a noise generator with defined noise power for applicaitons up in the microwave … archive/a... ·...
TRANSCRIPT
VHF-COMMUNICATIONS 3/84
Harald Fleckner, DC8 UG
A Noise Generatorwith Defined Noise Powerfor Applications up in the Microwave Range
In recent years, several construction al articleshave been published for noise generator s (1),(2), (3), fo r a noise measuring system (4), aswell CIS informati on regarding this measu ringtechnology (5), (6)
connection to an automatic noise measuring system , and gives further informationregarding operational experience.
1.NQISEDIQDE
", (01'1<."'''('', .o...ro"•• • t OJ)
Fig. 1: Case and dimensions of the noise diodeBAT 31
,,),10
)00
The noise diode used is a BAT 31, which ismanu tacturec by PhilipslMuila rd in a mic rowave case type SO 86 (Figure 1). This case is
The following article describes the construction of the source and provides technical specifications of the noise diode, aswell as describing the construction of apower supply with switching amplifier for
The sem iconductor noise generators describ ed in (1) and (2) use the base- emitter d iode 01an RF-tm nsis\or operated in the backwarddirection. The avalanche er-ect required forwideband noise is not very prevalent due tothe low breakdown voltages of the RFtransistors BFR 34a and BFR 96 of < 5 V. It ismainly a tunnel effect that appears here ,which means that the noise spectrum dropsoli cons ide rably towards higher freq uencies(> 1 GH z), (7).
The described noise generator operateswith a silicon avalanche diode type BAT 31which provides a wideband noise spectrum from < 10 Hz to > 18 GHz with a typi cal noise power (ENR == Excess NoiseRatio) of > 34 dB. This allows manual orautomatic noise measurements to be madeup to the satellite TV-band of 12.4 GHz withsufficient accuracy even for radio ama teurs.
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VHF-COMMUNICATIONS 3/84
Fig. 2:Noise power as a f unction of frequency whenInstall ed In a 50 () system1SmA noise diode b ios
Abso lut e error l or 50nsystem Im peda nce
2,0 5.0Freouenc y (G Hz)
o
I
""'"z ' 01;\2 38•2 36• ....-r-........1-•'g 34
• 32' 'O"---",,,-~~~""",';c~~...,!;;-III " 0•u<W
known from the varacto r and Gun n dio de techno logy
The no ise power as a function of frequency ina 50 D system is shown in Figure 2. The fluctuat ions of lhe ENR in the amateur bands inexcess of t GHz amount to ± 0.5 dB, whichcorresponds to an absolute value of 1 dBwhen referred to a nois e power of 35 dB: tfus is
a very low value with respect to the large frequency range,
Fig ure 3 gives the ENR as a function of d iod ecu rrent, and of the frequency. One will seecl early th at a current of 13 - 17 mA is requi redfor wideband opera tion . The breakdown voltage is between 17 and 22 V.
nc.aNoise powe l as a function ofavalan che cu rrent w ith frequencyas parameter when installed in a50 o system
toNoi5e d iode bi" 5 curren t (mA l
l GHz
,•6o
Reco m me"d..d brosrange for brood bondoperat io n up t o 12A GHz
138
VHF-COMMUl\ICATIOI\ S 3184 - - --- - - - - -'.\
c
5tf lpl lf'e,--
Curr@1l" SQl';@ e- DC- 3k:k lf1g e- 50FL ~ A t t enuato rP8 002BA T 31
" Q:>14d B
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S,, ';hi 'Q p"~.. . "o~ ·,Oe BUG
L. ,~.,... I- 36 Ylle ~2~2 N n19A ~2OC ~ 00
nJl.JL oft
--- - - ----- - _0 0 _9V f ,o ~ 00 '" '1" ""
Fig. 4 : B lo ck diagram of fhe noise source and us power supply wit h switching ampli fie r
2.CIRCUIT
When used as noise sou rce In swncneooperation (manual or autc-n anc}, the componentshould exhibit a sou rce imped ance of 50 natannro es.
In c roe r to obtain this. the diode is nu.tt into a50 n coaxial or stripl ine system and terminated with a wicebanc ette nuarcr of > '4 dB.VSWR < 1.2 (see Figure 4 ). The value ot thereturn loss has a decisive enect on the qua lityof the meas c-ement (see Sect ion 5) . In th is ci rcuit. It is exclusively cetermmed by the attenuator. A varco of 20 dBwith a VSWR :;; 1.15up to 4 GHl and VSWR :;; 1 2~ tram4 - 12.4 GHz nas been found to bu sufficie ntlygood (Weinschel, HP, Radiall, or Greer-panHeme-mace uttenuatcrs are not recom mendco here' With an attenuauon at 20 dB, theE:NR Will amount to app roximatoly '5 dB andits rreqcencv-oepencent uuctuanons Will nowbe determined by the diode and auenuatorGood coa xial atteouato rs have a tolerance of:; 0,5 dB.
Mosl norse measunnq systems are calib ratedlor use wilh an EXCQSS Noise Ratio ENR 0115,2 dB Thi s means that the source is virtuallycompatible,
In the author's prototype. the diode operatesin a 50 n stripline system as shown in Figures5 and 7. The switch or operating voltage is fed
to the stnpfme via three. series-co nnected150 n metal-film chip resistors. The position.dimensions. and quality 01 the restsiors havean enect on the wideband characteristics o fthe decoup ling .
Tho diod e current is adjusted with the aid of aFET·constant current source, which determine s the noise power. Two par allel-connecteo ATC·tOOchip capacitors decoup le the swlt-
h AT{: DO7 : SOJl
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h 1';oOfI-lh ip
IJATJ l
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.---1 820p
Fig . ~ : SOn strl pllrle syslem wit h DC-YOlt3<,j('blo cking et the ou tpul connecl O1. andw iOOb '..>dcho ke in lhe supply line
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VI-tF-eOM MUNICATIONS 3184
Fig_ 6: Power supply with swit ch ing ampli fi er and current source
ncU UG
Fig. 7: layou ' 01, 0.79 mm RTldumid boa rd
140
3.CONSTRUCTION
The diode system comprises mainly a doublecoated. PTFE-board (RT-duroid 0.79mm) ontowhich a 50 0 line has been provided , Thecoupling ot the diode to the line must be madeextremely accurately, since it determines thevalue of the available noise power at the upperIrequency limit.
Further details regarding this are given inFigu re 8. The diode mount comprises a goldplated Ma-screw with which the diode iscoupled to the stripline on the ground-side oftheboard via a Ma-nut. In order to achieve this,the line is provided with a 1.5 mm hole at theend. which is carefully extended to 2 mm oremeter.The centrical position of the nut is cueremeec by use of a dummy instead of thenoise diode (such as a defective varactor orGunn diode), After this, a small gold plate isprepared with a width of 23 rnm. lengthacoroc. 3 mm. and a 1.6 mm hole. and isplaced over the dummy and soldered to the
noise iig ure measuring system described in(II) , or with older noise figure measuringsystems [eq . tho GM 551 A)
AICU'
,,'SJRC ~io
J61
ching voltage from the attenuator. The switching voltaqe should amounl to at least 33 V,which is generated most favorably using aseperaie power supply_ Figure 6 shows thecncun diagram of me power supply withswitching ampliher and current source- If theamplifie r input is provided with a modulatorpulse01, lor instance,9 V.500 Hz squarewavovoltage, a current and voltage-controllablepulse ot the same frequency and sufficientpowor will be available at the output. Thisallows the source to be used together with the
VHF-COMMUNICATIONS 3184
Co,,:acts
Smell geld o'a:e w;th 1.6mrn hole ;I'.' _ 2.3 mm; I _ 3-4 m-n soldered to str ipline
~'/~~~~'7~,/,::,----- Copper coatingr PTFE (RTlduroid) 0.79 mm
-------- Cor r er coannq
BAT 31
-fF=='--- M4 con withhole of 1.6 mm (I;a ..:3 1.6 mmimi" 2.5 m m d" up
Fig.8:In stallation detailso f the no ise diode
BAT31
PTrE-~a rd with ~ mm hotA
Go ld-r imed M4-nut soldered into pj~c ~
ecro-ororor M4 bott wit ll llo l~
stnptme. The smal l gold plate can be obtainedtram a connect ion strip of a power trans istorThe dummy is now rernoveo, and the hole iswidened from the rear side thro ugh theM4-nut with the aid 01 a 2 .1 mm dr ill , so thatone can guarantee that the diode is coupled tothe strip line with the aid 01the lower cathodeconnection via the gold plate, With a carefu lconst ruction, one will achieve a good stnplinecoaxial transition
Alte r this, the PC-board is solde red into ametal box, The DC-switching voltage is nowfed in via a BNC-conne ctor. and the striplineshould be terminated with the lowest disconti nuity with an SMA or Nconnector (preferablyN-plu g). One sho uld only use preci sion Ncon nector s (clean thread, gold p in, maybeusing a smaliliange). If one expects to obtainreproducible measur ing results, it is importantthat no movable coa xial con nections are provided. For this reason, BNC sho uld not beused!
Final ly, the chip capacitors, chip resistors, andthe choke with the prec ision resisto r. and thetwo disk capacitors on the ground side, areinstalled.
On insertin g the diode, check for correct pola-
rity (blocked operatio n) using an ra-rneter.Fig ure 9 shows the author's prototype fromboth sidesThe power supp ly with switching amp lifier andco nstant current source can be accommodated on the epoxy PC-board show n in Figure 10.fj manual operat ion is to be used exclusively,one wil l not require tho switching amplifier,which means il can be deleted from the board
4.OPERATION
Check the power suppl y before con nect ing it10the noise sautee. This is achieved by replacing the noise diode by a 20 V zener diode andconnect ing it via a dropp er resisto r of 470 0 tothe power supply. At an out put voltage of 36 Vfrom the stabilizer L 200, it should be poss ibleto adjust a current of 15 mA with the aid of thetrimmer potentiometer of the cur rent sou rce.CAUTION' T 2 mu st block!
Furthermore, the switching amp lif ier shouldbe driven with the mod ulator pulse from thenoise meas uring system, and the waveform at
141
=--------~~-- - _ _ _ _ _ _ _ -'V"H"F,~C"O"M~MCU~N,ICATIONS 3184
Fig .9:On the upper side of t henoise sou rce (w ith outattenuator), one w ill se eth e b locking capacito rs .the th ree ch ip resistors,and the buil t·i n diode.On the lo wer side , on ecan see the diode mo untand t he precis ion resistorso ldered int o p lacebetween the two diskcap ac itors.
the output checked with the aid of an oscilloscope.
It everything is working correctly, the sourcecan be connected to the power supply. Thecurren t value should be aligned using thetrimmer of the current source. This can bechecked by measuring the voltage dropacross the precision resistor R" == 100 Q. Prel im inary, it is suff icient to adjust the cur rent to15 rnA ~ 1.5 V voltage drop. In the case of apulsed diode, one ca n measure th is best withthe aid of a RMS-voltmeler, or oscill oscope.
For relative noise measurements, or compari son measurements for opti mizing the receivesens it iv ity of amplifiers etc., it is not necessaryto r th e ENR to be ca librat ed exactly. However,for abso lute measurements, it is abso lutely
142
necessary for the source and attenuator to becompared to a cal ibrated no ise source overthe who le frequency range of interest.
5,OPERATIONAL EXPERIENCE
The au thor has been using a BAT at-noisesou rce w ith great success for more than threeyears. It was firstl y used in manual operationfor measuring the Y-facto r, and later used inconjunction with a noise measu ring system ,typ e 551 A, manufactured by Genera l Microwave
VHF-COMMUNICATIONS 3/84
,i'~'''i7 OOjJ 1 6r'~Vr-,
Fig. 10, Co mpo nent locat io ns on t he po wer supply /switching ~mp l jficr board DCa UG 006
For abso lute measurements, it is necessary toknow the ENR for the amateur radio bands.Comparisons to All-norse systems using Hj> .
sources showed that the source desc ribedhere, used in conjunction with a 20 dB attenua to r, exhib ited the follow ing ENR-values:
- i44l 1296 2320 3456 5760 10368 MHz144 432-
15.8 I 154 15.2 15.2 15.0 16.9 14.0dB-
Sutticient noise power is also available in theshortwave range. II is even possible to carryout alignment for maximum sensit ivity in the80 m band . However, exact noise power valueswere not determined .
The principle of noise measurements is not tobe desc ribed here, since suffic ient articleshave been publ ished regarding this subject(1), (2), (6) .
However, the author would like to give someinformation regard ing the measuring accuracy, espec ially in conjunction with automaticnoise measuring systems.
Amon g experts, there is no doubt that all automatic noise measurements have a considerable uncertainty, espec ially in the range below3 dB The following errors are added to the
ttuctuatio ns of the source-ENR as a funct ion offrequency:
1. Accuracy of the noise metere go AIL 7514 ± 0.15 dB, GM 551 A ±0.25 dB, HP 340 A ± 050 dB, all in thelowest range .
2 Temperature error± 0.1 dB with a ± 10degree dev iation from290 Kelvin
3. Image noise powerup to + 3 dB, this can be avoided by usingsuitable circui try
4 Mismatch sou rce - fest objectThis error is not considered carefullyenough during noise measurements in therange below 3 dB. For th is reason, Figure11 shows this street graphically.
143
VHf-COMMUNICATIQNS 3/84
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Fig , 11; Ma.imum measuring eflOI a s a function 01th e me.suled n o ise hgure, as we ll as 01the VSWR·Ylllu.. sQfthe nQise SQurc e an d test o b tet1 (UUT. Hewlel1·P.tk....d).
Example: In the case of a determined noisefigure of 2 dB, a source·VSWR of 1.2, and atest object-YSWR of 2.0, the measuringuncertainty will be in the order of ± 0 5 dB!
As can be seen further, the retom loss of
' 44
Ihe noise source cannot be large enough inorder to provide tolerable measurements inthe range below 2 dB, especially when thetest objects posse ss a high stand ing waveratio.
VHF-COMMUNICATIONS 3184
If these errors are added in the correct sign,one will arr ive in a tolerance range that makesthe decimal oa-veroee seem rid iculous Evenif one assumes that those can appe ar withopp osite signs, and the actual accura cy of themeasur ing system is greater than the tolerance value. one should note that diffe rencesof 05 dB must be accep ted, whe n using thesame preamplifie r at differ ent times with di fferent measuring set-ups on the same measuring system.
This just is to demonstrate how questionablemost noise figure values under 1 dB are it theyare not measured using an exacnv calibratedhot/cold standard, but have been autom aucauy measured , and tnat temns-cr-a-oavalues can be ignored.
To summarize, it can be said that the possib il ity of compariso n measurements in the SHFrange represents an important aid for thehome const ruc1or and cr itical amateu r consumer, especially consideri ng the incre asingact ivity. Absolute values should be of secondimportanc e to radio amateu rs, since the optimization of every receive system must bemade including the antenna , and Ihis IS
achieved best using solar noise(8)
6.REFERENCES
(1) Michael tnbrtcnt . DB 2GMA Noise Generator for VHF and UHFVHF COMM UNICATIONS 13.Ed ition 1/1982, Pages 38- 43
(2) Or A. Waxweiler:Rauschgenerator mit detinierterRauschleistungcq-DL 12/1981, Pages 585- 587
(3) H. Fleckner:No ise Generator with BAT31Dubus Info 3/1983. Page s 188 - 189
(4) Martin Dohlus:A Home-Made Autornanc Noise-PiqureMeasu ring SystemVHF COMMUNICATIONS 15.Ed ition 111983, Pages 2- 11VHF CO MMU NICATIONS 15.Edition 2/1983. Pages 66 -83
(5) Gannaway/Hol mes :Some Pitfa lls in Noisa-FlqureMeasurementsVHF COMM UNICATIONS 14.Editio n 1/1982. Pages 44 - 48
(6) B Stein:Automatic Noise-Fiq ure Measu rementsFacts and FancyHam Radio Magazine 1978.August. Pages 40 - 54
(7) Unger/Schultz:Elektrc niscbe aao erernenteund Netzwerke IVioweg-Verlag Braunschweig,ISBN 3-5281-3505- 0
(8) G. Hoch. DL6WU:Determining the Sensihvilyot Receive Systems with the Aidof Solar NoiseVHF COM MUNICATION S 12,Edition 2/1980, Pages 66-72
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Yours - UKW-BERICHTEIVHF COMMUN ICATIONS
145