5 13 8 DL6 D6 9 DL7 12 D7 OE CK 10 11 1 CONN 2 PSU PCB +5V 0V -5V P3.5 P3.6 P3.7 A000 7 V IN +5V YY 0V 12 C000 5 P3.7 4 11 R2 10k 2 5 C3 220 6 IC1 AD736JN 1 8 IC2a 3 TL072 R3 10k 8 1 4 -5V C E B VIEWED FROM BELOW IC6b C000 +5V 2 C2 10 R6 20k D-SINE 6 7 3 4 -5V +5V R4 20k VREF SINE OUT -5V IC6c +5V 74HC32 14 P3.6 13 7 R1 10k R7 10k RMS CAL VR2 20k +5V DL7 SINE OUT 7 IC2b 12 1 -5V DL6 A000 P3.4 DL3 DL5 5 C14 0.1 DL4 P3.3 P3.3 6 8 DL1 6 13 LIN NULL VR2 20k R9 3 68k IC4 4052 4 DL2 P3.4 CONN 4 FROM BOOT PCB P3.2 CONN 1 TO FRONT PCB DL0 10 9 11 -5V P3.2 AUDIO LAB A-D CONVERTER 76  Silicon Chip coloured green toggle between two options, and items coloured light blue signify that a selection of values is available. For example, on the “Lab” screen the options on the menu bar are: “—”, Input, Mike, Sine, Rms, Lin, *FREQ, *COMP, *SWEEP, Scroll and EXIT. To select an option, move the mouse Fig.2 (above): the A-D converter board is based on an ADC0804 8-bit A-D converter and also features the AD736 true-RMS converter. Fig.3 (right): the front panel board carries all the analog input circui­ try and CMOS switches which are controlled by the 80C31 proces­sor. 12 LEDs are used to indicate the various measurement modes. ▲ and the input voltage of the selected input, and finally, the Menu select bar at the bottom of the screen. All the screens use the same method to select the different options. Any red item on the Menu bar has another screen or further options available when that function is selected by clicking the left mouse button. Items 16 CONN 1 DL0 DL0 10 A DL1 R19 5.1k 1x 14 DL2 DL3 9 DL1 DL4 B 0x XX 500  R27 91k IC3 4052 DL7 7 R28 820k -5V SINE IN C10 0.1 V OUT +5V DC SINE IN DL6 R43 10k B IC2a TL072 1 -5V RMS LED7  R42 DL7 10k B C Q1 2N3904 E  VOUT C4 0.1 C1 0.1 R3 COARSE VR10 20k 680  5 IC2b C7 1 IC4a 16 11 A 4 IN/OUT 1 DL3 10 B 6 IN/OUT 2 DL4 9 C 5 IN/OUT 5 IC7 4051 15 2 IN/OUT 1 IN/OUT 0 IN/OUT 7 DL0 DL1 8 COMP LED12  A RANGE2 LED4  14 13 K   330   RANGE10 LED1 LED5 R45 8.2k SINE LED11 MIC LED8 R37 330  RANGE5 LED2 16 IC6 4052  x10 LED4 15 14 12 2x 1x 0x IN/OUT x A B OUT/IN 10 9 13 7 R46 8.2k ELECTRET +5V S1 +5V C2 1 C3 33pF 3 R2 100k 2 K B C E VIEWED FROM BELOW 8 IC1a TL072 5 R8 20k C6 1 1 MIC GAIN VR8 20k DL0 DL1 A 8 C13 10 MICROPHONE 8 OUT/IN DL2 11 A DL3 10 B DL4 9 C x1  6 IN/OUT R18 20k  x0.1 LED6 4 IN/OUT 3 DL4 R36 7 IN/OUT 7 3 -5V DL3 DL2 1 2 1 4 R40 330 R22 200k -5V 200k C8 0.1 4 R5 680  1.5pF R17 -5V R24 20k SINE ADJ VR1 20k 2 C5 47 C9 DL2 +5V 0 IN/OUT R11 10k 7 IC1b R23 20k 7 8 R4 10k 4 R6 8.2k IN914 LIN LED10 13 R12 10k SINE OUTPUT 6 5 +5V C12 0.1 0V C11 0.1 -5V R15 680 S2b FINE VR9 1k R7 8.2k R39 330  D1 R14 8.2k S2a LOW R38 330  +5V 14 1 IN/OUT R34 200k +5V COMPONENT A B 4 R10 8.2k Q2 2N3904 R35 100k VR4 500  6 HIGH 3 C E R44 200W LOW R16 100k R13 270k HIGH 2  7 R25 43k VR3 5k COMP CAL VR2 20k R9 10k IC4b TL072 IC5 4051 8 R41 330  FREQ LED9 6 R33 100k R26 820k -5V 4y IN/OUT 1 1y IN/OUT 5 +5V 8 15 2 IN/OUT 4 2 3y IN/OUT 2y IN/OUT -5V 5 R30 10k -5V AC 3 Y OUT/IN 0V R29 820k OFFSET VR7 5k D2 1N914 16 R32 100k R31 10k D3 1N914 R20 680 VR6 R21 12 560  DL5 DL6 +5V VR5 5k R1 680 AUDIO LAB FRONT PCB September 1995  77 mouse clicked then the screen reverts back to the previous setting. “OPTS” always has this function from any screen. Similarly, if “Mike” or “Sine” is selected then the mike input or the sine output is monitored and displayed. “Scroll” is light blue and toggles between “Plot” and “Scroll” which are two different modes on the “Scope” display. DB25 TO COM 2 CONN 4 TO BOOT PCB RESET OUT IN 0V +DC S1 RESET 2x1N4004 D2 IN 9-10VAC 1A D1 C1 3300 25VW REG1 7805 GND Measuring components OUT 1 TANT +5V 0V F1 1A 7805 7905 -5V C2 3300 25VW GND IN I GO GIO CONN 1 TO A-D PCB 1 TANT REG2 7905 OUT AUDIO LAB POWER SUPPLY Fig.4: the power supply is fed from a 9V AC plugpack and uses two halfwave rectifiers together with 3-terminal regulators to pro­duce the ±5V rails. until the desired selection is highlighted in yellow. Press the left mouse button and the selection is made. If “Input” has been selected, then the first item on the menu bar changes to “Range” and the monitor input is switched to the input terminals. The range is shown on the LEDs and also on the video screen. If RANGE is now selected, the menu bar changes to enable the range of the input voltage required to be selected; ie, 250mV, 500mV up to 100V. If “OPTS” is highlighted and the If “*COMP” is selected, the component measuring facility is loaded. To use this, the setting of the High/Low range switch must be entered and if the switch is changed at any time, the setting must be re-entered, otherwise the calculations will be meaningless. As described in last month’s article, a two-component potential divider with one of the components being accurately known (P/Res) is used to measure resistance, capacitance and inductance. The software defaults to measure resistance with P/Res at 100kΩ and frequency of 1kHz. The scope screen and Volts display indicate the voltage with respect to zero (Vx) at the junction of the potential divider. If the range switch is in the High position, then three values of P/Res are available: 1kΩ, 10kΩ and 100kΩ. By switching between these values, resistances from 200Ω to 10MΩ can be meas­ured and displayed, as well as impedance. For a pure resistance, the displayed resistance and impedance will remain the same irrespective of the frequency applied to the potential divider. If the resistor under test is not “pure”, (most wirewound resis­ tors have significant inductance, for example), then the im­ pedance will vary with frequency. Low value inductances The A-D board plugs into the Boot board as shown in this photograph, with the interconnections made via two 8-way pin connectors (CONN3 and CONN5). The A-D board is then secured in position using Nylon spacers and machine screws and nuts. 78  Silicon Chip Low value inductances can have significant resistance which causes misleading results when they are being measured. If LOW range is selected when measuring inductance, another option is available on the menu bar, namely S.res. When selected, this measures the impedance of low value inductors at 10Hz. The series resistance is measured, displayed, and then can be used when calculating the inductance. The X1 and X5 toggles on the menu bar increase the resolu­tion of the potential divider when measuring high value impedanc­ es with the range switch set high, or low value impedances with the range switch set low. 1 C3 J1 1 1 IC5 74HC257 IC4 RAM IC3 EPROM C15 C4 CONN5 1 J4 IC6 74HC00 CONN4 IC1 80C31 C14 CONN3 IC7 ADM232 C6 C9 C10 C11 C12 RST C5 REG3 OUT IN OV +DC 1 X1 C16 CONN2 1 2 3 C15 1 C13 1 2 3 1 IC2 74HC573 As the measurements taken by Audio Lab are obtained by an 8-bit analog to digital conversion, the data has a maximum reso­ lution of 255 steps. Also, because the equations used to calculate impedance are non-linear, the accuracy of the reading is a func­ tion of where in the range the measurements are taken. The “Scale%” display gives an indication of this accuracy. This is not an absolute indication of the accuracy of the system but is calculated by computing the value of the impedance of the device under test at the next quantised step and displaying it as a percentage of the actual reading. This enables a choice to be made as to which combination of P/Res and frequency to use to obtain the highest accuracy. The last selection on the menu bar is “*SWEEP”. This also appears on the “LAB” screen and its function is identical. Being coloured red, this takes us to another screen where frequency sweeps are performed. If entry to the Sweep screen is from the Component measure screen, then the selected active input is Component. If entry is from the LAB screen, then which ever input was enabled is active. The LEDs show which input is selected and this is also shown on the PC display. “G.Col” scrolls through the different colours of the graph plot. “Print” prints the graph to the printer. Toggle “.Xlin.” and “.Xlog.” selects either linear frequency sweep from the pre-selected start frequency with a selected frequency incre­ment, or a full log frequency sweep from 10Hz to 20kHz. To save a project, click on *SAVE and to load or delete a previously saved project, select *FILES. All the parameters associated with a given project are saved to disc, along with the sweep data, and if these parameters are not identical to the current parameters when loading a file, a warning of “Parameter Change” is given. This stops files that have been created and saved with different ranges, inputs, etc from creating misleading plots when multiple graphs are displayed simultaneously. However, this can be overridden if required. “* SETUP ” takes us to the last C2 C16 R1 Accuracy & resolution C1 C7 C8 CONN1 Fig.5: the component overlay for the boot board. Note that the crystal and 3-terminal regulator must lay flat on the board. screen which is used to select printer options and the start and increment frequencies for the linear frequency sweep mode. A printer choice of HP Deskjet+, Epson 9-pin or Epson 24-pin emulation in either draft, final, landscape or portrait is available. If HP is chosen, resolutions of 300, 150, 100 or 75DPI can also be selected which give different size printouts. At any time, a screen dump can be taken by pressing F10. This gives a different printout to the Print selected from the Sweep screen which only This view shows how the two RCA input sockets (DC & AC) are connected to the front panel board using short lengths of tinned copper wire. Note in particular how the two earth lugs are connect to the earth pattern on the board. September 1995  79 C6 C10 1 R14 ZD1 CONN3 VREF C13 CONN2 C12 -5V 0V +5V R15 C11 Boot board Fig.6: the component overlay for the A-D board. Care must be taken with the interboard connectors. It must be assembled onto the boot board before the interboard connectors are soldered in place. To keep the stray capacitance to a minimum, when assembled, all the boards should be cleaned using a proprietary flux clean­ing spray. The power supply board should be assembled first, taking note of component polarities and ensuring the correct positioning of the 3-terminal regulators. Do not fit the 25-pin D connec­tor yet. It is also important to observe the orientation of the connectors CONN1 and CONN4, making sure the pins face to the outside of the board. Connectors CONN2 and CONN5 are not used in this project. Assemble the socket for the AC plugpack into the hole on the back panel and connect it to the “AC in” connections on the PC board. Solder two wires into the holes marked “RST”, and connect them to the reset switch MIC S1 R6 R10 CONN1 S1 R46 R45 VR1 C12 1 R11 R12 R13 VR3 R14 R15 VR4 R16 1 C9 R17 IC3 4052 C2 C5 IC2 TL072 0V C1 R2 VR8 R1 C3 1 C8 VR2 C4 R9 VR9,10 R3 IC1 TL072 SINE OUT R5 R7 R8 R4 1 XX C11 A COMPONENT B 80  Silicon Chip D3 R30 D2 1 R31 R32 R33 R34 R35 LEDS ON OTHER SIDE OF PCB LED4 LED1 LED2 LED3 LED6 LED7 Q1 DC LED5 1 C10 AC S2 1 VR7 R20 R21 R22 R23 R24 C7 R44 C13 R25 R26 R27 R28 R29 C6 0V IC6 4052 Fig.7: the component overlay for the front panel board. Note that the LEDs, sockets and binding post terminals must line up with the front panel. IC5 4051 All the PC boards are double sided, with plated-through holes, screened component overlays and solder masks so construc­tion is quite straight forward. However, it is advisable to follow the sequence of assembly to ensure the correct align­ment of the boards, especially the Front PC board. All resistors are 1% with 4-band codes. It is a good idea to check each resistor’s value with a digital multimeter before it is soldered into place. IC4 TL072 Construction R19 R18 VR6 VR5 prints the currently active graph along with the setup data. This board should be assembled, using sockets for all the ICs. Take care not to overheat the crystal or ceramic capaci­tors. The 8-way socket strips are CONN3 and CONN2 while the 9-way strip is CONN5. CONN4 is not used. There are two links on the Boot board to select the running mode of the processor. On J1, link 2 and 3 to enable the run from RAM function, and on J4, link 2 and 3 to select the polarity of address A000H. Before inserting any ICs, connect the 6-way cable from the power board to the Boot board. Power up and check that +5V is present between pins 40 and 20 on the microprocessor socket. Switch off and insert all the integrated circuits, observing correct orientation. Do not mount the boot PC board into the case at this stage. If you are using the Test PC kit, the Boot and power supply boards can now be tested. After assembling the R42 R43 Q2 D1 LED9 R41 R38 R13 R39 LED10 R40 1 D2 D-SINE IC6 74HC32 IC7 ADC0804 R12 C9 D1 1 R37 C8 IC4 4052 IC7 4051 IC5 74HC574 R36 C14 R6 C7 1 R11 C4 CONN5 VR3 R8 1 1 VR2 R3 C2 after it has been assembled into the back panel. Apply power and check the +5V and -5V rails on CONN1 and check for about +12V to +15V between +V and 0V on CONN4. If all is correct, then mount the D connector onto the back panel and solder the power supply board into place. Assemble the 3-way and 6-way interconnecting cables, ob­serving the correct orientation of the connections. The way to be sure this is correct is to place the connectors flat, so that both ends are facing away from each other and the location guides on both connectors are facing up. Then wire the connectors to­gether with no twists in the cable. Q1 R5 IC3 4093 ICI AD736 R1 R2 C5 R7 CONN1 R4 1 R9 R10 C3 VR1 IC2 TL072 C1 YY LED8 LED11 LED12 The front panel board is secured using two sets of nuts on the binding post terminals. Be sure to orient the three potentiometers as shown here and note that the metal bodies of the pots are all earthed back to the PC board using tinned copper wire. test PC board plug the board into the bus connectors on the Boot PC. The eight interconnecting pins go to port P1 on CPU pins 1-8, which is CONN2, and the 5V supply connects to pin 40 on the 80C3 which is the top pin above CONN5. Apply power and the LED connected to pin 1 should flash. Press the reset switch and the LED should stop, either on or off, and when the reset switch is released the flashing should restart. If all is well, the CPU, EPROM, reset and decoding circuitry are functioning correctly. Power down and connect Audio Lab to the serial port on your the PC, using a standard 25-way maleto-female cable. On the Calibrate/test disc is a program called LABTEST.BAT. Power on Audio Lab and the same LED will flash. Now type LABTEST from A: drive. A small 8031 machine code program called “Test.bin” is now being dumped to the RAM at 9600 baud and the LED will flash faster. After installation, the LEDs will flash in order from P1.0 to P1.7 and an acknowledgment of cor- rect data transfer will be shown on the PC. The RAM and Serial interface have now been tested and the Boot PC is functioning correctly. If any of these tests fail, check all components and the power cables to the boards. A-to-D board Assemble this board as normal but do not insert the IC sock­ets for IC5, IC7 or the PC board interconnecting strips. Connect the 3-way power cable and disconnect the 6-way cable from the Boot PC board. When powered up, check for +5V between pins 16 and 8 on IC4 and -5V between pins 7 and 8 on IC4. Turn the power off and disconnect the 3-way power cable. Gently insert the ends from the plastic holder on the PC interconnecting strips into the two 8-way PC interconnecting sockets, CONN3 and CONN5 on the Boot PC, making sure they are seated all the way down. Note that the 8-way connector for CONN5 should start at the second pin down, leaving the top pin, marked +5, empty. Carefully position the A-to-D board and the pins into CONN3 and CONN5 C3 TO S1 RST OUT IN 0V +DC C4 -5V 0V +5V CONN1 CONN4 C1 C2 D2 REG1 D1 REG2 F1 DB25 AC IN Fig.8: the component overlay for the power supply board. This carries the 25-pin D socket which mounts to the rear panel. (on the A-to-D board). Place the nylon spacers between the two boards in the holes marked A and C and screw together, with the nuts on the reverse of the Boot board. Now solder the interboard connectors in place and also solder the nuts to the underside of the Boot board. The boards can now be disassembled and the remaining IC sockets soldered in place and the ICs inserted. The Boot board can now be screwed into the case. Carefully line up the A-to-D and Boot boards and bolt them together as before and connect the cables from the power supply to both boards. Front board assembly The interconnection between the A-to-D and Front boards is made via a 16-way cable which is provided ready assembled and tested. Assemble the Front PC board but do not insert the LEDs. The switch is mounted on the component side with the mounting nut on the copper side and links made between the holes adjacent to the relevant connector and the switch. The DC and AC RCA connectors should be fixed in place with the solder lug connectors facing down and slightly bent out to ease connecting to the Front board later. Assemble all the components on the front panel, using only one nut on the two binding post terminals, and noting the orien­tation of the three front mounting pots, as shown in the above photo. Insert, but do not solder, the 12 LEDs on the copper side of the board, noting their polarity and colour which is marked on the PC overlay. Do not push them too far into the board. The Front PC is now offered up to the front panel and held in place with the September 1995  81 The power supply board (bottom, left) is mounted on the DB25 connector and is wired to the A-D and Boot boards via 3-way and 6-way cable assemblies. A 16-way cable connects the A-D board to the Front-Panel board. remaining nuts on the two binding posts. The nuts should not be tightened but should just secure the board while allowing some movement and the switch should line up with the HIGH/LOW hole. The LEDs will now line up with the holes in the front panel and can be gently pushed through the clearance holes until they are flush and poking through the panel. Slightly tighten the nuts on the binding posts and carefully solder the LEDs in place. The front board can now be removed and the ICs inserted. Reassemble the front board to the front panel and wire the pots, connectors and the ELEC switch as per the circuit diagram and component overlay on the PC board, using screened cable for the Mike connection. Connect the two earth connectors on the DC and AC 82  Silicon Chip input connectors together and link to the 0V terminal on the PC board. Link the centre connectors of the RCA sockets to their respective points marked on the board and tighten the nuts on the two binding posts. They can be soldered to the board later when the system has been tested and proved to be functional. Calibration There are a total of 10 multi-turn Kit Availability Audio Lab is designed by R.S.K. Electronics Pty Ltd who hold the copyrights and sell the complete kit. Pricing details are shown in the advertisement elsewhere in this issue. pots in the Audio Lab but the calibration is made simple by means of the software. First, insert the Test/ calibrate disk into the A: drive. Type install and a direc­ tory called C:\ ALAB will be created and the software copied into this directory. Change directory to ALAB and type SETUP. This software steps through all the procedures involved in calibra­tion, with a full graphic interface showing which pots to adjust and the reason for the calibration. After calibration is complete, exit from the program and insert the disc marked ALAB into the A: drive and type INSTALL. The full suite of software will now be copied into C:\ALAB. To run the software simply type ALAB from the C:\ALAB directory. Once you have mastered the simple menu system you will be able to flip from screen to screen. You should find Audio Lab an invaluable addition to your test equipment. SC