An Automatic Noise-Figure Meter

Transcription

1 An Automatic Noie-Figure Meter Here i a project that will automatically meaure the noie figure of your preamplifier project. It ha an operating range from 3 MHz to over 400 MHz. Jim Koehler, Introduction Anyone intereted in building low-noie pre-amplifier will want to meaure the noie figure of the device. Commercial noie-figure meter, often called Preciion Automatic Noie Figure Indicator (PANFI), exit, but they are expenive even on the urplu or ued market becaue of the demand for them. There have been a few publihed contruction article in the ham literature, but they are either overly complex or don t compare to commercial tet intrument. 1 Thi article decribe the contruction of a imple verion, uitable for home contruction, but which ha many feature of the bet commercial intrument. See Photo A. Theory A good background to noie-figure meaurement i given in a Hewlett-Packard (now Agilent Technologie) Application Note: AN Thi note i available on the Agilent Web ite, and i worth reading. 2 Noie figure, in an amplifier, i a meaure of how much exce noie an amplifier add to any incoming ignal. The lower the noie figure, the le noie it add. Noie figure can be expreed in a number of way, but I prefer to think of it in term of temperature. Every paive reitive electronic component generate ome noie becaue of the random motion of charge carrier in it. The greater the temperature, the greater the motion of the charge, and therefore, the greater the random noie power generated by the component. The noie power generated by any component depend only on it temperature and the bandwidth that you are 1 Note appear on page 46. conidering; it i given by the equation: P = ktb [Eq 1] where k i Boltzmann contant, which i equal to joule / kelvin, B i the bandwidth in Hz and T i the temperature in kelvin. A perfect amplifier would add no noie to incoming ignal, but real one do. We can pecify thi exce noie in term of the temperature of a reitor at the temperature that would give that ame amount of noie. Thi temperature, called the exce noie temperature, T e, i a meaure of how good the amplifier i; the lower the exce noie temperature, the better the amplifier. The amplifier may alo be characterized by a noie figure, which i the ratio of the exce noie temperature (plu room temperature) to room temperature. ( T e +Tr ) NF = T r [Eq 2] where T r i the room temperature. Thi ratio i often expreed in db. An ideal amplifier would have a T e of zero and therefore a noie figure of 1.00, or 0.0 db. Modern noie-figure meter meaure the noie figure of an amplifier by connecting a noie ource, which can be turned off and on, to the amplifier input. A noie ource i jut that; a device with an output that i broadband noie of a known level, correponding to a reitor at ome temperature. Noie ource are characterized by their effective noie ratio, ENR, which i given by: ( Th Tr ) ENR = [Eq 3] T r 2258 June Rd Courtenay, BC V9J-1X9 Canada jark@haw.ca Photo A The front panel of the automatic noie-figure meter. May/June

2 where T h i the hot temperature that a reitor would have to produce the ame noie power a the noie ource when it i witched on. When the noie ource i witched off, it produce only the amount of noie that a reitor at room temperature, T r, would produce. Now, imagine an amplifier with a noie ource at it input and a power meter at it output to meaure the power coming out of the amplifier. When the noie ource i turned off, the power going into the amplifier will conit of the power from the noie ource at room temperature plu the amplifier exce noie power. The output power will be thi um multiplied by the amplifier amplification factor. When the noie ource i turned on, the input power to the amplifier will be the noie power of the noie ource when it i on, plu, again, the exce noie power due to the amplifier alone; the output will be thi new um again multiplied by the amplification factor of the amplifier. The ratio of the on-power to the off-power at the output of the amplifier i the Y-factor: ( T h +Te ) Y = [Eq 4] T +T ( ) r e Noie-figure meter determine T e by meauring the Y-factor and by knowing the value of T h and T r. T r i normally aumed to be 290 K, o, rearranging Equation 4, we get; ( T h 290 Y ) T e = [Eq 5] Y 1 ( ) A PANFI i therefore jut an intrument that ha a power detector to accurately meaure the output noie power of an amplifier and a mean of turning the noie ource on and off. It mut then do the mathematic to diplay the exce noie temperature either a a temperature or a a noie figure. Noie ource are motly olid tate device and mot are deigned to be turned on when they are upplied with V dc. A more detailed dicuion of how a PANFI work i given in the Appendix. Before leaving thi topic, let me make a few remark about the preciion of the meaurement. What i being meaured i the power due to wide-band noie. Noie i random and o there i ome uncertainty in meauring it power level; the level will fluctuate over time. It i a well-known fact that the accuracy with which noie power may be meaured depend on two thing: the amount of time over which the power i averaged, τ, and the bandwidth of the noie, B, itelf; the meaurement error i proportional to 1 Bτ. The fact that the accuracy depend on the bandwidth may eem a bit trange but if you think of it tatitically, the wider the bandwidth, the more ample of the noie you are taking, and hence the more accurate the average. So, to increae the accuracy, the noie power mut be averaged over longer time and/or the bandwidth of the meauring intrument mut be increaed. Commercial PANFI may average over period of a few econd and typically have meaurement bandwidth of a few MHz. Older PANFI uually operated at a fixed frequency, often 30 MHz. To ue them at any other frequency, it wa neceary to convert the ignal to thi pecific frequency. Many modern intrument have a much wider frequency range. The intrument decribed here operate over a frequency range from about 3 MHz to over 400 MHz. Block Diagram A block diagram of the intrument i hown in Figure 1. There i a wide-band amplifier with eparate input and output that may be added to the output of the device being teted to increae the ignal level. The detector i baed on the Analog Device AD8307 logarithmic detector. 3 Thi wonderful little device ha a frequency repone extending up to 500 MHz, and i linear (in logarithmic power) over 8 decade of power. The microproceor board control all apect of the device operation. In the meaurement mode, it turn on the external noie ource by upplying +28 V dc to it and, uing it internal A/D converter, meaure the ignal level detected by the AD8307. It tore thi value and then turn off the noie ource and again meaure the level detected by the AD8307. Thi meaurement i repeated 50 time over a period of about 1 econd. Then the noie figure of the device i calculated and diplayed on a 2-line, 16-character liquid Figure 1 Thi block diagram how the ection of the automatic noie-figure meter. Figure 2 The chematic diagram of the broadband amplifier how that it ue three MiniCircuit MAR-3SM monolithic amplifier. 40 May/June 2007

3 crytal diplay (LCD). The A/D converter ha a baic reolution of 1 part in 1024 (10 bit) but each of the 50 meaurement i an average of 100 A/D converion. Becaue of thi, in the abence of noie, the expected overall reolution of each ingle power determination would be about 1 part in 70,000. Ultimately, the final calculated noie figure ha an accuracy that depend motly on the accuracy of the logarithmic repone of the AD8307, and the fact that ince the ignal being meaured i random noie, it ha a limiting accuracy that depend on the bandwidth and the averaging time (in thi cae, about 1 econd). Detailed Circuit Decription The Broadband Amplifier The circuit of the broadband amplifier i hown in Figure 2. It ue three MiniCircuit MAR-3SM monolithic amplifier. 4 The overall gain i about 37 db and the circuit, if laid out properly uing trip line of the correct width, ha a bandwidth from about 3 MHz to cloe to 2 GHz. Uing dead-bug type of contruction, the bandwidth will probably be reduced, but ince the AD8307 repone tart to fall off at 500 MHz, thi won t matter. The MAR-3 amplifier i pecified to have a noie figure of about 6 db in the range from a few MHz to 2 GHz. Each tage draw about 35 ma and the total power diipated in each tage bia reitance of 200 Ω i about 1 4 W. I laid out my amplifier uing trip line and urface mount component. The 0805 ize reitor I ued were only rated at 0.1 W; therefore I ued four 51-Ω reitor to make up thi reitance. Uing dead-bug contruction, you would probably want to ue a 200 Ω, ½ W reitor or two 100 Ω, ¼ W reitor. The Detector Circuit The circuit for the detector i hown in Figure 3. The AD8307 require a upply voltage of 5 V but with a current drain of only a few ma, o a low power 78L05 regulator wa ued. The 51-Ω reitor at the input of the AD8307 hould be connected between pin 1 and 8 with a cloe to zero length lead a you can manage. I ued a urface mount verion of the AD8307 and a urface mount 0805 ize reitor located jut 1 or 2 mm from thee lead. The detector i very enitive and o the circuit hould be well hielded. I put both the detector and the wide-band amplifier in a mall box made of double-ided circuitboard material and oldered the lid on. See Photo B. The detector and amplifier are in eparate compartment in thi box. The coaxial input to and from the amplifier and to the detector have the hield braid oldered to the ide of the box. The dc upply voltage to the circuit and the detector output are fed into the box by feed-through capacitor. Photo B The broadband amplifier i built into a circuit board box mounted inide the bottom of the project cae. Figure 3 An Analog Device AD8307 logarithmic detector i the heart of the detector circuit. May/June

4 The Microproceor Module The microproceor ued wa an Atmel AVR-erie ATMega32. 5 Thi microproceor ha eight channel of A/D and 24 other pin of I/O. I like the AVR-erie of microproceor becaue of their low price, their good performance and the fact that there are ome very good development tool available for them. The ource code for thi project wa all written for the Gnu C compiler, GCC. There i a verion of thi compiler that produce code for the AVR erie of microproceor. A complete package of the compiler and all the needed utilitie i available a a package called WINAVR for intallation on Window computer. It i a firt-cla profeional tool et and i completely free! I ued a SIMM-100 circuit board, which i available from Dontronic for about $11 Autralian and i a bargain. 6 Dontronic hip to the USA and the ordering and ubequent hipping i painle. The board hould be aembled according to the accompanying direction. See Photo C. The board wa originally deigned for the AT90S8535 microproceor but it i pin-compatible with the ATMega32. Ue the 16PI verion of the ATMega32, a it i rated for the indutrial temperature range and for clock frequencie of up to 16 MHz. It cot jut a few pennie more than the commercial grade. Ue a good quality ocket for the microproceor. The LCD require even I/O pin; there are two A/D channel ued (of eight avail- Photo C Thi photo how the aembled SIMM100 circuit board, with the ATMega32 Figure 4 An ATMega32 microproceor on a SIMM100 circuit board provide the brain for the noie-figure meter. Thi chematic diagram alo how the power upply, diplay and control circuitry. 42 May/June 2007

5 able) and there are two other digital I/O line ued. There i an RS-232 interface circuit built onto thi board and it i ued to provide communication to an external computer or terminal, which i needed for calibration of the finihed unit. I ued a MHz crytal in the ocillator and, with thi clock, the erial RS-232 connection i at baud. The Ret of the Circuit The ret of the circuit i hown in Figure 4. There are two front panel witche. One of thee i a pring-loaded toggle witch that i ued to et level prior to a meaurement. A puh-button witch could alo have been ued here. The other witch i a ingle-pole three poition witch ued to et the mode of operation. Again I ued a toggle witch. Three-poition toggle witche are not common and you could a eaily ue a rotary witch intead. There are a large number of LCD available. Since they all ue the ame protocol, it doen t much matter which one you chooe. I built the intrument in a mall cae that had once houed a piece of Tektronix tet gear. Photo D how the circuit board and wiring inide that cae. If I were doing it over again, I d put the intrument into a much larger box and ue an LCD with larger digit becaue I m getting old and my eyeight in t what it ued to be! The contrat trimmer potentiometer for the LCD hould be adjuted to give the bet contrat for the diplay. It will be et o that the voltage at the wiper i cloe to 0 V. The power upply provide +12 V and +28 V dc. I happened to have an old wall wart on hand, which provided 24 V ac, center-tapped. Any 24 V ac center-tapped tranformer capable of upplying a total power of a watt or two i good enough. The trimmer potentiometer in the +28 V circuit i ued to et the output voltage to exactly 28 V. Do thi a accurately a you can. If you do have thee, then the calibration procedure i a follow: 1. Connect a 50 Ω ignal generator operating at the deired frequency to the SIGNAL INPUT connector on the intrument. Set the output level to a fairly high level around 0 dbm. 2. Type the character c (upper or lower cae) on the terminal and the microproceor will repond with a line aking for the input level. Type the level being produced by your ignal generator in dbm. Then, pre the SET Calibration and Operating Calibration of the Power Detector You need an external terminal or computer to do the calibration of the power detector. The RS-232 connector i deigned to connect to a PC and you may ue ome terminal program to communicate with the intrument. The internal intrumental calibration, which convert from AD8307 output level to a dbm cale, i tored in the internal EPROM of the microproceor. The programmed microproceor ha a default calibration baed on the value pecified in the AD8307 data heet. Thee are fairly accurate for a broad range of input frequencie and mot uer will not want to bother trying to improve thi accuracy. I have made proviion for a more accurate calibration, however, for thoe who have good quality ignal generator and coaxial attenuator and who wih to calibrate the intrument preciely at ome pecific frequency. Photo D Thi view how the top of the project cae, with the SIMM100 board, LCD and variou wiring of the noie-figure meter. Photo E Thi photo how my homebrewed noie ource. May/June

6 witch on the intrument. 3. The intrument will repond by aking for the next data point. 4. Set the generator to a new level about 10 or 20 db lower. The preferred way to do thi i jut to leave the generator etting a it wa before but to add a known attenuation into the line. Again, type the new ignal level and pre the SET witch. 5. Repeat the tep above three more time for a total of five etting of the ignal generator. Chooe the five ignal level o that they are ditributed fairly evenly (in db) between 70 dbm and 0 dbm. The order in which you et the level doen t matter. Internally, the five value are ued to calculate the leat-quare bet fit of a traight line to the data and the two parameter decribing thi line are tored in the intrument in EEPROM. Thee two parameter are ubequently ued to tranlate from AD8307 output to dbm. While connected to the terminal, you mut alo now tell the intrument the noie level of your noie ource. Type a ingle character e (again, upper or lower cae) and the intrument will repond by aking you to type the ENR, in db, of your noie ource. Thi value i alo tored internally in EEPROM o it i there whenever the intrument i turned on again. While connected to the terminal, you can alo change whether the noie figure are diplayed a temperature or in db. Type a b to elect db or a t to elect temperature. The mode i alo aved in EEPROM o it will remain in whatever tate you left it the next time you turn it on. Finally, you may diplay all the internal etting of the intrument by typing the character d. I have not dicued noie ource here ince they are a ubject requiring a eparate article. Paul Wade ha written an excellent article about noie ource that tell you how you may make your own. 7 My own noie ource i a homebrew deign baed on information from that article. See Photo E. if the device i a down-converter with an output at 144 MHz or a 144 MHz amplifier, the filter hould be tuned to 144 MHz. After preing the SET witch, the diplay will how the meaured level with the noie ource on and off and alo will how the exce noie temperature of the broadband amplifier in the intrument. Thi hould be omething in the neighborhood of 900 K. MiniCircuit pecifie the MAR-3SM a having a noie figure of 6 db, which correpond to about that value. Then, connect the device-under-tet (DUT) to the intrument a hown in Figure 5B. With the ON-AUTO-OFF witch in the OFF poition, note the ignal level meaured by the intrument. It hould be greater than 70 dbm and no more than about 0 dbm. If it i omewhere in thi range, you are ready to go. Jut put the ON- AUTO-OFF witch into the AUTO poition and it will give you a meaure of the exce noie temperature of the DUT. You may convert thi to db, if you wih, uing Equation 2. The intrument will alo how a value for the gain of the DUT. The noie temperature diplayed ha been corrected for the gain of the DUT and the noie contribution of the following tage. If the ignal level i lower than 70 dbm with the configuration hown in Figure 5B, you Operation Before making any meaurement, it i adviable to check to make ure that the detector and amplifier are hielded well enough. With the ON-AUTO-OFF witch in the ON poition, make a note of the indicated ignal level when you have a termination on the SIGNAL INPUT connector. Then witch it to the OFF poition and again note the ignal level. It hould be the ame to within 0.1 db or o. If it in t, you haven t hielded the detector or amplifier well enough and your meaurement will not be accurate. Auming all i well, you are ready to make a meaurement. Firt, connect the noie ource to the intrument a hown in Figure 5A and pre the SET witch. The band-pa filter hown hould be appropriate to the device being meaured. For example, Figure 5 Thi drawing how the tep involved in making a noie-figure meaurement. Part A how the connection to meaure the noie ource. Part B how the device under tet connected between the noie ource and the noie-figure meter. Part C how that ometime an additional amplifier will be needed if the device under tet doe not produce a ignal with high enough power output. 44 May/June 2007

7 will need to ue an additional external amplifier to get the level up to where the meaurement can be made accurately. Thi amplifier hould be inerted at the output of the DUT and before the input to the broadband amplifier. Connect the circuit a hown in Figure 5C and now look at the level when the ON-AUTO-OFF witch i in the OFF poition. If it i till too low, you will need to add another external amplifier. If the ignal level i too high, don t ue the broadband amplifier and/or jut add ome attenuation before the SIGNAL INPUT connector. If the DUT i a down-converter, the gain hown will be correct if the noie ource ENR i the ame for both the output frequency of the DUT and the input frequency. That i becaue in the SET meaurement, the noie level wa meaured at the output frequency of the DUT wherea in the final noie meaurement, the noie ource i connected to the input of the DUT, which ee it at it input frequency. If you want to meaure the noie temperature of a LNA ahead of a down-converter, you mut firt SET the intrument with the down-converter in the circuit, a hown in Figure 6A. Then connect the LNA in front of the down-converter a hown in Figure 6B and et the witch to the AUTO poition to read the noie temperature and gain of the LNA. If there i too much overall gain, you may want to by-pa the broadband amplifier and connect the output of the down-converter directly to the SIGNAL INPUT of the intrument. Other Conideration You will need to have a range of bandpa filter for the frequencie that you normally ue. I made up ome fixed doubletuned LC filter for 144 and 30 MHz and a mall fixed cavity filter for 432 MHz. I alo made a tunable cavity filter by modifying a urplu HP5253B frequency converter. Photo F how the front panel of thi filter. Thee unit plugged into ome older HP frequency counter and they have a very well-made tunable ilver-plated cavity. They make very nice 50 MHz to 500 MHz tunable band-pa filter. One often ee thee unit for ale at hamfet flea market for jut a few dollar and they are worth picking up. One diadvantage of the commercial PANFI i that it i very difficult to tune omething where the output i given by changing figure in a text diplay. Human are better at evaluating an output if there i ome analog value aociated with it, uch a a voltage that can be diplayed by a meter. I added an analog output to the ytem with a very imple circuit addition. A low-pa filter coniting of a 5.6 kω reitor going to a 0.1 µf capacitor to ground i connected to pin 3, J5 on the SIMM100 board. Thi pin correpond to PD5 of the microproceor. I added oftware to pule-width modulate thi output to produce an analog voltage that i proportional to the meaured value of Y. The voltage varie from 0 to 5 V a Y change from N.00 to N.9999 volt, where N i an integer. For example, if Y i 2.4, the voltage will be = 2.0 V. If Y were to vary from 2.95 to 3.05, the voltage would tart at = 4.75 V, increae up to 5 V, jump down to zero and then go up to = 0.25 V. Jut uing the non-integer portion of Y i an eay way to make an expanded-cale voltmeter. To tune a ytem for lowet overall noie temperature, you jut tune for maximum Y uing a voltmeter on thi output. I added a pin jack onto the front panel on my intrument for thi output. Concluion Thi intrument will make accurate noiefigure meaurement if you take jut a little care in the contruction and operation. The circuit i very imple and eaily reproduced. Analog Device made it all poible with the AD8307 logarithmic detector. That, and the ue of a microproceor to maage the data, reult in a firt-cla laboratory intrument. Both the Agilent Application Note and the article by Paul Wade dicu ome conideration to be oberved when making noie-figure meaurement and I recommend that you read them if you re eriou about interpreting the reult you get. The ource code and the HEX object code for the microproceor are available on the ARRL QEX Web ite. 8 The author will program ATMega32 microproceor for the cot of return potage. 9 Appendix: Meauring Noie Temperature Conider the baic meauring ytem, which conit of an amplifier and a detector to meaure output power a hown in Figure A1. The ytem i deigned to meaure the ratio of the two output power when the external noie ource i turned on, to the output power when the external noie ource i turned off. Thi ratio i called the Photo F I built a tunable cavity filter into an HP frequency counter box. Figure 6 Thi drawing how the connection required if a down-converter i needed to bring the output frequency of the device under tet into the range of the noie-figure meter. Part A how that the noie ource and down-converter are meaured firt, and then the device under tet i added, a hown at Part B. May/June

8 Y-factor. The gain of the ytem amplifier i G and the ytem noie temperature i T. The latter, the exce noie of the meauring ytem, can be repreented by a reitance at temperature T at the input whoe noie power i ummed with the external input. When the external noie ource i turned on, it act a a reitance at temperature T h and when it i turned off, it act a a reitance at room temperature, T c. When the external noie ource i turned on, the power output at the detector will be: P oh = (T h + T )G [Eq A1] When the noie ource i turned off, the power output at the detector will be: P oc = (T c + T )G [Eq A2] T c i commonly taken to be 290 K. The ratio of thee two power, P oh / P oc, i the Y-factor for the ytem; Y. Thi factor i thu: Figure A1 A Preciion Automatic Noie Figure Indicator (PANFI) witche the noie ource on and off, and meaure the noie power. Y = P P = T +T oh h T +T oc c [Eq A3] We can rearrange the term of Equation A3 to give the noie temperature of the ytem in term of the Y-factor and the external noie temperature: T = T Y T h c Y 1 [Eq A4] Knowing T from Equation A4, we can alo determine the meauring ytem gain by ubtituting it into Equation A1: Figure A2 With the device under tet placed between the noie ource and PANFI, the noie figure i meaured. G = P oh T +T h [Eq A5] G will be in unit of W / K. To meaure the noie temperature of an amplifier or a converter (or, the noie figure, which may be derived from the meaured noie temperature), the noie temperature and gain of the meauring ytem are firt meaured by connecting the external noie ource a hown in Figure A1. Thi provide the initial calibration of the meauring ytem. The amplifier or converter whoe noie figure i to be meaured i then inerted between the meauring ytem and the external noie ource a hown in Figure A2. The Y-factor for the whole amplifier/converter meauring ytem combination i then meaured. From thi Y-factor, we can determine both the overall ytem noie temperature a well a the amplifier/converter noie temperature alone. Let the overall Y-factor be Y. Then, the overall noie temperature of the combined ytem, T, i given by: T = T Y T h c [Eq A6] Y 1 The true noie temperature of the amplifier/converter alone i given by: T T = T [Eq A7] G Therefore, to determine the true noie temperature of the device, we mut know the device gain. Thi can be found by an equation that ue the overall power output when the external noie ource i turned on, P oh, and when it i turned off, P oc, a well a the ytem gain, G : P oh P G = G T T oc ( ) h c [Eq A8] G ha been meaured previouly. In ummary, in order to determine the true noie figure of an amplifier or a converter, you mut firt meaure the meauring ytem noie temperature and gain uing the et-up hown in Figure A1, where the external noie ource i connected to the input of the meauring ytem. Thi i the calibration phae of the meaurement. Then, you diconnect the external noie ource from the input and connect the output of the amplifier or converter to the input of the meauring ytem and put the external noie ource onto the input of the amplifier or converter. Then, the intrument will how the true noie temperature of the amplifier or converter a well a it gain. Note 1 Harke Smit, PAØHRK, A Noie/Gain Analyzer, QEX, Nov/Dec 1999, pp See alo reference therein. 2 The Agilent Technologie Web ite i at and the direct link to the Application Note i agilent.com/litweb/pdf/ e.pdf 3 AD8307,00.html card.ap?part_id= Paul Wade, N1BWT (now W1GHZ), Noie Meaurement and Generation, QEX, November, 1996, pp Alo, ee reference therein. 8 The ATMega32 microproceor ource code and the HEX object code are available on the QEX Web ite. Go to and look for the file 5x07_Koehler.zip. 9 I can only handle the DIP verion of the microproceor. Send it in a padded envelope, along with an addreed return padded envelope, to J. Koehler, 2258 June Rd, Courtenay, BC, V9J-1X9, Canada. I cannot ue US tamp o pleae encloe enough IRC or $2 US to cover the potage. 46 May/June 2007