A Simplified GPS-Derived Frequency Standard - …

1 14 Sep/Oct 2006242 Robert-Martial StGatineau, Qu bec J9J Simplified GPS-DerivedFrequency StandardBertrand Zauhar, VE2 ZAZHere is a simple and modern approach to a 10-MHz many reasons, an accurate frequencystandard at an Amateur Radio stationis desirable. For weak-signal operationsuch as EME (moonbounce) at microwavefrequencies, you must be transmitting and lis-tening exactly at the right Frequency ; other-wise the narrow receive filters used will makeyou miss that weak signal. With such a setup,a 10-MHz Frequency reference feeds the fre-quency synthesizer that generates the radio soperating Frequency .

2 An accurate 10-MHzreference is also useful for test equipmentadjustment. With an accurate Standard , youcan put Frequency counters and signal gen-erators on advent of the Global Positioning Sys-tem (GPS) has allowed a Simplified approachto time and Frequency accuracy. Several com-mercially available GPS receiving units pro-vide a 1 pulse-per-second (pps) signal. Thissignal typically exhibits a short-term accu-racy of 1 microsecond (1 ppm or 1 10 6).Figure 1 Block diagram of the GPS-Derived Frequency with permission; copyright ARRL Sep/Oct 2006 15By averaging it over a long period, a muchbetter accuracy can be achieved.

3 This is whatthis project does: it locks an external 10-MHzvoltage-controlled signal source to the 1 ppsGPS good work of Brooks Shera1 has gen-erated a lot of interest within a broad commu-nity of experimenters who want to increase thelevel of Frequency accuracy available to themat low cost. His system uses a PLL techniqueto lock an external oscillator to a GPS receiverand obtain an accurate Frequency project I present here provides asimpler and more modern approach to aGPS- derived 10-MHz Frequency found in today s technologyoffer the following benefits: solid perfor-mance, more features and a reduction in thenumber of design differentiates itself from otherpreviously published designs because: It uses a simpler Frequency measurementtechnique, as opposed to phase measure-ment.

4 It provides on-board reference buffering andfan-out with 50- output impedance. It provides the three most common refer-ence frequencies of 10 MHz, 5 MHz and1 MHz. It provides full software control of the fre-quency acquisition and control processes,without DIP switches. It has fewer components and does not re-quire an external DAC or external inputcounter chips. It runs off only one supply voltage: +5 V dc(excluding the VCXO supplies).Tests have shown that this system consis-tently produces a short-term reference accu-racy in the 1 10 10 range.

5 This is derived us-ing Standard automotive-grade GPS range of accuracy does not rival cesium-based references. It is much better than mostof the Standard built-in, free-running oscilla-1 Notes appear on page 2 Schematic diagram of the GPS-Derived Frequency Standard Sep/Oct 2006tors seen in commercial test instruments, how-ever. Just to give you an idea of the type ofaccuracy, one part in 1010 represents an errorof one hertz on a 10 GHz signal!System DescriptionFigure 1 shows a block diagram of myGPS- derived Frequency Standard .

6 The systemoperates a hardware/firmware Frequency -locked loop (FLL). In essence, the system com-pares a local Frequency source (an externaloscillator) to a GPS-Derived reference. It willadjust the local 10-MHz variable source tomatch the GPS-Derived 1 pps reference. The10-MHz source is kept aligned with respectthe 1-Hz GPS reference on a real-time basisby the firmware. The resulting 10-MHzreference is fanned out, Frequency -divided andprovided to the user for high-accuracy appli-cations. System control and monitoring areachieved using a bicolor LED and a serial portconnected to a terminal (PC).

7 Hardware DescriptionFigure 2 shows the system circuit sche-matic. The main operation consists of count-ing the number of rising edges produced bythe 10-MHz voltage-controlled crystal oscil-lator (VCXO) signal over a 16 s period (16 GPS pulses). If the GPS and the VCXO areat the same Frequency , exactly 160,000,000pulses will be counted ( 1 pulse, inherent tocounter technology).Prior to entering the microcontroller, the10-MHz VCXO signal is buffered and am-plified by U1, a receiver chip. An optionalinput termination resistor R1 can be added ifthe VCXO s output circuit calls for one.

8 Thebuffered 10-MHz reference signal is fannedout to several locations on the , the Microchip PIC18F2220 micro-controller, has a built-in 16-bit counterincremented by an external source, theVCXO. The counter value is latched by an-other external signal rising edge, the GPS1 pps signal in our application. This processis totally autonomous and independent fromfirmware. The microcontroller s task in thisprocess is to analyze the results and adjustthe VCXO Frequency Frequency ControlThe Microchip PIC18F2220 microcon-troller does not have an integrated digital-to-analog converter (DAC).

9 To produce anadjustable voltage source to vary the VCXO Frequency , the built-in 10-bit pulse-widthmodulator (PWM) is used instead. A continu-ous rectangular-wave output is produced by thePWM. A downstream external 1-Hz, two-stagelow-pass filter (U5A, U5B and discrete com-ponents) is used to recover the average dcvalue of the PWM output. By varying the dutycycle of the PWM, it is possible to producean accurate analog dc voltage with 210 or 1024steps over the DAC of 14-bit resolution is achievedby precisely controlling the duty cycle of thePWM output.

10 This translates to a tuninggranularity of 6 10 5 Hz for a VCXO thathas a 1-Hz tuning range. Achieving a 14-bitDAC using a 10-bit PWM requires additionalfirmware processing. The idea is to dither the pulse width within a 16-cycle 16 cycles translate into an additional4-bit resolution. For example, increasing the14-bit DAC output by one step involves in-creasing the 10-bit PWM output width by oneincrement on one of the 16 pulses. Increas-ing the DAC by two steps means increasingthe 10-bit PWM output width by one incre-ment on two of the 16 pulses, and so add flexibility for interfacing with vari-ous VCXOs, the filtering stages have a supplybypass feature that allows you to feed theoperational amplifiers with different upperand lower rail voltages.