Fullscreen HTML5Med Res (??MB)HTML4

L-o-o-o-n-g gating times for the 12-Digit High-Resolution Counter By JIM ROWE This add-on PCB module enables higher resolution measurements with the 12-Digit Frequency/ Period Counter described in the December 2012 & January 2013 issues of SILICON CHIP. It adds an additional decade divider for the external timebase input to allow measurements using a gating time of 10,000 seconds (nearly three hours) and includes front-panel LEDs for gating indication. D URING OUR RECENT work in calibrating an ex-telecoms rubidium frequency standard (SILICON CHIP, April 2014 issue), it became apparent that it was possible to improve the 12-Digit High Resolution Counter to make it better for this type of very high resolution frequency measurement. This would involve a small module which could be built inside the counter’s case. The end result provides three separate improvements, as outlined below. One of the functions I wasn’t able to provide on the original 12-digit counter was any indication of when the counter’s gate is open and counting is under way. This doesn’t matter much when you’re making measurements at short gating times like 1 second or 10 seconds, because each new reading follows the last in relatively short order. But it becomes a drawback when you’re using longer gating 80  Silicon Chip times for higher resolution frequency measurements. For example, if you want to measure with a resolution of 1mHz (0.001Hz), each measurement involves a gating time of 1000 seconds and there’s also a gap of 1000 seconds between measurements, because of the way the counter works. Without any indication of when the gate is actually open and counting is taking place, it’s not possible to tell whether it’s counting or ‘waiting between counts’. So one of the improvements provided by the new add-on module is to provide an indication of when the counter’s gate is open and counting is under way. It does this with a bi-colour LED, which is red when the gate is open for odd counts and green when the gate is open for alternate even counts. Because it doesn’t light at all during the gaps between counts, this makes it quite easy to tell at a glance what the counter’s status is at any particular time. But what if you’re over the other side of the room, or perhaps in another room – so you can’t be glancing over at the counter all the time? To solve this, the module includes a simple beeper circuit, which operates a piezo buzzer for a short time at the start of each new counting period. So all you need to do is keep an ear out for the beep, to let you know when a new count has begun. Then you can go over to the counter and record the previous count (which continues to be displayed during the new count). The circuit of the add-on module has been arranged so that the beeper is only activated when the counter is set for gating times of 100s or more. For the shorter gating times, it’s disabled. The beeper circuit is also linkprogrammable with respect to the actual beep duration. There’s a choice siliconchip.com.au of four different beep durations: 0.5 seconds, 2 seconds, 16 seconds or 128 seconds. So you can easily select the duration that’s most suitable for your application. The third improvement provided by the new module enables the counter to make really high resolution measurements. It’s an additional synchronous decade divider for the counter’s external timebase input, so that the maximum gating time/counting period can be extended to 10,000 seconds – allowing you to make frequency measurements with a resolution of 100µHz (100 microhertz or 10-4 Hz). But there’s a price to pay for making this type of measurement with the counter: each count will take 2 hours and 47 minutes, with a gap of the same duration between counts. So you’ll need to be patient but at least the indicator LED and beeper will let you get on with other things! Note that the additional timebase divider can be switched out of circuit when it’s not needed and another LED indicates when the additional divider is being used. This LED will also remind you to ‘bump up’ the decimal point in the counter’s display, because the counter itself has no way of knowing that the additional divider is in use. Circuit details The full circuit for the new add-on module is shown in Fig.1. The circuitry for the ‘gate open’ indication is at the top, the ‘beep at the start of each count’ function is in the centre and the additional timebase divider circuitry is at the bottom. The ‘gate open’ indication circuitry involves gate IC1b, flipflop IC2a and transistors Q1, Q2 & Q3. IC1b is used as an inverting buffer, which takes a ‘GATE OPEN-bar’ signal derived from pin 2 of IC18a on the counter’s main PCB and inverts it to provide an active high ‘GATE OPEN’ signal with a positive-going leading edge. This leading edge is then used to toggle flipflop IC2a, which therefore changes state at the start of each new count. The flipflop’s two outputs (Q and Qbar) are then used to control transistors Q2 & Q3, so only one of these is able to conduct at any time to allow current to flow through either LED1a or LED1b. This depends on whether the counter is performing an odd count or an even count, although these labels are purely arbitrary. siliconchip.com.au The main add-on logic module is mounted on the lid of the counter’s case, while the smaller add-on LED board is attached to one end of the counter’s display PCB. An extra switch is also mounted on the rear panel. Whether either of the two LEDs is able to conduct current doesn’t just depend on transistors Q2 & Q3, however; because neither LED can pass current unless transistor Q1 is also conducting. Q1 is only able to conduct current when it is provided with forward base current via the 22kΩ series resistor connected to pin 4 of IC1b. So Q1 only conducts when the ‘GATE OPEN’ signal on pin 4 of IC1b is high (ie, during counting). Hence LED1a lights only during odd counting periods and LED1b is on only during even counting periods. In the gaps between counting periods, both LEDs remain dark. Gating beeper circuit The beeper section involves gates IC1a, IC1c & IC1d, together with timer IC3 and transistor Q4 to switch the piezo buzzer on and off. The input gating circuitry may look a little strange but it’s really quite straightforward. IC1a is being used as another inverter, to derive a ‘GATE OPEN-bar’ signal from the signal at pin 4 of IC1b. This is then fed to pin 13 of IC1d, used here as a negative input logic AND gate. We don’t want the beeper to function when the counter is being used with the shorter gating periods, so the second input of IC1d (pin 12) is connected to IC1c’s output pin (pin 10), while IC1c’s inputs are connected to two pins of IC23 on the main counter PCB: pin 2, which carries the 100s gating signal (H = 100s gating) and pin 6 which carries the 1000s gating signal (H = 1000s gating). Since IC1c a NOR gate, this means that its pin 10 output will only switch low when the counter is set for either 100s or 1000s gating. Accordingly, pin 12 of IC1d will only be taken low for these gating times also, and will be kept high for the shorter gating times. So even when pin 13 of IC1d drops low during GATE OPEN’ periods, IC1d’s pin 11 output will not be able to switch high unless the counter is set for either 100s or 1000s gating. The output from pin 11 of IC1d is coupled to the MRST input (pin 6) of timer chip IC3 via a differentiator circuit using a 470nF capacitor and 10kΩ resistor. This is done so that IC3 only receives a short triggering pulse, derived from the leading edge of the gated positive-going signal from IC1d. I’ll explain the reason for this shortly. IC3 is a 4541B programmable digital CMOS counter/timer, used here to time the duration of our ‘start of a new count’ beeper. It’s triggered via MRST input pin 6, while its output at pin 8 is used to control the piezo buzzer via transistor Q4. The duration of the output pulse (and therefore the length of the beep) is determined by July 2014  81 Parts List 1 PCB, code 04106141, 169 x 45mm (cut into two boards, 137 x 45mm & 30.5 x 45mm) 3 6-pin PCB-mount right-angle polarised locking headers, 0.1-inch spacing 3 6-pin polarised locking plug sockets, 0.1-inch spacing 2 2-pin SIL headers (or 1 x 4-pin DIL header) for LK1 & LK2) 2 2-pin jumper shunts 1 piezo buzzer, 24mm diameter, PCB mounting 1 DPDT panel-mount mini toggle switch 1 1m-length of 6-conductor rainbow ribbon cable 3 6G x 6mm self-tapping screws 2 M3 x 9mm machine screws 2 M3 flat washers Semiconductors 1 4001B quad CMOS NOR gate (IC1) 1 4013B dual D-type flipflop (IC2) 1 4541B programmable CMOS timer (IC3) 1 4017B Johnson decade counter/divider (IC4) 2 PN100 NPN transistors (Q1,Q4) 2 PN200 PNP transistors (Q2,Q3) 1 5mm 3-pin common-cathode red/green bicolour LED (LED1) (Altronics Z0885) 1 3mm blue LED, waterclear (LED2) 3 1N4148 silicon diodes (D1-D3) Capacitors 1 10µF 16V RB electrolytic 1 470nF MKT or MMC 6 100nF MMC (multilayer monolithic ceramic) Resistors (0.25W 1%) 1 33kΩ 1 2.2kΩ 4 22kΩ 2 1kΩ 2 15kΩ 1 470Ω 6 10kΩ 1 100Ω the timing components connected to pins 1, 2 & 3, which set the frequency of the 4541B’s internal clock oscillator and also by links LK1 and LK2 which program the 4541B in terms of its timing count setting. As you can see from the small table at centre left of Fig.1, the link combinations provide a choice of four beep durations: from half a second up to 128 seconds. 82  Silicon Chip But why did we have to provide a short triggering pulse for IC3 – why couldn’t we simply use the logic output signal from IC1d directly? That’s because IC3 only provides its ‘end of timing count’ output pulse from pin 8 if the input triggering pulse supplied to pin 6 has ended. And in this circuit, the output signal from IC1d can of course stay at the high logic level for as long as 100s or 1000s (or even 10,000s), which would prevent IC3 from ever activating the beeper. So by using the simple differentiator shown, we derive a relatively short trigger pulse from the rising edge of the output signal from IC1d, ensuring that the triggering signal at pin 6 of IC3 has dropped back to zero in no more than about 100ms. This allows correct beeper operation, even with a beep duration of only 0.5 seconds. By the way, diode D3 is provided simply to ensure that any negative pulse appearing at input pin 6 of IC3 when the output of IC1d does drop back to zero (when the counter’s gate finally closes) is limited to an amplitude of -0.6V. Additional divider stage Now let’s look at the circuitry at the bottom of Fig.1, which provides the additional ‘divide-by-10’ function to extend the counting duration when using an external timebase, eg, the 1Hz pulses from a GPS receiver or a rub­ idium time and frequency standard. This uses a 4017B Johnson-type synchronous CMOS decade counter (IC4). Its CP0 input (pin 14) is connected directly to CON3 at the rear of the main counter PCB, which is disconnected from the original external timebase input by removing the 1kΩ series resistor just to the front of CON3. Diodes D1 and D2, together with the 100Ω and 22kΩ resistors, are used to protect the input of IC4 from possible over-voltage damage. This additional timebase divider is always fed with the external timebase signal from CON3. However, whether or not its output is fed to the external timebase input of the counter is controlled by added switch S1, which is fitted to the counter’s rear panel. This is a double-pole switch, with its ‘a’ section used to select either the raw external timebase signal from CON3 or alternatively, the pin 12 output of IC4. S1a’s common terminal is connected back to the external timebase input of the counter via a 1kΩ series resistor, which replaces the one that’s removed to tap into the signal from CON3. S1b is simply used to switch LED2 in or out of circuit, so that this LED only glows when S1a has been set to make use of the additional timebase divider. One last point: the 100nF capacitor and 10kΩ resistor connected to pin 15 of IC4 are simply there to deliver a short reset pulse to this IC when power is first applied to the counter, so that IC4 starts off ‘on the right foot’. Construction As shown in Fig.2 and the photos, virtually all the components used in the add-on module are fitted onto two small PCBs, which are cut apart from a single board measuring 169 x 45mm and coded 04106141. The larger PCB (coded 04106141a) carries most of the components and circuitry, and mounts up inside the righthand end of the counter’s lid. The smaller PCB (coded 04106141b) carries only the two extra LEDs and is mounted at the righthand end of the counter’s display PCB, behind the front panel and with the extra LEDs just protruding through two additional holes in the panel. Three ribbon cables are used to make the connections. These go between the two add-on PCBs, between the larger add-on PCB and the main counter PCB, and to added switch S1 and the external timebase input circuitry (CON3). This should all be fairly clear from the overlay diagram of Fig.2 and also the internal photos. Cut the two boards apart and smooth their cut edges with a small file before you add any of the components. You can then begin the assembly of the larger board by installing the two wire links, followed by the resistors, diodes, capacitors, transistors and ICs. The three 6-pin 90° header plugs and the two 2-pin SIL headers for LK1 and LK2 can then go in. The piezo buzzer can be left until last, as it’s relatively large and makes it hard to access some of the other components once it’s in place. Next, you can fit the two LEDs to the smaller PCB, both with their ‘flat’ sides (cathodes) downwards as shown in Fig.2. Leave their leads about 14mm long above the top surface of the PCB, as this will be about the right length for the LEDs to protrude slightly through siliconchip.com.au +5V 1 +5V FROM IC23 PIN 20 GATE OPEN SIGNAL FROM 3 IC18a PIN 2 5 14 6 100nF 10 µF IC2: 4013B 6 GATE OPEN TIME 5 D C 22k B 4 E Q1 PN100 4 470Ω CON7 10k 2 Q Vss 7 R C 1 Q3 PN200 22k B GATE OPEN ODD OR EVEN COUNTS 3 GA RA LED1a GROUNDS FROM COUNTER’S MAIN PCB 2 C 10k IC 2 a IC1b E B 14 Vdd 1 Q S CLK 3 4 Q2 PN200 E 22k λ 2 LED1b 100nF λ K GATE OPEN & COUNTING INDICATION 2 100s GATING SIGNAL 5 FROM IC23 PIN2 6 IC1a +5V GATE OPEN TIME 3 14 VDD 5 9 AUTORST Q/Q SEL 1 1000s GATING SIGNAL FROM IC23 PIN6 IC1: 4001B 13 12 CON6 9 Q1, Q4: PN100 Q2, Q3: PN200 IC1d 11 470nF 6 K 7 10k IC1c A LK1 LK2 OUT IN OUT IN 16 SECONDS OUT IN IN 3 15k BEEP DURATION OUT 33k 100nF 2 LOW = 100s OR 1000s GATING SELECTED CSEL B CSEL A D3 10 8 MRST 1 IC3 4541B RS OUT 13 + PIEZO BUZZER 12 C 15k 8 B RTC MODE 10 E Q4 PN100 2x 10k Vss 7 +5V 0.5 SECONDS 128 SECONDS K 16 100nF D1 A 100Ω Vdd 14 CP0 O7 O6 15 D2 22k A 1k MR O5 O4 O3 O2 CP1 O1 Vss 3 2 IC4 4017B 10k 13 4 100nF O9 O8 K 1 2.2k LK2 CTC START NEW COUNT BEEPER (100s GATING & ABOVE) 2 SECONDS 100nF LK1 8 CON5 O5-9 12 O0 1k 11 9 A 6 λ LED2 6 5 K 5 1 (CON7) 10 7 4 2 3 8 9 5 6 (CON5) 11 ÷1 S1a ÷10 EXT TB 10 ADDITIONAL ÷10 GATING TIME DIVIDER ÷1 ÷10 S D Q IC 2 b CLK R 10 Q 13 12 S1b K (CON3) D6 1k (REMOVE FROM PCB) A K 22k D5 NOTE: LED1 AND LED2 ARE MOUNTED ON THE SMALL ADDED DISPLAY PCB. ALL OTHER PARTS ARE ON THE LARGER ADD-ON PCB. A AG K SC K A AR (ON MAIN COUNTER PCB) 20 1 4 LED2 LED1 PN100, PN200 ADD-ON MODULE FOR HI-RES COUNTER D1– D3: 1N4148 A K B C E Fig.1: the add-on module is based on four CMOS ICs (IC1-IC4). The ‘gate open’ indication circuitry is at the top, the ‘beep at the start of each count’ circuit is in the centre and the added timebase divider circuitry is at the bottom. the front panel when the board is mounted in position. Once both PCBs have been fully assiliconchip.com.au sembled, the larger one can be fitted inside the upper half of the counter’s case, at the righthand end. Note that it’s mounted upside-down, with the copper side towards the case lid and the component side facing inwards. July 2014  83 RIBBON CABLE TO MAIN COUNTER PCB +5V, GND, IC18 PIN 2, IC23 PINS 2, 6 * ON MAIN COUNTER PCB 100nF 100nF 3 x 6mm LONG SELF-TAPPING SCREWS MOUNTING THIS ADD-ON LOGIC PCB INSIDE RIGHT-HAND END OF UPPER HALF OF COUNTER CASE b14150140 EXT TB x10 A K LED2 Led1ga Led1k Led1Ra Q2 PN200 GA K RA LED1 Led2k Led2a 6 Led2k Led2a 5 Q3 PN200 22k 22k IC2 IC1 100nF 3 4 Led1ga Led1k Led1Ra 470Ω 22k 10k 10k 470nF 1 2 6 4013B 5 LINK 3 4 GATE OPEN ODD/EVEN COUNTS 1k LINK D3 2 15k 33k IC3 LK1 4541B + 4017B 100nF 2.2k 1 RET NU O C SER-I H ELUD O M N O-DDA a14150140 Q1 PN100 100nF D2 D1 IC4 1k 100Ω 22k 4148 4148 10k 4148 BUZZER CUT THE TWO PCBS APART HERE CON7 +5V GND GATEOPEN GND 10 0 sGATE 1 000sGATE 6 LK2 5 10k 10k 4 100nF 3 10k 2 CON3 & S1a x1 S1aROT S1a x10 CtrTB I/P S1bROT S1b x10 1 RIBBON CABLE TO ADD-ON LED PCB (LEDS 1 & 2) CON6 + 15k PIEZO 4 1 0 2 C PN100 Q4 10 µF CON5 4001B RIBBON CABLE TO S1, CON3* , CATHODE OF D5* RIBBON CABLE FROM CON7 ON ADD-ON LOGIC PCB Fig.2: cut the PCB into two sections as indicated, before installing the parts on the two modules. Make sure that all polarised parts are correctly orientated and be careful not to get the ICs mixed up. The photo at right shows the completed add-on logic module mounted in place on the case lid. The PCB is held in place using three small 6mm long self-tapping screws which mate with three of the small standoffs moulded inside the case lid at that end. After that, you need to drill two additional holes in the counter’s front panel (to allow the two extra LEDs to protrude and be visible) and also a single additional hole in the rear panel to accept toggle switch S1. Fig.5 shows the size and location of these two additional holes in the front panel. Note that these line up horizontally with the uppermost and lowest of the three existing LED holes in the panel just to the right of the main display window, but are only 10mm in from the righthand end of the panel. The single additional hole in the rear panel should have a diameter of 6.5mm to allow S1 to be fitted but its exact location is not critical. This should be located directly above CON3, about 55mm up from the bottom of the rear panel. This allows S1 to be activated quite easily by reaching over the case top. The next step is to make up the three interconnecting ribbon cables. One of these (the one to connect from the centre connector to various points on the main counter PCB) should be about 300mm long, while the other two can be around 230mm long. One of the two shorter cables (the one used to connect to the small LED display PCB) needs only five conductors rather than six. To assemble each cable, bare all of their conductors for about 4mm at each end. Then you need to crimp and solder each conductor (at one end) to one of the pins of a 6-way polarised and locking header socket. That done, you can cut the pins from their carrier strip and push each one into the slots of the plastic socket moulding. Make sure that you push each clip fully home, ie, until its small barb clicks into the slot near the end. If you don’t do this, the clips won’t remain in position. Make sure also that with the 5-wire shorter cable, you push the five clips into slots 1, 2, 3, 5 & 6 of the header socket. Leave slot 4 empty, because the corresponding pin of CON7 (the connector with which this cable socket mates) isn’t used. Once the header sockets have been attached to one end of each cable, you’re then ready to connect the free ends of each cable to the designated points on either the small add-on display PCB, the counter’s main PCB or the added toggle switch S1, on the rear panel. For example, the conductors of the short 5-way cable are connected to their matching holes along the bottom of the LED display PCB (04106141b), as shown at bottom right in Fig.2. Once all five have been soldered to their  Table 2: Capacitor Codes Value µF Value IEC Code EIA Code 470nF 0.47µF   470n   474 100nF 0.1µF   100n   104 Table 1: Resistor Colour Codes   o o o o o o o o o No.   1   4   2   6   1   2   1   1 84  Silicon Chip Value 33kΩ 22kΩ 15kΩ 10kΩ 2.2kΩ 1kΩ 470Ω 100Ω 4-Band Code (1%) orange orange orange brown red red orange brown brown green orange brown brown black orange brown red red red brown brown black red brown yellow violet brown brown brown black brown brown 5-Band Code (1%) orange orange black red brown red red black red brown brown green black red brown brown black black red brown red red black brown brown brown black black brown brown yellow violet black black brown brown black black black brown siliconchip.com.au pads on the rear of the add-on LED PCB, this board can be fitted into the counter case. ADDED LED DISPLAY BOARD (SPACED BEHIND MAIN DISPLAY PCB USING TWO M3 FLAT WASHERS) Mounting the LED board As you can see from Fig.3 and one of the inside photos, the added LED display PCB is mounted just behind the counter’s existing main display PCB, at its righthand end (as viewed from the front of the counter). This is done by removing the two existing M3 x 6mm screws attaching the main display PCB to the Nylon spacers at that end, and then replacing them with two M3 x 9mm screws and M3 flat washers. Each of these screws pass through a mounting hole of the add-on LED PCB (from the copper side), then through an M3 flat washer (acting as a thin spacer) before passing through the holes in the main display PCB and then into the Nylon spacers as before. When both screws are tightened, both boards will then be mounted securely behind the front panel. Once the other shorter ribbon cable has been fitted with its 6-way header socket and clips, you can then solder the free ends of two of its conductors to the main counter PCB, just behind CON3 at the right rear and to the pads at each end of the position where the 1kΩ resistor was removed (see photo). Basically, the conductor coming from pin 4 of CON5 (on the add-on logic PCB) connects to the pad on the left, while the conductor from pin 1 of CON5 connects to the pad on the right. The latter also connects to the siliconchip.com.au LED1 M3 x 9mm SCREWS REPLACING ORIGINAL SCREWS AT RH END OF MAIN DISPLAY PCB LED2 RIBBON CABLE TO MAIN ADD-ON PCB MAIN COUNTER DISPLAY PCB MAIN PCB FRONT PANEL BOTTOM OF CASE Fig.3: this cross-section diagram shows how the add-on LED display PCB is attached to the righthand end of the original display PCB (as viewed from the front) using two M3 flat washers plus two M3 x 9mm machine screws into the original Nylon spacers. upper left lug of switch S1, although you may wish to make this second connection using another short length of hookup wire. The other four conductors of this ribbon cable connect to the other lugs of S1. The one from CON5 pin 2 goes to the centre lefthand lug, while the one from CON5 pin 3 goes to the lowest lug on the lefthand side. Similarly, the one from CON5 pin 6 goes to the lowest lug on the righthand side of S1, while the one from CON5 pin 5 goes to the centre-right lug of switch S1 (the common terminal). All that remains after the two shorter ribbon cables have been fitted is to do the same with the longest of the three cables. This is the one used to make the connections between CON6 of the add-on logic PCB to various points on the counter’s main PCB. All of these points are pins on two of the ICs and it’s quite easy to solder each lead to its corresponding pin using a fine-tipped soldering iron. Here’s how these conductors are wired. First, the wire from CON6 pin 1 July 2014  85 10 E Fig.4: follow this diagram to mark out and drill the two additional holes in the front panel. 13 A 45.75 This photo shows the wiring to DPDT toggle switch S1, to CON3 and to the main PCB where the 1kΩ resistor was removed (ie, just behind CON3). HOLE A: 3.5mm DIAMETER; HOLE E: 5.0 mm DIAMETER ALL DIMENSIONS IN MILLIMETRES goes to pin 20 of IC23, on the lefthand rear side of the main PCB. The wire from CON6 pin 2 then goes to pin 1 of the same IC, while the wires from CON6 pins 5 & 6 go to pins 6 & 2 of IC23, respectively. The remaining two conductors are taken to two pins of IC18. In this case, the wire from CON6 pin 3 goes to pin 2 of IC18, while the one from CON6 pin 4 goes to pin 7 of IC18. All that remains is to fit whatever combination of jumper shunts you wish to LK1 and LK2 (just to the left of IC3 on the add-on logic PCB), to program it for the beep duration you want, and then plug each of the three interconnecting ribbon cables into their correct pin headers (ie, into CON5, CON6 & CON7). That done, you should be able to refit the case lid to the counter and apply power to get it all working again. Don’t be surprised when the counter emits a beep as soon as power is turned back on. This doesn’t indicate any sort of problem; it’s just a quirk of the 4541B timer IC, which produces an output pulse as soon as power is applied. Final comments This circuit is a worthwhile addition to the 12-Digit High-Resolution Counter, especially if you will be making high-resolution measurements. Admittedly, such measurements will be very time-consuming compared to a counter which has interpolation technology instead of the long-term averaging system we used in the December 2012/ SC January 2013 design. The connections from the add-on logic module to the main counter PCB, the add-on LED PCB and to other components are run using ribbon cable. 86  Silicon Chip siliconchip.com.au