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6GHz+ Touchscreen Frequency and Period Counter Having described our new 6GHz+ Touchscreen Frequency/Period Counter in the first article (October) and then built and tested it (November), we shall now show how to use it and explain what it can do. Apart from its very wide frequency range, it offers outstanding accuracy. Part 3: by Nicholas Vinen A went well, your unit should be operaa few small tweaks as the software has ssuming you’ve managed to tional. The rest of this article explains been finalised. source the components for how to use the software and its touchThere is information shown in each the Frequency Meter (most of screen interface. corner of the screen, plus the large frewhich are available from either the quency/period display in the centre. SILICON CHIP Online Shop or Digi-Key) Main screen display The frequency/period is auto-ranging and successfully put it together, you Pretty much all the functions of the with frequency using units of mHz can then program the Explore 100 with Frequency Counter are available on (millihertz, ie, 1/1000th of one hertz), the software. the one main screen, shown in Fig.5. Hz, kHz, MHz or GHz and period havWe don’t supply the PIC32 pre-proThis is similar to the prototype screen ing units of ps, ns, µs, ms or s. grammed with the BASIC code beshown in the last two articles but with You can switch between frequency cause the Explore 100 provides a USB and period disinterface that makes play by touching loading it quite easy. the centre of the The PIC32 which screen. is supplied in our ExChanging beplore 100 kit (or the tween frequency one from Rictech in and period disNew Zealand) does play does not afalready have the fect the way the MMBasic firmware measurement is loaded. So you just being taken; both need to connect it to readings are calyour PC, download culated based on the software from the number of our website (free for pulses received subscribers) and load from the referit into the Micromite ence clock and Plus chip. the input signal The procedures for in a given period. doing that, as well as The frequency setting up the LCD is simply calcutouchscreen, were Fig.5: the default main screen, showing the frequency reading in large digits at lated as Fin/Fref given in last month’s the centre and various additional information below that, and in the corners of while the period article. the display. To change the settings in the corners, it’s generally just a matter of is Fref/Fin. Assuming that all touching that area of the screen. 84 Silicon Chip Celebrating 30 Years siliconchip.com.au Note that all settings, including this one, are retained in Flash memory automatically so that the configuration is retained for the next time the unit is powered up. Accuracy and precision estimate display Another indication of reading accuracy is the fact that the last couple of decimal places in the reading may be dimmed, indicating that they have a degree of uncertainty and even with a stable signal, you may see these digits fluctuate. If averaging is active then over time, the reading will become more certain and these digits will become lighter. With a stable signal, white digits should be quite stable. these update rates by touching the update line near the lower right-hand corner of the screen. The update rate is independent of the averaging setting. Say you select 30s averaging with a 2s update rate. You will get a reading after two seconds but it will only be based on two seconds of data. Then you will get a reading two seconds later which will be slightly more accurate (and the accuracy and precision figures will reflect this). The time span over which the signal has averaged so far is shown in parentheses ( ) at the end of the Mode line. The reading accuracy will continue to improve until the 30-second mark, at which point the precision and accuracy figures will not improve. The reading will continue to change though, representing the average signal frequency over a time “window” spanning the last 30 seconds. In other words, the displayed value is a moving average. If the signal frequency changes, you would have to wait 30 seconds for the new reading to be accurate. Alternatively, you can simply touch at the end of the Mode line, where the averaging time so far is displayed, to reset it to zero and start the averaging window anew. To change the maximum window (ie, averaging) time, simply touch the left side of that line instead. This will cycle through a series of different time values from one second up to ten minutes. To turn averaging off, you can keep pressing this until you get back to the “immediate” setting or alternative, to save time, hold your finger on the Mode line for a couple of seconds. Regardless of what is being displayed, the precision and accuracy estimates are shown below. Precision indicates repeatability, ie, if you Input switching measured the same exact signal using the same settings on two different ocThe current input is shown in the casions, this is the maximum differlower left-hand corner of the screen ence you could get between the two and you can switch inputs simply readings. by touching it. Make sure you press This relates to the stability of the far enough down the screen that you reference oscillator and how its frearen’t pressing the Mode line above; quency changes over time and with changing mode will be explained temperature. shortly. It’s computed based on the reference Mode switching is simple since it clock tolerance and measurement pejust toggles between the SMB (high riod and shown as both the parts per frequency) input and the BNC (low fremillion/billion error and a frequency quency) input. If you’re using averagor period uncertainty. ing, it will reset when changing inputs. When using averaging, the uncerThe SMB input impedance is fixed tainty will drop over time until it at 50Ω but the BNC input impedance reaches a minimum value, once the can be switched between 75Ω and programmed averaging time period about 1MΩ. This can be changed simhas passed. ply by touching that part of the Mode The accuracy shown automatically line when the BNC input is selected improves quite dramatically if you’re and like the other settings, it is reusing GPS disciplining since this will tained even when power is lost. allow the unit to compensate almost Update rate and averaging entirely for long-term drift (since GPS timekeeping is much more stable) and The range of update rates has been temperature drift will also be reduced expanded to include one update every (but not eliminated). three, two or one second or five times The accuracy figure is shown in a per second. You can cycle through similar manner but this also takes into account the initial error in the reference oscillator frequency. This can be reduced if you have a more accurate reference source to calibrate the TXCO. When using GPS disciplining, the accuracy figure will generally match the precision figure (or come close) since the accuracy provided by the GPS Fig.6: a similar display but this time with the output shown as a period rather time signal is ex- than a frequency, and with averaging enabled. Most of the operation and interaction with the unit is done via this screen. cellent. siliconchip.com.au Celebrating 30 Years Changing the display brightness To change the LCD backlight display brightness, press and hold your finger on the lower right-hand corner of the screen, where the current brightness percentage is December 2017  85 displayed. While still pressing on the screen, swipe your finger up or right to increase the brightness, or left or down to decrease the brightness. Because you’re starting in the lowerright corner, it’s easiest to swipe up to increase and left to decrease. But if you swipe up and increase the brightness too much, you can go either down or left to bring it back to the desired value. Reducing the brightness to the minimum will drop power consumption by around 200-250mA compared to maximum brightness. The estimated current drawn from the DC supply for a given configuration is shown in the upper-left corner of the screen. Frequency reference calibration There are three ways to do this. The first is the simplest but needs to be done with the case open and requires an accurate frequency meter. It needs to be more accurate than the one you are calibrating, eg, around 1ppm or ±0.0001% accuracy or better. Measure the frequency at pin 9 of the Explore 100 header, relative to pin 1 (ground). Then press on the TCXO frequency at upper-left and hold your finger down for a couple of seconds, then lift it. A keypad will appear and you can enter the precise TCXO frequency in Hz. It will then ask you for a second figure, the accuracy of your frequency meter, in ppb (parts per billion). 1ppm = 1000ppb = 0.0001%. This is used to provide the estimated precision and accuracy figures when making a measurement. If you don’t know, abort entering this number and the default value for an uncalibrated TCXO will be used, but the calibration itself will still be performed. The new figures will be stored and displayed but you can recalibrate again at a later date if necessary. The second option can be done with the case closed and all you need is an accurate frequency source. For example, you could use the 10MHz reference output from another piece of test equipment. Make sure the TCXO frequency is set to the default value of 16.368MHz; if not, set it using the above procedure. Feed the signal in and measure its frequency with reasonably long averaging (eg, one minute). Take note of the figure shown on the screen. Let’s call it Fmeas and the expected frequency Fexact. Now perform the following calculation, with all values in Hz: TCXO = 16368000 x Fexact / Fmeas You can now program the resulting figure in as the new measured TCXO frequency using the procedure given above. If you know the accuracy of your reference signal frequency, enter that in when prompted for the “ppb” figure (in parts per billion). The third method is a combination of the above two methods and requires a stable (but not necessarily accurate) frequency source along with an accurate frequency meter. You simply measure the frequency of your signal source using the accurate meter, then feed that same signal into your newly built Frequency Meter and measure it as stated immediately above. You can then perform the same calculation, using the figure you got from your known-accurate meter in place of Fexact and the figure from your new Meter as Fmeas. As before, the accuracy (ppb) figure should reflect the accuracy of the meter you’re using for calibration. The upper-left corner also shows the TCXO frequency and measured CPU (PIC32) operating frequency. The latter is mainly for interest’s sake. The CPU is typically operated at 80MHz as a compromise between screen update speed and power consumption/stability. The PIC32 itself is perfectly stable at higher speeds but we saw some display glitches when driving the touchscreen at faster rates (the LCD bus speed is determined by the CPU clock rate). The TCXO specified operates at a nominal 16.368MHz and this will be the default value at power-up. It can change for two reasons: either you’ve manually calibrated it (as described GPS disciplining below) or the GPS 1PPS signal has been used to determine the actual TCXO If you fit a GPS module, this is all frequency. So when GPS disciplining pretty much automatic. The PIC32 is available, the should detect a TCXO setting autovalid serial stream matically updates from the GPS unit when necessary. and display some These changfigures in the upes are saved to per-right corner the PIC32’s Flash of the screen. If memory so that not, check that you the frequency can haven’t transposed be accurate the the TX and RX pins next time the unit of the GPS unit or is powered up bemade some other fore it’s been runmistake with the ning long enough wiring. Check also to get an accurate that the power LED reading of the GPS on your GPS unit time base. is lit. For manual caliMost GPS bration (eg, if you units (including have not fitted a the recommendFig.7: using the on-screen keypad to calibrate the onboard oscillator for greater GPS unit), you must accuracy. There are three different calibration methods given in the text, with ed VK2828) also first measure the the simplest involving measuring the oscillator frequency with a more accurate has an LED which TCXO frequency. flashes when it has meter and then typing it in as shown here. 86 Silicon Chip Celebrating 30 Years siliconchip.com.au a good satellite inputs. You still SILICON CHIP 6GHz+ Touchscreen Frequency/Period Meter lock. just need to substiTimestamp,Hz,Freq,PrecHz,AccHz,TCXO,Input,Imped,Mode,AvgSec,GPSSats,UTC,Date If you’re gettute the units when 6239317,5260135255,5.26013526GHz,240,370,16367993,SMB,50,1,5,124837,03112017 ting some indireading the divided 6239817,5260134170,5.26013417GHz,230,360,16367993,SMB,50,1,5,124837,03112017 cation in the upoutput to get the ac6240317,5260134285,5.26013429GHz,220,350,16367993,SMB,50,1,5,124838,03112017 per-right corner tual frequency. 6240817,5260133925,5.26013393GHz,210,340,16367993,SMB,50,1,5,124838,03112017 that the GPS unit Note that while the 6241317,5260133910,5.26013391GHz,200,330,16367993,SMB,50,1,5,124839,03112017 has been detectaverage frequency 6241817,5260133965,5.26013397GHz,200,330,16367993,SMB,50,1,5,124839,03112017 ed but you aren’t produced from the 6242317,5260133940,5.26013394GHz,195,325,16367993,SMB,50,1,5,124840,03112017 seeing a proper reference output 6242817,5260133995,5.26013400GHz,190,320,16367994,SMB,50,1,5,124840,03112017 fix (latitude, lonshould be very ac6243317,5260133965,5.26013397GHz,190,320,16367994,SMB,50,1,5,124841,03112017 gitude, time, date, curate, there could Table 1: sample output from the unit over the serial console, captured with etc) then you may be some jitter bea terminal emulator. The result is in a CSV format so you can save, plot need to move the cause of the Pulse and analyse it easily using standard software such as Microsoft Excel or unit closer to a Diffusion technique LibreOffice/OpenOffice Calc. window or conused to provide an sider fitting a GPS module with an and leave it powered up for at least accurate division ratio. So it’s best to external antenna. half an hour to allow it to calibrate feed it to equipment with a reasonably Note that it may take several min- the TCXO frequency to a reasonable long acquisition window (say at least utes to get a lock even with a good sig- accuracy. 100ms) to get good results. nal, especially if the GPS module has If you’re only using it in short bursts not been used for many days. later, it may not have enough time to Serial output Once a signal has been found, a cir- get a good lock and so doing this periOne feature we haven’t mentioned cle is displayed which should flash at odically (eg, every couple of months) so far is that the measured frequency/ 1Hz, concurrent with the 1PPS signal will help it continue to provide good period, TCXO oscillator frequency and from the GPS unit. It will be red if a accuracy. general configuration are also printed satellite lock has not yet been achieved to the serial console in CSV format. Reference output or green if it has. So if you want to hook the MeOnce it’s green, the unit will start inAs stated in the earlier articles, the ter up to your PC, you can do so and ternally “time stamping” each pulse. reference (BNC) output can produce capture/log/process the resulting data If the lock remains good for at least a one of three signals: a fixed 1Hz or quite easily. few minutes, the time stamps will be 1kHz reference signal, or a frequency You can see the output of the unit in used to improve the TCXO frequency that is equal to the measured frequency MMEdit’s “MMChat” window or you and thus the reading accuracy and divided by 1000 (for the BNC input) or could use a serial console program precision. like Tera Term Pro to view and capture 1,000,000 (for the SMB input). The length of time that the unit has This varying division ratio is nec- this data. Set its baud rate to 115,200 had a good satellite lock is shown be- essary to keep the output frequency and make sure the correct COM port low the latitude, longitude and alti- within reason at the upper end of the is selected. Make sure to close MMEdtude information (which are provided device’s measurement range and is it before launching Tera Term Pro so merely for your curiosity). shown on-screen when you switch that the COM port isn’t already in use. Also, it’s imOnce captured, save portant to realthe data to a CSV file ise that the time so you can open it latand date giver for analysis. en are for UTC Conclusion (GMT). They’re also Despite all the preprovided for vious explanation, your reference; this meter is quite you need to simple to use, espeknow your curcially if you are usrent local time ing GPS disciplinzone offset to ing since there is no convert them to need for manual calilocal time. bration. By the way, All you need to do we suggest once is connect your signal you get the Meup to one of its inputs, ter up and runpower it up, select the ning, you leave appropriate input and it in a location Fig.8: having entered the measured TCXO frequency, you also have the option averaging time, then of providing an accuracy figure to go along with it. This allows the unit to with a good compute and display the new, better accuracy figure for any given reading. wait a few seconds and GPS signal lock Press the Save button and the new calibration figures will take effect. read off the result. SC siliconchip.com.au Celebrating 30 Years December 2017  87