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Build this superb December 1987 1GHz Digital Frequency Meter In this second article on our state-of-theart 1GHz Digital Frequency Counter we continue describing the special circuit features which make it easy to use and give it such a high performance. By STEVE PAYOR Last month we described the circuitry on the front panel board, up to the point where the incoming signal had been squared up and prescaled to provide a 0-10MHz signal for the ICM7216A counter IC. Before it is fed into the counter though, the signal is converted from ECL to CMOS voltage levels by Q2, a 2N4258 switching transistor. When the ECL output from IC4b (pin 15} is low, Q2 is turned on and the collector voltage rises to just over + 4V. When the ECL output is high, the transistor is turned off and the 27011 collector resistor pulls the output down to 0V. The BAW62 diode is important here. Without it, Q2 would not turn off fully and if that diode has a forward voltage drop greater than 0.9V at 15mA, Q2 will not turn on fully. This means that a BAW62 diode must be used here instead of the more common 1N914 or 1N4148. In other words, don't substitute. Similarly, do not substitute an or- Comments on the Parts List (1 ). The five miniature red LEDs specified in the parts list should be the 3mm round type. If highefficiency types are used here (eg, Hewlett-Packard HLMP-1301), the 27011 current limiting resistor should be increased to 56011 so that the LED brightness matches the display brightness. (2) . Do not substitute for the specified HDSP-5501 ?-segment LED displays. The display brightness will be disappointing it you do. 76 SILICON CHIP (3) . The red LED used tor setting up and testing should be a standard type with a forward voltage of 1. 7V. (4). The 0 .047µ,F input capacitor was listed as a ceramic in the parts list. The author would prefer a highvoltage plastic type here. Either a 250V or 400V DC rating should do, depending on the intended use of the meter. Physical size must be checked as the space available for this capacitor is limited. Metallised dinary PNP transistor for the 2N4258 (or PN4258}, as this is a special type designed for highspeed saturated switching. One final note - we have chosen a fairly low value of collector load (27011} so that the input capacitance of the 7216A will not slow the fall time of the waveform too much. However, any additional capacitance at this point will kill the high frequency response. For example, connecting a CRO lead to observe the 4V p-p waveform at 10MHz will stop the counter from functioning. The 7216A counter IC Those readers not familiar with the basics of digital frequency and period measurement can refer to the accompanying panel for a brief summary. Although this summary makes the task look fairly simple, a vast amount of support circuitry is required to make a practical instrupolyester layer-type construction is the most compact. (5). The 4-40pF trimmer capacitor should be a Philips type if it is to fit into the PCB. Stability- both electrical and mechanical - is very important here. (6). The mains switch must be rated for 240V AC. Don't use a switch with a 125V, 5A rating - it might fail after a short time. An alternative to the push-on push-off type is the standard C & K miniature toggle switch, which has a more than adequate rating. The bottom corner of the display PCB is This view shows the neat and orderly layout of the parts on the display PCB. Keep all leads as short as possible and make sure that the LED bar modules line up with the front panel displays. ment. Fortunately, most of this circuitry is contained within the 7216A. Essentially, the 7216A is a fullyintegrated 10MHz universal counter IC. It contains a highfrequency oscillator, a decade timebase counter, an 8-decade data counter, and all the circuitry necessary to generate gating signals, latch data and drive an 8-digit multiplexed LED display. The 7216A is designed to drive common anode LED displays. The inbuilt segment drivers sink a condesigned to be cut away when using this type of switch. The switch itself is mounted directly on the front panel. (7). A small transparency with the "µsec" and "kHz" display annunciators should be added to the parts list. (8) . Use 24 x 0.2mm hook-up wire for all wiring. It is important to minimise the resistance between the power supply and the counter board, otherwise noise from the power supply will appear on the +5V supply at the counter board. trolled current of 25mA per segment, while the digit driver outputs can source up to 200mA each, when all the segments on a digit are lit. The display multiplex frequency is 500Hz, and so each digit has a time-slot of 250µs. In actual fact, each digit is only on for a period of 244µs. An inter-digit blanking time of 6µs is used to prevent " ghosting" between digits. We have specified high efficiency 7-segment displays (HewlettPackard HDSP-5501). These are much brighter than the normal variety, and the 7216A counter runs somewhat cooler because of their higher forward voltage drop (2.2V vs 1.7V for normal displays). The 7216A also provides an output for driving decimal points on the display but we have had to design a separate decimal point driver circuit (more about this later). The 7216A decimal point output is used instead to drive a LED which indicates that the counter has overflowed. Frequency mode Fig.8 shows a simplified frequency meter with a counter , a reference frequency and a gating circuit. The reference signal is needed to open and close the gate for precise time intervals. The 7216A provides gating times of 0.01, 0.1, 1 and 10 seconds. All timing signals within this IC are derived from a clock frequency of 10MHz which is generated by the crystal controlled on-chip oscillator (pins 25 and 26 of the 7216A). The 10MHz signal is divided internally to 100Hz and then further divided by 1, 10, 100 or 1000 to give the required gating time intervals of 0.01, 0.1, 1 and 10 seconds. Ref erring to our simplified block diagram (Fig.BJ, imagine a programmable divider (-:- 1, 10, 100 or 1000) between the reference frequency (now 100Hz) and the gate control flipflop. The division ratio of this programmable divider - and hence the gating time - is controlled by the 7216A's Range input (pin 14). Connecting the Range input (pin 14) to the Dl digit driver output (pin 23) instructs the 7216A to select the -:- 1 option for its programmable divider. Similarly, connecting it to D2 [pin 22) selects -:- 10, while D3 selects -:- 100 and D4 selects -:- 1000. DECEMBER1987 77 Frequency and Period Measurement - "!"hertz INPUT REFERENCE FREQUENCY Fig. 8 How does a digital frequency meter work? In its simplest form, it consists of three basic blocks: a gate, which can be opened and closed to let the signal through to a counter, and an accurate timing signal to control the opening and closing of the gate. We call this the reference frequency . If we open the gate for exactly one second and, say, 123 pulses get through to the counter during this time, then the counter will show a final reading of " 123". This is the frequency measurement in Hertz. If we want greater accuracy, we can increase the gating time . For example, if the gate is opened for 10 seconds, and the counter registers 1234 pulses, then the frequency is 123.4Hz. J -"p" seconds --+-/ I _J INPUT \ p pulses ,- __ JLJl_fl___ GATE I 1 .-!-- - - - - ~ 17nn17,-,nn :J ---1-~-'-----...l---l flip- 110 '---+-----' I\ ~_u, u u u u u u u _, COUNTER gate open I ~~:ed LJlJlJ7_f7 Fig. 9 REFERENCE FREQUENCY I-;!~ If the signal frequency is very low, it is more convenient to measure the period of the signal. The same building blocks are used as for frequency measurements, but are slightly rearranged . The incoming signal is now used to open and close the gate, and it is the reference signal that is gated through to the counter. For example, if three cycles of the 1 Hz reference get through to the counter during the time the gate is open , then the period of the incoming signal is three seconds and the frequency is 1 ..,.. 3 = 0.3333Hz. If greater resolution is required , we can leave the gate open for say 10 cycles of the input signal, in which case a counter reading of 34 pulses gives us a period of 3.4 seconds . The divider output is used to gate through the input signal to the counter circuit (see Fig.8) for either 0.01, 0.1, 1 or 10 seconds. In period mode, the signal flow is _rearranged. In this case, the 7216A's programmable divider is connected to input A (instead of the lOOHz internal reference) and so the gate is now opened for 1, 10, 100 or 1000 78 SILICON CHIP cycles of the incoming signal, to gate through the reference frequency to the counter circuit (see Fig.9). Frequency ratio mode When function F4 (frequency to 1GHz) is selected, a + 128 prescaler is used. In order to obtain a direct reading in kHz on this range, some DFM designs take the rather drastic step of switching to another timebase. This design overcomes the problem in a more elegant manner by making use of another operating mode which the 7216A provides - the Frequency Ratio Mode. In this mode, the 7216A takes the input for its reference counter from Input B (pin 2), instead of the internal l00Hz reference. Apart from that, the internal configuration is exactly the same as for frequency mode. Therefore, if we were to feed lO0Hz into input B, we would obtain the same result as for a normal frequency measurement. If, however, we feed a signal of 100/1.28 = 78.125Hz into Input B, the gating periods will be 1.28 times longer. Similarly, by feeding a signal of 7.8125Hz into Input B, the reading can be scaled up by a factor of 12.8, and the gating times are now 0.128, 1.28, 12.8 and 128 seconds. This is how we solved the problem of having a + 128 prescaler on the 1GHz (one gigahertz) range. We simply shifted the decimal point one place to the right (xlO) and increased the gating times by a factor of 12.8 to give the desired x128 correction factor. But just where do we get a frequency of 7.8125Hz from? We could try dividing down the 10MHz crystal oscillator frequency by 1,280,000, but there is no need to go to so much trouble. The display multiplex frequency is 500Hz, and the 7216A obtains this frequency by dividing the crystal oscillator frequency by 20,000. So all we need is an additional + 64 stage to divide the 500Hz and we have our 7.8125Hz reference. The 500Hz signal is derived from digit driver DB, filtered by a lOkn resistor and 0.0022µF capacitor, and buffered by transistors Q15 and Ql6. The signal then goes to the clock input (pin 1) of a 4024 CMOS 7-stage binary counter (IC7). The output of the sixth stage (pin 4) is the required 500Hz 64 = 7.8125Hz. Controlling the 7216A The various operating modes of the 7216A - ie, Frequency, Frequency Ratio and Period - are selected by connecting the Function Fig.10: parts layout and wiring diagram for the 1GHz DFM. Note that the lOµF capacitors on the display PCB must be installed so that they lie flat against the board. Take care with the mains wiring. input (pin 3) to digit drivers Dl, DZ or DB respectively. This job is performed by CMOS analog switches ICBa, 9a, 10a and 11a. These connect the 7216A Function input to DB, Dt, Dl or DZ when control lines Fl, FZ, F3 and F4 are high respectively. The Range selection works in similar fashion. In this case, Rt, RZ, R3 and R4 control analog switches IC8c, 9c, 10c and 1 lc. These connect the 7Z16A Range input (pin 14) to digit drivers D4, D3, DZ and Dl respectively, selecting the gating times described earlier. Note that the circuit diagram (Fig.5) published last month is in error here. Pushbutton switch Rl should be connected to pin 6 of ICBc, while R2 should be connected to pin 9 of IC9c (these connections are transposed on the circuit diagram). The PCB artwork and component layout drawings are correct. Decimal points In either Period or Frequency mode, the 7Z16A delivers a decimal point drive pulse for digits DZ, D3, D4 or D5. Unfortunately, the 7Z16A's decimal point driver cannot be used because we need to shift the decimal point one place to the right for functions F3 and F4 (100MHz and 1GHz ranges). We solved this problem by designing an external decimal point driver circuit consisting of tran- sistors Qt 7 and Q18 and a number of CMOS switches. Here's how the circuit works. Q17 and QlB are driven by the digit driver outputs of the 7Z16A via the CMOS switches. These digit driver signals are selected by ICBb, 9b, 10b or llb when Rt, RZ, R3 or R4 are high respectively. Similarly, ICBd, 9d, 10d and lld are selected by Ft, FZ, F3 and F4. Let's say, for example, that Fl and Rl have been pressed. ICBb and ICBd both close and the D5 output of the 7Z16A drives Qt7 and QlB, thus turning on the decimal point at digit 5. If RZ is now pressed ICBb opens and IC9b closes. The D4 signal now drives Q17 and Q18 to light the decimal point at digit 4. DECEMBER1987 79 Specifications Operational modes Period, Frequency to 1 0MHz, Frequency to 1 00MHz, Frequency to 1GHz Frequency range 1OHz-1 GHz ( 1. 7GHz typ.) in frequency mode; 1 0Hz-2.5MHz (5MHz typ .) in period mode Gating times 0.01, 0.1 , 1 & 10 seconds (10MHz & 100MHz ranges); 0 .128, 1.28, 12 .8 and 128 seconds (1 GHz range) Maximum Resolution 0.1 Hz (10MHz range); 1 Hz (100MHz & 1GHz ranges) ; 0.0001 µ,s (Period mode) Display High-brightness 8-digit LED display, overflow indicator, usec and kHz annunciators, leading zero blanking Sensitivity 1 MO input: better than 20mV RMS from 10Hz to 20MHz, rising to 70mV RMS at 100MHz (see Fig.6) 500 input: better than 1 0mV RMS from 70MHz to 1 GHz. Typical peak sensitivity is 50µ,V RMS at 1.2GHz (see Fig. 7) Input impedance 1 MO input: 1 M0// 10pF for signals less than 1 V p-p; 180k0//20pF for large amplitude signals 500 input: 300//1.5pF at 1GHz Overload 1 MO input: 250V RMS for frequencies up to 10MHz (short term only), dropping to 25V RMS at 100MHz 500 input: 300mV RMS Accuracy/stability Typical uncalibrated accuracy with a good quality crystal is ± 1 0 parts per million (ppm) at 25 °c with a temperature stability of ± 12 .5ppm from -20°C to +70°C. Prototypes were found to have a total warm-up drift of considerably less than 1 ppm Power requirements 240V AC, 50/60Hz Similarly, R3 and R4 select IClOb and IC11b to light the decimal points at D3 and DZ. When F3 or F4 is selected, the decimal point must be shifted one place further to the right. These digit driver lines have already been selected by the 7 216A Range selection switches (ICBc, 9c, 10c and 11c), so the output of this network is simply connected to the inputs of IClOd and 11d. These then pass the signal along to the decimal point driver circuit when either F3 or F4 is high. Pushbutton latching Control signals R1-R4 and F1-F4 are vital to the function of the entire circuit. They are obtained from a circuit which simulates the action of a bank of mechanically interlocked pushbutton switches. This greatly simplifies the physical construc80 SILICON CHIP tion and allows the use of low-cost, click-action, PCB-mounted switches. The latching circuit uses the ubiquitous 4017 CMOS decoded decimal counter as a simple "keyboard scanner". One 4017 is used for each bank of switches (IC12 and 13). Both 4017s are clocked continuously by the 500Hz clock derived from the multiplexed display. However, they are prevented from counting continuously by the 47k0 resistors which pull the CE-bar inputs (pin 13) high, inhibiting the clocking. Thus, if no buttons are depressed, the counters remain in their current state indefinitely. Suppose now that the "1" output (pin 2) of IC12 is high. If Fl is now pressed, this high is connected to CE-bar which is already high, and so nothing happens. In other words, once signal Fl has been latched, further pressing of the Fl button has no effect. Now suppose we press button F4. Initially, the "3" output (pin 7) is low, so CE-bar immediately goes low and enables the 4017 counter. Counting then proceeds from "1" to "2" to "3", but when the "3" output (ie, signal F4) goes high, CE-bar also goes high and stops the counter. F4 now remains high, even after we've stopped pressing the F4 button. All this happens very quickly, although you may sometimes just catch a glimpse of some of the function LEDs flashing briefly as the counter chases the selected push button. Power-on At power-on, we have arranged a "default" setting for the counter which selects the 10MHz Frequency mode, with the gating time set for one second. This is achieved by cannecting the F2 and R2 pushbuttons to the " O" outputs of the 4017s, and arranging for these ICs to be reset during power-up. This default setting is both desirable and necessary, since otherwise the counter could be powered on in virtually any mode. The 0.022µ,F capacitor and lMO resistor connected to pin 15 of IC12 and IC13 provide the power-on reset function. At switch-on, the 0.022µ,F capacitor pulls the Reset inputs high, thus resetting the counters. The capacitor then charges via the lMO resistor which pulls the Reset inputs low again after about 20ms. Diode DlO discharges the capacitor after switch off so that the circuit is ready for the next power-on reset cycle. Power supply The power supply is a straightforward transformer, bridge rectifier and 3-terminal regulator arrangement which provides a fixed + 5V output. The entire circuit is attached to the rear panel of the instrument case, in order to reduce heating of the 10MHz crystal and subsequent frequency drift. Ventilation holes are provided in the top and bottom of The rear panel carries the power supply components. Sleeve all mains connections to prevent accidental electric shock. Note that the mains earth wiring differs slightly from that shown here. the case to remove heat produced by the power supply. Warm air is vented from the top rear of the case, while fresh air enters the bottom of the case, just under the crystal, thereby minimising changes in crystal temperature during warm-up. If you follow our instructions for drilling and ventilating the rear panel and case, you can expect a frequency drift of less than 1 part-per-million, even with a cheap crystal. Putting it together Now let's build a frequency meter. Ease of construction has been a major design objective. The circuit is built on two self-contained modules: the power supply module, which slides into the rear panel slot; and the frequency meter module which slides into the case along with the attached front panel. The frequency meter module itself is built on two PCBs which are soldered together at right angles. These boards are the display PCB (code sc041-1187-1) and the counter PCB (code sc041-1187-2). The display PCB inserts into the third row of PCB slots in the case, behind the front panel. We '11 begin construction with the power supply assembly. If you haven't bought a kit, the first job is to prepare the rear panel. This is Close-up view of the power supply PCB showing how the 3-terminal regulator is mounted. Take care of the orientation of the four diodes. cut from 2mm thick aluminium sheet to the exact size shown in Fig.11. Round the corners very slightly so that it will slide freely into the slot at the rear of the plastic case. A row of 6mm holes across the case provides essential ventilation. You will also have to drill four 3mm holes for mounting the power transformer, power supply PCB and earth solder lug. Finally, a hole for the cord-clamp grommet is required in the lower right-hand corner of the panel. Take care in filing this hole - the grommet should compress the mains cord securely and it should not rotate. Power supply PCB The power supply PCB is coded sc041-1187-3 and measures 54 x 44mm. Solder all the parts onto the board as shown in Fig.10, but don't mount the 7805 3-terminal regulator at this stage. Make sure that the four diodes and the 2200µF electrolytic capacitor are correctly oriented. DECEMBER1987 81 CI ose-up view of the dispIay PCB . The " µ, sec"and "kHz" legends are made from film transparencies which are glued · to the LED bar modules. Once these parts have been installed, solder two short lengths (about 100mm) of hookup wire to the AC inputs, and install two PCB stakes at the + 5V and 0V outputs. Now for the 3-terminal regulator. The leads of the 7805 need to be pre-bent before it is soldered to the board. The first bend is 2.5mm from where the leads enter the plastic body. Bend the leads upwards 90 degrees, gripping the part of each lead closest to the body with a pair of needle-nosed pliers. Another 90-degree bend in the same direction can now be made about 3mm from the ends of the leads. Now solder the 7805 into the PCB. Finally, bend the leads over the edge of the PCB so that the body of the regulator sits under the board as shown in the accompanying photograph. The various items of hardware can now be installed on the rear 15 panel (see Fig.10) but first smear the underside of the 7805 regulator with thermal grease. You should also solder a 150mm-length of green hookup wire to a solder lug. Now mount the transformer on the panel, using 25mm-long screws. On the screw closest to the mains cord entry, fit a 2-way mains terminal block and secure using a washer and nut. On the other screw, fit a washer and a 5mm spacer then slip the PCB over the mounting screw and secure with a nut. The other side of the power supply PCB is secured with a screw through the rear panel, the 7805 regulator, the solder lug with the earthing wire, a 5mm spacer, the PCB and a nut in that order. Check that the PCB is reasonably level and that the regulator sits flat against the panel before tightening the screws. 33 80 194.5 1.6-2mm ALUMINIUM DIMENSIONS IN MILLIMETRES Fig.11: drilling details for the rear panel. 82 SILICON CHIP 45 Power supply wiring All mains voltage connections should be completely covered with heatshrink tubing or plastic sleeving. This is to avoid the possibility of accidental contact with any of the mains connections. The mains cord enters through the hole in the bottom right of the panel. First, strip back about 150mm of outer insulation [enough to reach the front panel switch), then secure the cord to the rear panel with the cord-clamp grommet. Terminate the active mains lead [brown) in the 2-way terminal block and the neutral lead [blue) direct to one of the transformer primary lugs. The other primary lug is connected via a short lead to the other side of the terminal block [see Fig.10). The earth lead (green/yellow) is connected to a solder lug bolted to the rear panel adjacent to the terminal block. Connect the earth lead from the 7805 regulator tab to this point also. Finally, connect the two lowvoltage AC leads from the PCB to the 0V and 7.5V lugs on the transformer secondary. The power supply can now be 'fired-up' and the + 5V rail checked. To do this, secure a 100 0.25W resistor in the terminal block in place of the front panel switch (ie, in series with the transformer primary). The resistor will act as a The counter PCB is soldered at rightangles to the display PCB. Don't forget the earth strap over the crystal and orient the trimmer so that its outer terminal goes to the + 5V rail. safety fuse should anything be amiss. Now plug-in, switch on and verify that the DC output from the PCB is close to + 5V. If you don't get the correct reading, switch off immediately and check for wiring errors. Be careful not to touch the 100 resistor during this procedure - its leads will be at 240V AC. Assuming all is well, switch off, pull the plug from the mains socket, and disconnect the 100 resistor. That completes the power supply assembly. Main counter PCB Before installing any parts on this board, carefully inspect the PC pattern for possible shorts or breaks in the copper tracks. Note also that some of the pads are quite close together, so use a fine-tipped soldering iron and take special care to avoid solder bridges. Apart from that, assembly of the main counter PCB is straightforward. The first job is to install the numerous wire links (0.6mm tinned copper wire is ideal). Once all the links are in place, the resistors can be installed, followed by the remaining components. Be sure to keep all component leads as short as possible. The transistors should be pushed down onto the PCB as far as they will comfortably go before soldering. Install two PC stakes for the power supply connections. Make sure that the transistors, electrolytics and ICs are all correctly positioned and oriented. The notch in the end of each IC, adjacent to pin 1, goes towards the rear of the PCB. The crystal is mounted flat against the PCB and is earthed using a U-shaped wire link. Note that the outer terminal of the trimmer should be connected to the + 5V side of the circuit. Display PCB The display PCB (sc041-1187-1) can now be assembled. As before, install the wire links first, followed by the resistors, diodes, trimpot VR1, and the capacitors. Note that the three 101,tF tantalum capacitors should have their leads bent so that the capacitor bodies lie flat against the PCB. The ICs can be installed next, then the transistors and pushbutton switches (R1-R4 and F1-F4). Be sure to orient the transistors correctly and check that the correct type is used at each location. The switches should be pushed into the PCB as far as they will go, with the flat side of each switch facing upwards. Now install the eight 7-segment LED displays. These must be mounted flush against the PCB. Check that the decimal point of each display is in the bottom right hand corner before soldering the pins. The two light bar modules (LED 2 and LED 3) can now be installed and adjusted so that they sit flush with the front surface of the adjacent 7-segment display. Finally, install two PC stakes on the back of the board in the LED 1 position, then install the five 3mm indicator LEDs. The latter should be stood off the PCB so that they align with the front of the 7-segment displays. Note that the cathode lead is the shorter of the two and is adjacent to the flat on the LED body. The two BNC input sockets should be left off the board for the time being. They are installed later, when the front panel is attached. That's all we have space for this month. Next month, we'll complete construction and describe the test procedure. tt DECEMBER1987 83