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Low-Cost 50MHz Frequency Meter; Mk.2 By JOHN CLARKE This update of our very popular compact 50MHz Frequency Meter now has an internal battery pack or can run from a DC plugpack supply. It also incorporates a 10kHz rounding mode to enable 36MHz R/C transmitters using pulse position modulation (PPM) to be measured with an unambiguous reading. 58  Silicon Chip siliconchip.com.au measure radio control transmitters, the modulation used will often result in an incorrect value. More information on this feature is detailed in an accompanying panel. As before, the design is easy to build, since it uses the programmed PIC microcontroller to perform all the complex logic. Apart from that, there’s an LCD readout, a couple of low-cost ICs, two transistors, the 3-terminal low dropout regulator and a few sundry bits and pieces. Note that although we have specified this Frequency Meter at 50MHz maximum, typical units will be capable of measuring frequencies somewhat higher than this. In fact, our prototype meter was good for measurements to above 64MHz. LCD readout S INCE MANY of our readers will not be familiar with the 50MHz Frequency Meter presented in the October 2003 issue, we are presenting the design in full. No doubt kitset suppliers will bring out the updated version of the kit but many readers will want to update a kit they have already built. This is easy to do because there are only a few circuit and hardware changes and the PC board itself is unchanged. As far as the circuit is concerned, the major change is in the PIC microcontroller. We have used a PIC16F628A instead of the originally specified PIC16F84P, because we needed a larger memory. Other changes include an LM2940CT-5 low dropout regulator instead of the 78L05, an additional toggle switch on the front panel and the aforementioned internal battery pack. In other respects, the circuit is unchanged. Frequency meters are used in virtually all areas of electronics and are invaluable for servicing and diagnostics. Among other things, they are ideal for checking the operation of oscillators, counters and signal generators. This unit is auto-ranging and displays the frequency in Hz, kHz or MHz. This makes it easy to read, as it automatically selects the correct range for any frequency between 0.1Hz and 50MHz and inserts the decimal point in the correct place for each reading. Provision for prescaler If you want to measure frequencies siliconchip.com.au Main Features • Compact size (130 x 67 x 44mm) • • 8-digit reading (LCD) Automatic Hz, kHz or MHz indicator units • Prescaler kHz, MHz and GHz indicator units • Three resolution modes including 10kHz rounding • • • • 0.1Hz resolution up to 150Hz 1Hz resolution up to 16MHz 10Hz resolution above 16MHz Battery or DC plugpack supply above 50MHz you will need a prescaler that divides the input frequency to a range that the frequency meter can accept. A good example is our UHF 1000:1 Prescaler, presented in the October 2006 issue. Accordingly, our updated version of the 50MHz Frequency Meter includes a prescaler switch which changes the units from MHz to GHz, kHz to MHz and Hz to kHz. As already mentioned, for radio control modellers, the 50MHz Frequency Meter Mk.2 can be set to display the reading in 10kHz steps for frequencies above 16MHz. This is an important feature because when a standard frequency meter is used to A 2-line 16-character Liquid Crystal Display (LCD) shows the frequency reading. This has several advantages over LED displays, including much lower current consumption. In addition, the LCD can show all the units without resorting to the use of separate annunciators, as would be required with a LED display. Resolution modes Three resolution modes are available: (1) a low-resolution mode with fast updates, suitable for most measurements; (2) a high-resolution mode for greater precision when required; and (3) the 10kHz rounding up feature. In low-resolution mode, the resolution is 1Hz for frequencies from 1-999Hz and 10Hz for frequencies above this. The corresponding display update times are 1s from 1-999Hz and 200ms from 1kHz-50MHz. High-resolution mode provides 1Hz resolution for frequencies from 150Hz16MHz. Above 16MHz, the resolution reverts to 10Hz. The display update time is 1s. Below 150Hz in the high-resolution mode, the display has 0.1Hz resolution and a nominal 1s update time for frequencies above 10Hz. This 0.1Hz resolution makes the unit ideal for testing loudspeakers, where the resonance frequency needs to be accurately measured. Note that the update time is longer than 1s for frequencies below 10Hz. The three resolution modes are selected by pressing the Resolution switch. The meter displays “LOW”, February 2007  59 Parts List 1 PC board, code 04110031 for Dick Smith Electronics version; code 04110032 for Altronics version; 04110033 for Jaycar version – 121 x 61mm 1 plastic case, 130 x 67 x 44mm 1 front panel label to suit version, 125 x 64mm 1 LCD module (DSE Cat. Z 4170, Altronics Cat. Z 7000A or Jaycar Cat. QP 5515) 1 SPST toggle switch (S1) 1 pushbutton momentary contact switch (S2) 1 miniature SPDT toggle switch (S3) 1 panel-mount BNC socket 1 low-drift 4MHz crystal (Hy-Q HC49/U 4000.00kHz GG03E) (X1) 1 PC-mount 2.5mm DC socket 1 18-pin dual-wipe contact DIP socket (for IC3) 1 28-pin dual-wipe contact DIP socket (for DSE & Altronics LCD modules; see text); OR 1 14-pin dual-wipe contact DIP socket (for Jaycar LCD module) 1 14-way SIL pin header for the LCD sockets 4 M3 x 10mm countersunk screws 4 M3 nuts 4 M3 x 6mm cheesehead screws 4 M3 x 12mm tapped Nylon spacers 4 M3 Nylon washers 10 PC stakes 1 300mm length of 0.7mm tinned copper wire “HIGH” or “LOW 10kHz<at>>16MHz” to indicate which mode is currently selected. In addition, the selected resolution mode is stored in memory and is automatically restored if the meter is switched off and on again. In the 10kHz rounding mode, the frequency is rounded up to just show the next 10kHz frequency band for frequencies between 16MHz and 50MHz. When the display is showing frequency rounding the second line of the display indicates this with a “(10kHz Rounding)” indication. In low-resolution mode, the display will show 0Hz if the frequency is below 1Hz. By contrast, in the high60  Silicon Chip 1 60mm length of 75W coax 1 100mm length of hookup wire 1 1kW horizontal trimpot (code 102) (VR1) 1 10kW horizontal trimpot (code 103) (VR2) Semiconductors 1 MC10116N triple ECL differential line receiver (IC1) 1 74HC132 quad Schmitt trigger (IC2) 1 PIC16F628A/P microcontroller programmed with freqenc2. hex (IC3) 1 LM2940CT-5 low dropout regulator (REG1) 1 2N5485 N-channel VHF JFET (Q1) 1 BF450 PNP transistor (Q2) 3 BAW62 diodes (D1-D3) 1 1N4004 1A diode (D4) Capacitors 2 100mF 16V PC electrolytic 3 10mF 16V PC electrolytic 1 470nF MKT polyester 1 100nF MKT polyester 8 10nF ceramic 1 470pF ceramic 1 33pF NP0 ceramic 1 22pF ceramic 1 10-60pF trimmer (VC1) Resistors (1%, 0.25W) 1 910kW 7 470W 1 100kW 1 330W 1 47kW 4 100W 2 10kW 1 15W1W (optional) 2 2.2kW resolution mode, the display will show “No Signal” for frequencies below 0.1Hz. If the frequency is below 0.5Hz, the display will initially show an “Await Signal” indication before displaying the frequency. If there is no signal, the display will then show “No Signal” after about 16.6s. The 0.1Hz resolution mode for frequencies below 150Hz operates in a different manner to those measurements made at 1Hz and 10Hz resolution. Obtaining 0.1Hz resolution in a conventional frequency meter normally means measuring the test frequency over a 10s period. And that means that the update time is slightly longer than 10s. This is a long time to wait if you are adjusting a signal generator to a precise frequency. However, in this frequency meter, the display update period is 1s for frequencies above 10.0Hz, increasing gradually to 10s for frequencies down to 0.1Hz. So for normal audio frequencies, the display will update at 1s intervals. Just how this is achieved is explained below, when we discuss the block diagrams for the unit. The Prescaler switch causes the display to show the prescaler units in the LOW and HIGH resolution selections. When selected, the words “Prescaler units” are shown on the second line of the LCD. The prescaler units feature is not available for the 10kHz rounding feature because it is not required and would confuse the reading. Block diagrams Fig.1 shows the general arrangement of the frequency meter. It’s based mainly on the microcontroller (IC3). In operation, the input signal is processed and applied directly to a divide-by-256 prescaler inside IC3. The divided signal then clocks timer TMR0 which counts up to 256 before clocking Register A, an 8-bit register that counts up to 256 before returning to zero. Combining all three counters (the prescaler, TMR0 and register A) allows the circuit to count up to 24 bits, or a total of 16,777,216. By counting over a 1s period, it follows that the unit can make readings up to about 16.7MHz. However, if the frequency is counted over a 100ms period, the theoretical maximum that can be measured is just over 167MHz. As shown in Fig.1, the input signal is amplified (by Q1, IC1 & Q2) and fed to gating stage IC2a. This drives clocking stage IC2b which is controlled by IC3’s RA3 output. Normally, IC2b allows the signal to pass through to the prescaler at IC3’s RA4 input. IC3’s RB2 output controls gating stage IC2a so that signal passes through for either a 100ms period or a 1s period. During the selected period, the signal frequency is counted using the prescaler, timer TMR0 and register A. Initially, the prescaler, the timer and register A are all cleared to 0 and the RB2 output is then set to allow the input signal to pass through to the prescaler for the gating period (ie, for 100ms or 1s). siliconchip.com.au Fig.1: the block diagram of the 50MHz Frequency Meter for “normal” frequency measurements. The incoming signal is first amplified, then fed through a gating circuit to clocking stage IC2b. This then drives a divide-by-256 prescaler inside PIC microcontroller IC3 (ie, at the RA4 input). Fig.2: this is the alternative configuration for making high-resolution (ie, to 0.1Hz) measurements below 150Hz. In this case, the input signal is applied to the RA4 input as before. However, the prescaler is no longer clocked by the RA4 input but by an internal 1MHz clock instead. During this period, the prescaler counts the incoming signal applied to RA4. Each time its count overflows from 255 to 0, it automatically clocks timer TMR0 by one count. Similarly, whenever the timer output overflows from 255 to 0, it sets a Timer Overflow Interrupt Flag (TOIF) which in turn clocks Register A. At the end of the gating period, IC3’s RB2 output is cleared, thus stopping any further signal from passing through to the prescaler. The value of the count in TMR0 is now transferred to Register B. Unfortunately, the value in the prescaler cannot be directly read by IC3 and so we need to derive the value. This is done by first presetting register C with a count of 255. That done, the RA3 output is taken low to clock the prescaler and timer TMR0 checked siliconchip.com.au to see if it’s count has changed. If TMR0 hasn’t changed, the prescaler is clocked again with RA3. During this process, register C is decreased by 1 each time the prescaler is clocked. The process continues, with RA3 clocking the prescaler until timer TMR0 changes by one count. When this happens, it indicates that the prescaler has reached its maximum count. The value in Register C will now be the value that was in the prescaler at the end of the counting period. The processing block now reads the values in registers A, B and C. Based on this information, it then decides where to place the decimal point and whether to show Hz, kHz or MHz. The required value is then written to the LCD via the data and control lines (RB4-RB7 and (RA0-RA2). For the Prescaler units selection, the Hz units are shown as kHz, the kHz units are shown as MHz and the MHz units are shown as GHz. In the 10kHz rounding mode, frequencies above 16MHz are rounded up to the next 10kHz band. So for example a 36.44659MHz signal is rounded up to 36.450MHz. Alternative configuration If the input signal frequency is greater than 16MHz and the gating period is 1s, register A will initially have overflowed. In this case, the gating period is automatically changed to 100ms. Alternatively, if the high-resolution mode is selected and the frequency is below 150Hz, the frequency meter changes its configuration to that shown in Fig.2. In this case, the input signal is applied to the RA4 input as before. February 2007  61 Specifications • Input sensitivity: typically less than 20mV RMS from 1Hz to 100kHz rising to 50mV at 20MHz and 85mV at 50MHz. • • • • • Input Impedance: 1.1MW in parallel with about 10pF Frequency range: 0.1Hz to 50MHz or better Untrimmed accuracy: ±20ppm equivalent to 1000Hz at 50MHz Trimmed accuracy: ±10ppm from -20°C to 70°C Resolution: High Resolution Mode: 0.1Hz from 0.1-150Hz; 1Hz from 150Hz-16MHz; 10Hz from 16-50MHz. Low Resolution Mode: 1Hz from 1-999Hz; 10Hz from 1kHz-50MHz • Update time (approx.): 200ms for 10Hz resolution; 1s for 1Hz resolution; 1s for 0.1Hz resolution down to 10Hz, increasing to 10s at 0.1Hz • Display units: Hz from 0.1-999Hz; kHz from 1-999.999kHz; MHz from 1-50MHz • Current consumption: 65mA with 7.5-12V input Fig.3 (right): the circuit is based on microcontroller IC3. This processes the signals from the preceding amplifier stages and drives the LCD. Power comes either from a 9-12V DC plugpack or from a 7.5V battery. at the source. This loss is more than compensated for in the following amplifier stages. Next, the signal is AC-coupled to pin 4 of amplifier stage IC1a via a 100mF electrolytic capacitor and a parallel 10nF capacitor. The 100mF capacitor is sufficiently large to allow for a low frequency response of less than 1Hz. However, this capacitor loses its effectiveness at higher frequencies due to its high internal inductance and the signal is coupled via the 10nF capacitor instead. Differential line receivers However, the prescaler is no longer clocked by the RA4 input but by an internal 1MHz clock. Basically, what happens is that the RA4 input is monitored for a change in state – ie, from a low voltage to a high voltage – which indicates a signal at the input. When this happens, the prescaler is cleared and begins counting the 1MHz internal clock signal. The overflows from the prescaler and timer TMR0 are carried to register A as before. Counting continues until the input signal goes low and then high again, at which point counting stops. If the counting causes register A to overflow, then the display will show no signal (this will happen after 16.7s if the signal does not go low and high again). Conversely, if the counting is within range, the prescaler value is determined by clocking IC2b using the RA3 output as before. From this, it follows that if the input frequency is 1Hz (ie, a 1s period), the value in the A, B and C registers will be 1,000,000. That’s because the prescaler is clocked at 1MHz for 1s. Similarly, the count will be 100,000 for a 10Hz signal and 10,000 for a 100Hz input signal. Finally, the value in the registers is divided into 10,000,000 and the decimal point placed immediately to the left of the righthand digit. This gives a direct readout in Hz with 0.1Hz resolution on the LCD. 62  Silicon Chip This technique cannot be used for measuring very high frequencies because the value in the counter becomes smaller as the frequency increases and so we begin to lose accuracy. For example, at 500Hz, the counted value would be 2000 and at 500.1Hz the counted value would be 1999. The result of the division of 1999 into 10,000,000 would be 500.2 instead of the 500.1 required. The 0.1Hz resolution has therefore been restricted to readings below 150Hz to ensure accuracy of the calculation. Circuit details Refer now to Fig.3 for the full circuit details. The input signal is AC-coupled to the unit via a 470nF capacitor to remove any DC component. This signal is then clipped to about 0.6V peak-to-peak using diodes D1 & D2, with current limiting provided by the 100kW series resistor. The 22pF capacitor across the 100kW resistor compensates for the capacitive load of the diodes. From there, the signal is fed to the gate of Q1, a 2N5485 JFET. This transistor provides high input impedance, which is necessary to ensure a wide frequency response. Q1 is self-biased using a 910kW resistor from gate to ground and a 470W source resistor. It operates with a voltage gain of about 0.7, which means that the signal is slightly attenuated IC1a is one of three differential line receivers in an MC10116N IC. It’s biased via the DC output at pin 11 and this is decoupled using a 10mF electrolytic capacitor and a paralleled 10nF ceramic capacitor. The voltage is then applied to the wiper of trimpot VR1 (Offset Adjust) and this allows adjustment of the input bias voltage. In operation, IC1a is run open-loop (ie, without feedback) so that it provides as much gain as possible. Even so, it only operates with a voltage gain of about seven times. It’s differential output signals appear at pins 2 & 3 and are applied to the differential inputs (pins 12 & 13) of IC1b. Note that the differential outputs have 470W pull down resistors, as they are open emitters. In fact, the MC10116 IC is an emitter-coupled logic (ECL) device. Unlike IC1a, IC1b has negative feedback provided by the two associated 100W resistors. This reduces the gain of this stage to just below two. The third stage using IC1c employs positive feedback and so it functions as a Schmitt trigger rather than as an amplifier. Its hysteresis is around 450mV and this means that the signal swing on its differential inputs must be greater than this in order to provide an output. In operation, the output signal at pins 6 & 7 swings from 4.3V when high to 3.4V when low. This needs to be level-shifted to provide normal CMOS input levels to the gating cirsiliconchip.com.au siliconchip.com.au February 2007  63 Fig.4: follow this layout diagram to build the Altronics version. Note that trimmer capacitor VC1 mounts on the track side of the board – see photos. 3 output remains high and the input signal is blocked. So, in summary, the signal is allowed through to IC2b when RB2 is high and is blocked when RB2 is low, as described previously. IC2b normally has its pin 5 input held high via IC3’s RA3 output, so that the signal from IC2a is again inverted at pin 6. When RB2 is brought low, pin 3 of IC2a remains high and so pin 4 of IC2b is also high. This allows RA3 to clock the RA4 input via IC2b. Fig.5: this is the layout to follow if you are building the Jaycar version. Fig.6: this is the layout for the Dick Smith Electronics version. Note that DSE may not be offering a kit for the new Mk.2 unit. However, this layout lets you upgrade existing Mk.1 versions of the DSE kit. cuit (IC2a) and this is done using PNP transistor Q2. IC2a is a Schmitt NAND gate. It inverts the signal on its pin 1 input 64  Silicon Chip when pin 2 is held at +5V by IC3’s RB2 output (ie, the signal passes through to the pin 3 output but is inverted). Conversely, when RB2 is at 0V, IC2a’s pin Driving the LCD IC3’s RA0-RA2 outputs drive the control inputs to the LCD module and select the line and the position of the character to be displayed. Similarly, RB4-RB7 drive the data inputs (DB4DB7) on the LCD module. A 470pF capacitor on the E-bar (enable control line) is included to slow down the rise and fall times of the square wave from IC3. A 4MHz crystal connected between pins 15 & 16 of IC3 provides the clock signals for IC3. The recommended crystal has low drift but a standard 4MHz crystal could be used if accuracy is not critical. The capacitors at pins 15 & 16 provide the necessary loading for the crystal, while VC1 allows the clock frequency to be “tweaked” slightly to provide calibration. Power supply Power for the circuit is derived from either a 9-12V DC plugpack or a 7.5V battery made up using five AA cells. siliconchip.com.au The LCD module is secured to the lid of the case using four M3 x 6mm cheesehead screws, four M3 nuts and four M3 x 10mm tapped Nylon spacers. Make sure that all polarised parts on the counter board are correctly orientated. You can choose to operate from batteries or a DC supply but not both. Diode D4 protects the circuit against reverse polarity protection when using a plugpack supply, while regulator REG1 provides a +5V supply rail to power the circuit. The specified regulator is a low dropout type so that the meter will still operate when the batteries have dropped to 5V. If a battery is used, it connects to the cathode side of D4; ie, it bypasses the reverse polarity protection. This means that D4 can be left out of circuit (along with the DC socket) if the unit is to be battery powered. If you wish to use rechargeable cells, then it is recommended to use an extra cell to obtain more voltage. In this case you could replace D4 with a 15W 1W resistor to enable charging. Make sure you get the polarity correct. If you are concerned about polarity, a Schottky diode (1N5819) could also be included in series with the resistor. LM2940CT-5 low dropout regulator. In addition, you will need to drill an extra hole in the front panel to accommodate the additional switch. Each LCD plugs directly into its intended PC board, which means that there are no external wiring connections except to the BNC input socket, Construction The SILICON CHIP 50MHz Frequency Meter Mk.2 can be made in one of three versions, depending on where you buy the kit. That’s because the LCD modules available from Dick Smith Electronics (DSE), Altronics and Jaycar are all different and so a different PC board has been designed to suit each module. These boards are coded 04108031 (DSE), 04108032 (Altronics) and 04108033 (Jaycar). If you are buying a kit, make sure you get the updated version and not the original version described in October 2003. If you decide to purchase the earlier kit or you are modifying an existing kit, you will need a new programmed PIC16F628A, a miniature SPDT toggle switch and an Table 2: Capacitor Codes Value 470nF 100nF 10nF 470pF 33pF 22pF mF code EIA Code 0.47mF 474 0.1mF 104 .01mF 103 NA 471 NA   33 NA   22 IEC Code 470n 100n   10n 470p   33p   22p Table 1: Resistor Colour Codes o o o o o o o o o o siliconchip.com.au No.   1   1   1   2   2   7   1   4   1 Value 910kW 100kW 47kW 10kW 2.2kW 470W 330W 100W 15W 4-Band Code (1%) white brown yellow brown brown black yellow brown yellow violet orange brown brown black orange brown red red red brown yellow violet brown brown orange orange brown brown brown black brown brown brown green black brown 5-Band Code (1%) white brown black orange brown brown black black orange brown yellow violet black red brown brown black black red brown red red black brown brown yellow violet black black brown orange orange black black brown brown black black black brown brown green black gold brown February 2007  65 Fig.7: this diagram shows how the unit is installed inside the case. Be sure to use Nylon spacers and washers where indicated. switch S3 and to the battery holders. The unit is housed in a plastic case measuring 130 x 67 x 44mm, with the LCD module protruding through a cutout in the front panel. The Dick Smith version has the power switch on the righthand side and the signal input applied to the BNC socket at the top left of the box. By contrast, both the Altronics and the Jaycar versions have the power switch at the top left, while the input socket is mounted on the top right of the box. This difference comes about because the display readout for the DSE LCD module is upside down compared to the other two modules in relation to the input terminals. Note that the unit shown in the photos is the Jaycar version but both the Altronics and DSE modules were fully tested. Figs.4-6 shows the PC board layouts for the three versions. Begin by check- ing that you have the correct PC board for the LCD module you are using. That done, check the mounting holes for the LCD module against those on the PC board (the holes must be 3mm in diameter). Check also that holes are large enough to mount switch S2 and the DC input socket. Next, install all the wire links and resistors, using the accompanying resistor colour code table as a guide to selecting each value. It’s also a good idea to check the resistors with a digital multimeter just to make sure. IC1 and IC2 can go in next, taking care to ensure that they are correctly oriented. Then install a socket for IC3 but don’t install the microcontroller yet. The diodes and capacitors can now all be installed, followed by REG1 and transistors Q1 & Q2. Note that REG1 mounts using PC stakes and is mounted horizontally to cover IC3. Note also that some of the parts must sit with their bodies parallel to the PC board. These include crystal X1, its adjacent 470pF capacitor and the 10mF capacitor adjacent to switch S1 on the Jaycar version (so it doesn’t later foul S3), plus the 10mF and 100mF capacitors on the Altronics version (so that they don’t foul the LCD module). It’s just a matter of bending their leads at right angles before installing them on the board. Similarly, the top of transistor Q2 must be no higher than 10mm above the PC board so that it doesn’t interfere with the LCD module (all versions). The next step is to install the socket for the LCD module. Both the DSE and Altronics versions use a 28-pin DIL IC socket that is cut in half to obtain a 14-way strip socket which is then soldered in place. By contrast, the Jaycar Here’s another view of the completed PC board. Note how regulator REG1 is mounted face down over IC3 and the adjacent 100nF capacitor, with its three leads soldered to PC stakes. 66  Silicon Chip siliconchip.com.au version uses a 14-pin IC socket which is cut into two 7-way strips which are then installed side-by-side. Once the sockets are in, install PC stakes for the “+” and “-” supply connections (near D4) and for the signal input and GND connections. These PC stakes should all be installed from the copper side of the board. PC stakes are also used to mount switch S1. These should be trimmed so that when the switch is mounted, its top face is 20mm above the top surface of the PC board. Be sure to orient S1 with its flat section facing towards the right, as shown in Figs.4-6. The remaining parts can now be installed on the board. These parts include switch S2, the DC socket, trimpots VR1 & VR2, crystal X1 and trimmer capacitor VC1. Note that VC1 is mounted on the underside of the PC board, so that it can be adjusted without having to remove the LCD module. Front panel The front panel (ie, the case lid) must be drilled and a cutout made to accommodate the three switches and the display. However, if you have purchased a kit, then you probably won’t have to worry about this. It will also be necessary to drill the mounting holes for the LCD module. Note that these should be countersunk so that the intended screws sit flush with the surface of the lid – see Fig.7. That done, the adhesive label can be attached to the panel with the cut-outs made using a utility knife. Kit versions are supplied with screen-printed labelling. In that case, countersunk screws will not be necessary. BNC SOCKET NYLON WASHER NYLON WASHER TRIMMER VC1 The PC board is secured by plugging it into the matching header pins on the LCD module and installing four screws to fasten it to the spacers. Note the Nylon washers under the top two screw heads – these are necessary to prevent shorts to adjacent tracks. The inset at top-left shows an enlarged view of VC1. Now press the Resolution switch – the display should show “HIGH”. It should then show “Await Signal” when the switch is released. If the switch is then pressed again, the display should show “LOW”. A third press will bring up the LOW 10kHz<at>>16MHz mode. Final assembly Refer to Fig.7 for the final assembly details. As shown, the LCD module is secured to the case lid using four M3 x 10mm CSK screws, four M3 nuts (used as spacers) and four 12mm-long tapped Nylon spacers. The PC board is then secured to the bottom ends of the four spacers. Use Nylon washers for the underside of the PC board to prevent shorting any tracks with the screws (see above photo). You will have to drill a 9mm dia­ Testing Now for an initial test before IC3 or the LCD are plugged in. Apply power and check that +5V is present on pin 16 of IC1, pin 14 of IC2 and pins 4 & 14 of IC3. If this is OK, disconnect power and install IC3 in its socket, taking care to ensure it goes in the right way around. Plug the LCD module into its matching socket and temporarily fit a couple of 12mm tapped Nylon spacers to support it on the PC board. Apply the power again and check that the display shows either 1Hz or 0Hz. If not, adjust VR2 for best display contrast. VR1 should be adjusted so that the display shows 0Hz when the signal input terminals are shorted. siliconchip.com.au Fig.8: the two battery holders (4 x AA and 1 x AA) must be wired in series as shown here. Add an extra AA holder if you are using rechargeables. February 2007  67 Checking The Frequency Of Radio Control Transmitters W HEN MODEL ENTHUSIASTS get together they often want to fly their radio-controlled aeroplanes (or drive their radio-controlled cars or boats) in a competition-based meet. With so many radio controls being used they must each operate on a different band to avoid interference between the controllers. Having a device that can immediately check each transmitter’s operating frequency is a great asset because it can tell immediately if there is going to be a frequency conflict. In that case, they can change the crystal frequency on one of the transmitters and for its receiver. Radio transmitters operate on the 27MHz band, 29MHz band, 36MHz band and 40MHz band. However, the synthesised modules (crystal controlled) are only available on the 36MHz band and this is by far the most popular band. On this band, there is PPM (pulse position modulation) and PCM (pulse code modulation) used for the transmission. With PCM the frequency reading on a meter will be correct since the modulation is symmetrical and the frequency swings will average out. For PPM the frequency reading on a meter will be a few kHz low because of the asymmetrical dwell times on the high low parts of the modulation. The PPM frequency reading can be most confusing at times. To understand why let’s consider an example. The 36MHz band runs in 10kHz steps from 36.010MHz to 36.590MHz. If we have a crystal in the transmitter that is set at 36.450MHz, the reading on a standard frequency meter will show a lower value at say, 36.44646MHz. But with our frequency meter in 10kHz rounding mode, it converts the 36.44646MHz count to 36.450MHz. There is no need to connect the RC transmitter directly to the frequency meter for these readings. Just bring the transmitter’s antenna close to an antenna that’s connected to the meter as shown in one of the photos. We made our antenna from an old Ethernet cable. Just cut the cable so that you have a length of 200mm or so from the BNC socket, then strip off the other sheath insulation and the woven shield, leaving just the inner insulated wire. This can then be covered with a length of heatshrink sleeving to provide extra stiffening and protection. The battery holders are attached to the bottom of the case using epoxy adhesive. 68  Silicon Chip meter hole in one side of the box to provide access to the DC socket if you are powering the unit from a plugpack. This hole should be positioned midway along one side and about 10mm down from the top edge of the case. If the unit is to be battery-powered, you will need to solder the battery leads to the supply PC stakes on the underside of the board. The batteries can be secured to the bottom of the case by mounting them in suitable holders. We used a 4 x AA holder and a single AA holder – see Fig.8. Use an extra AA holder if you are using rechargeables. The BNC input socket is mounted in a slot in the top side of the case (see photo below) and is wired using 75W cable to the two signal input PC stakes on the underside of the PC board. In practice, the slot must be made so that the socket can be either slid in or out, along with the PC board and display assembly. We made the slot just wide enough to allow the “flat” side of the BNC socket to fit. This prevents the socket from turning in the slot when an input connector is attached. Finally, switch S3 is wired to its terminals as shown using hookup wire. Calibration The completed 50MHz Frequency Meter can be calibrated against the 15.625kHz line oscillator frequency in a colour TV set. Fortunately, you don’t need to remove the back of the set to do this. Just connect a long insulated wire lead to the input socket and dangle it near the back of the TV set. It’s then just a matter or adjusting VC1 so that the meter reads 15.625kHz when the resolution is set to “High”. Note: the TV must be showing a PAL program, not NTSC (15.750kHz). If there is insufficient adjustment on VC1 to allow calibration, the 33pF capacitor at pin 15 of IC3 can be altered. Use a smaller value if the frequency reading is too high and a larger value if the frequency reading is too low. Usually, the next value up or down from 33pF will be sufficient; use either 27pF or 39pF. If you require greater accuracy, the unit can be calibrated against the standard 4.43MHz colour burst frequency that’s transmitted with TV signals. The best place to access this frequency is right at the colour siliconchip.com.au Fig.9: this front-panel artwork suits both the Altronics and the Jaycar versions. Fig.10: use this artwork to upgrade an existing Dick Smith Electronics Mk.1 version. burst crystal inside a colour TV set. This crystal will usually operate at 8.8672375MHz (ie, twice the colour burst frequency), although some sets use a 4.43361875MHz crystal. Be warned though: the inside of a colour TV set is dangerous, so don’t attempt to do this unless you are an experienced technician. There are lots of high voltages floating around inside a colour TV set and you could easily electrocute yourself if you don’t know what you are doing. In particular, note that much of the circuitry in a switchmode power supply circuit (as used in virtually all late-model TV sets) operates at mains potential (ie, many of the parts operate at 240VAC). In addition, the line output stages in some TV sets also operate at mains potential – and siliconchip.com.au that’s in addition to the lethal EHT voltages that are always present in such stages. Note too that some TV sets (particularly older European models) even have a “live” chassis, in which all the circuitry (including the chassis itself) operates at mains potential (ie, 240V AC). Usually, there will be a label on the back of the set advising of this but don’t take it for granted. Don’t even think of messing about with this type of set. In short, don’t attempt the following calibration procedure unless you are very experienced and know exactly what you are doing. Assuming that you know what you are doing (and the set has a grounded chassis), you will need to make up an insulated probe with a 10MW resistor in series with the input plus a ground lead. This probe is then connected to one side of the colour burst crystal and VC1 adjusted so that the meter reads either 8.867237MHz or 4.433618MHz (resolution set to high mode). Make sure that the probe has no effect on the colour on the TV screen when it is connected to the colour burst crystal. If it does, it means that the probe is loading the crystal and altering its frequency. In that case, try connecting the probe to the other terminal of the crystal. That’s it – your new 50MHz Frequency Meter Mk.2 is now calibrated SC and ready for action. Footnote: a complete kit of parts for the 50MHz Frequency Meter MK.2 is available from Jaycar Electronics (Cat. KC-5440). February 2007  69