Universal Square Wave Generator Stable all the way up to RF

Transcription

1 Elektor Post Project No. Universal Square Wave Generator Stable all the way up to RF A square wave generator is useful in a test environment, and often the harmonic content of a simple square wave signal is advantageous. With an ever widening range of specialized integrated circuits it is now a simple job to build a high quality generator without too much expense. Add to that a display to indicate the signal output frequency and amplitude, and you have a useful piece of test gear. By Kai Riedel (Germany) A universal signal generator is a useful addition to any workbench. They are ideal for squirting signals into electronic circuits and checking to see if the circuit behaves in the way you had anticipated. Of course for any generator it s useful to be able to select the frequency of interest and to adjust the output to give the appropriate signal level. For testing digital circuits it would also be useful to output a TTL compatible signal. All of these criteria are fulfilled with this design. With a potential frequency range extending from 0 khz to 0 MHz its spectrum is so wide that it really does deserve the title universal. This universal square wave generator has a classic DC-free 0 Ω output adjustable from 0 V to approximately V. In addition there is a digital TTL output with fixed amplitude. A two line LCD display indicates output frequency and amplitude. Power for the generator can be supplied from a standard AC/ DC wall adapter. Square wave generation A crystal or ceramic resonator would normally be high up on a list of components you would first reach for to build a stable frequency generator. One disadvantage of this approach is that if you wanted it to produce more than just one fixed frequency it would be necessary to include programmable frequency dividers which would add considerably to the circuit complexity and expense. RC oscillators are a simple solution and can be made adjustable but their long term stability is not sufficient for many applications. One solution to this dilemma is to use a specialist oscillator IC like the examples shown here from the IC manufacturer Linear Technology. There are two pin-compatible variants of this chip which cover different frequency bands. This has the advantage that by just swapping an IC the same circuit board can be used to make a signal generator which covers two frequency bands. Variant V fitted with an LTC99 [] provides an impressively wide range from khz up to 0 MHz. For pure RF use variant V using the LTC90 [] covers a range from MHz to 0 MHz. The generators are adjustable and exhibit very low frequency drift. The chip s output frequency is set by the value of resistance R set between + V and pin. Pin of the chip also provides access to an in-built frequency divider N. In variant V the divider has the value (pin to ground), 0 (pin O/C) or 00 (pin to + V). Variant V with the same levels on pin divides the frequency by, or. The frequency output of the two variants is given by: and f V 0 MHz 0 kω = R N f. MHz 0 kω = + MHz R V. N set set elektor post Project No.

2 Elektor Post Project No. R k P k P 00R K R k GND C 0n VCC SET IC OUT LTC90 DIV C u 0V R 0R IC = AC IC.A IC.F IC.D IC.E 0 IC.B R R C u K R 00R f out TTL P 00R C 00n 0k 0k R R IC LMH9 C9 00n C0 R9 R 00n R0 9R9 R R 9R9 9R9 9R9 R K f out 0 9 IC.C D S C IC R D S C IC R K TTL_OUT/ C 0n C 00p C u C K0 D BAT C 0u V C9 00n IC 0 C0 00n C 0u V C IC 00n IC IC, IC = LVCGDC IC C C C 00n 00n 00n C 0u 0V C 0n 0n VPOS FLTR IREF IC VRMS RFIN AD SREF PWDN COMM DC: 9...V / 00mA DC_OUT R R K K ICSP R 0k RB RB R 0k k R RC 0k C 00n 0 VDD MCLR/VPP IC RA0/AN0 RA/AN RA/AN/CVREF/VREF RA/AN/VREF+ RA/COUT/T0CKI RA/AN/COUT/HLVDIN/SS PICF-I/SO RC0/TCKI/TOSO RB0/AN/FLT0/INT0 RC/CCP/TOSI RB/AN0/INT RC/CCP RC/SCL/SCK RC/SDA/SD RC/SDO RB/AN/INT RB/AN9/CCP RB/AN/KBI0 RB/PGM/KBI RC/CK/TX RB/PGC/KBI RC/DT/RX RB/PGD/KBI RA/ RA/ VSS OSC OSC VSS 9 X 0 9 C 0n K9 TTL_OUT/ RB0 RB RB RB RB RB RB RB RB RB RB RB RB RB RB RB 0 0 K K 9 9 R RB RB0 RC RB RB0 RC P 0k LED C 0n C p 0MHz C p 00 - For best stability V should not be used with N = at frequencies below 0. MHz or above 0 MHz and for variant V not above 0 MHz. The circuit For simplicity the diagram in Figure shows the complete circuit for the variant V. The three-way pin header K allows a jumper to select the division factor N as, or. With N = the values of P, P, R and R will give an output range between MHz and MHz. The binary dividers :: produces three overlapping frequency bands. P allows fine frequency adjustment of a few percent. To change the circuit to variant it will not only be necessary to swap IC for an LTC99 Figure. The universal square wave circuit looks as complicated as it is: IC generates the signal, IC amplifies it and IC measures it. elektor post Project No.

3 Elektor Post Project No. Timer0 counts the external clock pulses at input T0CKI. Timer generates an interrupt after 00 ms. In the corresponding interrupt service routine the counter register of Timer0 is read to give the measured frequency value. An A/D conversion is initiated to measure the output signal level. The frequency measurebut also both pots and resistors responsible for the value of R set. Now using a. kω for R, omitting R and using a value of. kω for P and 00 kω for P gives a frequency range (with N = ) from to 0 MHz which with the decimal divider :0:00 again provides three overlapping ranges. The components values for variant are shown in brackets in the parts list. The output signal from IC is now buffered by a gate of IC. This TTL compatible signal is available at connector K. It is also buffered by IC (a video amplifier with a gain of ) to give a DC-free level 0 Ω impedance output from connector K adjustable (via P) between 0 V and approximately 00 mv eff. For the purposes of generating a signal the circuit description is now more or less complete. If we didn t go any further with the circuit then we would not know the frequency or amplitude of the generated or the signal without hooking up some test gear. To add these features we need a display and some electronics to measure the signal frequency and amplitude. We have used a PICF (IC) microcontroller for this task, it has an in-built frequency counter good for signals up to approximately 0 MHz. In addition it has a -bit A/D converter allowing an amplitude measurement of 0. mv HF /bit. To measure the output signal the microcontroller IC requires a little assistance. For variant V a frequency prescaler is required, which is not necessary for V (where it is bridged by a 0 Ω resistor). IC and IC together divide the signal frequency from IC by four. This allows the microcontroller to measure signals up to 0 MHz. The 00 ms gate period used to sample the frequency is indicated on LED. The divided-down signal from IC is wired to the microcontroller input pin using a short length of coax from K to K9. The Firmware The firmware is, as usual available to download for free by going to the Elektor web page for this article []. It was developed using Microchip s free evaluation version of the C compiler MPLAB C. The main features of the software can be summarized: Figure. SMD components mean that the finished design is neat and compact. Figure. The prototype generator with display. The signal amplitude is sampled by the AD which is a very linear high frequency RMS value detector producing a DC output voltage proportional to the RMS value of the measured signal. A low level on pin of K indicates variant configuration to the controller while a high indicates variant. K- input RC is available for your own use. elektor post Project No.

4 Elektor Post Project No. Technical Data A compact universal square wave generator with adjustable frequency and amplitude. Frequency range: V = 0 khz to 0 MHz; V = MHz to MHz Divided into three bands: V: 0 to 00 khz; 0. to MHz and to 0 MHz V: to MHz; to MHz and to MHz Frequency stability: V = ±0 ppm/k; V = ±0 ppm/k Output waveform: Square wave Mark/space ratio: 0 % Two BNC outputs: K: TTL compatible ( V) K: Adjustable 0 to 00 mveff into 0 Ω, DC free Display: Two-line LCD display showing signal frequency and amplitude. Power requirements: 9 V to V, max. 00 ma Figure. The square wave output signal has a good shape at this frequency. ment is then adjusted by a scale factor and together with the level (effective value) of the square wave signal, sent to the display. Once this is complete the timers are again loaded with their initial values and the process repeats. The pin header K is included to allow the controller to be programmed. The microcontroller s memory has a useful amount of free space just waiting for your own improvements and modifications to be flashed to memory. The assembly As you can see in Figure it is possible to build a very compact and neat universal square wave generator thanks mainly to the use of small footprint SMD components. IC measures the frequency and amplitude of the generated signal and displays the values on a -line LCD. When the LCD uses a flex-print connector like the HMCSG LCD assembly it can be attached over to the PCB using stand-off mounts and connected to K. Alternatively Figure shows a standard x display Figure. At the upper frequency limit using variant V (at MHz) the signal shape is not as ideal but is still quite useable for undemanding applications. elektor post Project No.

5 Elektor Post Project No. connected to pin header K using a flat band cable. And finally A list of the main technical features of both variants is given in the text box. The two oscillograms shown in Figure and Figure are output signals from the prototype design of variant at MHz and MHz. While the lower frequency trace shows a very respectable square wave the edges of the higher frequency trace are a little less well defined. This is only to be expected at this frequency and the signal is never the less still useful. It must also be considered that at this frequency in the VHF band it is difficult, even with an expensive scope to get a good representation of the signal. Finally it s worth noting that despite the relatively low signal levels involved any flying leads connected to the outputs will act as COMPONENT LIST Values for V (V in brackets) Resistors Default shape: 00 R = kω (.kω) R = 0Ω R = not fitted (0Ω, refer to PCB overlay) R = Ω R = kω (not fitted) R = 00Ω R,R,R,R,R = 0kΩ R9 = Ω R0,R,R,R = 9.9Ω R = Ω R = kω P = 00Ω (.kω) 0-turn trimpot, vertical * P = kω (00kΩ) 0-turn trimpot, vertical * P = 00Ω 0-turn trimpot, vertical * P = 0kΩ 0-turn trimpot, vertical * Capacitors Default shape: 00 C,C,C,C,C,C = 0nF, XR C,C,C,C,C,C9,C0,C = 00nF, XR C = 0µF 0V, tantalum, Type A C,C = µf, XR C = 00pF, NP0 C,C = pf, NP0 C9,C0 = 00nF, MKT, mm pitch C = 0µF V, electrolytic,.mm pitch, mm diam. C = 0µF V, electrolytic, mm lead pitch, 0mm diam. C =.µf 0V, tantalum, Type A Semiconductors D = BAT, DO- IC = LTC90CS (LTC99CS), TSOT case IC = CDACM9G, SO- case IC,IC = LVCGDC (not fitted), VSSOP- case IC = LMH9/MA, SO- case IC = ADARMZ, MSOP- case IC = LM0, TO-0 case IC = PICF-I/SO, SOICW-, programmed, Elektor # 00- LED = LED, green, mm Miscellaneous K,K = BNC socket, PCB mount, right angled, TE connectivity 0 K,K = -pin pinheader, 0. pitch K = -pin pinheader, 0. pitch K = -pin FFC socket, SMD, mm pitch, MOLEX 0-9 * K = -pin (x) DIL pinheader, 0. pitch * K,K9 = -pin pinheader, 0. pitch K0 = wall adapter socket, PCB mount,.mm X = 0MHz quartz crystal, HC9/SMD LCD, Pollin type HMCSG or Elektor # 00- * PCB # 00- * see text elektor post Project No.

6 Elektor Post Project No. antennae. Unshielded, un-terminated connections at the TTL output in particular will splatter in the RF bands. Proper full size pots and an enclosure can be used in place of the presets to make the unit look more professional but be aware that all such wiring must be kept as short as possible otherwise the unit s performance will suffer. This applies to both variants. Pin of IC is particularly sensitive to any form of interference. A frequency band selector switch can also be mounted on a front panel instead of the PCB jumper position K; in this case it will only be necessary to bring out a single wire from the center pin of K. Pin of IC is not as sensitive to interference as pin. As we already mentioned the firmware is available from the web page [] but that s not all; a bare PCB can also be ordered, as well as a pre-programmed microcontroller and a suitable -line LCD display module. Power for the unit can be supplied from a standard wall-wart adapter. There is no requirement for the supply voltage to be stabilized. Internet Links [] [] [] (00) The Author Kai Riedel works as a design engineer at a company by the name of Turck in Beierfeld, Germany. He has always had a fascination with all things electronic. His specialty is in the design of test and measurement equipment incorporating microcontrollers, experimenting with RF technology and repairing diverse electrical equipment. He likes to spend his free time playing pianos and church organs. elektor post Project No.