a low noise preamplifier for the 70cm band with a gain of ... · this design was presented at the...

This design was presented at the 2012VHF meeting in Bensheim, it describedthe development of low noise LNAs forthe frequency range from 1 to 1.7GHzThe lecture was published in expandedform in VHF reports. It documentedthe current state in development of lownoise MMICs. The approach for asuccessful design was demonstratedusing a model amplifier. Measureme-nts confirmed the good simulation res-ults. The performance of the circuit atlow frequencies was also tested.

1.

Overview

The properties of modern MMICs, fromthe article mentioned above [1] for a 1 to1.7GHz amplifier, are briefly givenbelow for the 1 to 1.7 GHz amplifier. Theadvantages are: • Input and output are matched for 50Ω • Noise figures are below 0.5dB even

at frequencies below 500MHz • More than 20dB gain up to 2GHz. • Minimal external circuitry.

The author struggles with the followingdisadvantages: • The dimensions are now tiny and

only tiny solder pads are used instead

of connecting pins. • The common ground is a small spot

of solder in the middle of the unders-ide of the package.

• The layout design requires very highaccuracy; tracks and connection padson the IC are typically 0.25mm witha maximum width of 0.5mm

• The supporting components must beSMD size 0603 (1.25 x 0.75mm) orless to work.

• The cut-off frequencies of the comp-onents are so high that stability cont-rol is necessary even when operatingbelow 1GHz and up to 10GHz theref-ore appropriate measures must betaken.

• The operating point must be carefullycontrolled and very carefully stabili-sed partly due to the high currents(often over 50mA per device). Thesupply voltages are decoupled evenmore carefully and for a broadband

The thickness of the PCB was reduced,for all applications, to 0.25mm due to theextended frequency range; this preventsthe emergence of unwanted modes of thesignals on the strip lines. The time forvias made from silver plated tubularrivets is over - now you have to havePrinted Circuit Boards professionalmade.

The LNA development described in [1]was very successful; therefore, a step

A low noise preamplifier for the70cm band with a gain of 25dBand a noise figure ofapproximately 0.4dB

Gunthard Kraus, DG8GB

VHF COMMUNICATIONS 4/2013

201

down in frequency has been examinedfor the possibilities of use on the 70cmband. This is more difficult, because themanufacturer of this MMIC only givesusage data above 1GHz therefore theMGA635-P8 documentation for the prop-erties at low frequencies (e.g. noise para-meters) is very, very poor. So, a requestlist was formulated and checked how farit can be met: • Noise figure: maximum 0.4dB • Gain (S21) at least 20dB • Absolute stability (k greater than 1 up

to 10GHz) • The output reflection S22 should be

as low as possible (-20dB at 438MHz= dream value).

2.

The circuit development

The LNA for 1 to 1.7GHz (Fig 1) and the

PCB layout were used. A cascode GaAs-pHEMT amplifier and bias circuit areinside the MMIC device. Pin 1 sets theoperating point of 55mA through the biascircuit with a resistor (Rbias = R1 + R2 +R3 = 3.6kΩ). The generated bias feedsthe first gate on pin 2 via L1. Theinductor L2 on pin 7 forms the loadresistance of the second stage.

A major problem of the HEMT compon-ents is stability at low frequencies - theirtendency to oscillate. So a simple trick isused: with decreasing frequency the 50Ωvalue of R1 is presented more and moreto the input pin 2. This is effective andprevents oscillation. The way this works: • The reactance of L1 decreases with

decreasing frequency, but the reacta-nce of C3 increases. So at some pointR1 = 50Ω is the only thing active atthe input pin 2.

The circuit was adjusted for noise at438MHz using ANSOFT Designer SV.The values of L1, L2 and C3 weregradually changed using the simulation

Fig 1: The circuit diagram of the predecessor to this design.


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results for the noise figure, "NF in dB”,and under a permanent control of thestability.

The intention was to create the minimumnoise at 438MHz and to optimise thenoise figure NF. The minimum valueachieved was considerably lower than0.4dB (simulation result: NF = 0.12dB)with L1 = 180nH / L2 = 120nH and C3 =39pF. Up to 10GHz, a small resistor of

10Ω in the output circuit gave sufficientstability (fitted closed to the output pin ofthe MMIC). The output micro strip line(more correctly: "Grounded CoplanarWaveguide") with a width of 0.59mmand a "gap" of 1mm on each side and alength of 32.5mm should not be omittedfrom the simulation. The final simulationresults were obtained using the simulat-ion schematic shown in Fig 2 with thenoise data as shown in Fig 3. This is a

Fig 2: The simulation circuit diagram for Ansoft Designer SV. It offers nogreat mystery for the optimisation of noise figure and stability.

Fig 3: These noiselevels would be adream but are theyrealistic?


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dream, of course it must actually bechecked using a prototype to see if this istrue. Finally, no noise data is containedin the S parameter file for this frequencyrange so the simulation program simplyused a linear decrease of the noise figurewith decreasing frequency!

The required stability (k greater than 1 to10GHz) was no problem as shown in Fig

4 and the simulated S parameters for thisfrequency range gives no cause for conc-ern (Fig 5).

No extra development work was necess-ary for the board because the versiondeveloped for the 1.7GHz version couldbe used. The material is the “flameretardant version" of the familiar andproven Rogers material RO4003 called

Fig 4: No stabilityproblem up to 10GHz.

Fig 5: You can besatisfied with thesesimulated Sparameters up to1GHz.


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RO4350B and in stock at the circuitboard manufacturer [2] in the desiredthickness of 10 mil = 0.254mm. Toproduce such a thin PCB with countlessvias in perfect quality a Proffesional PCBmanufacturer is necessary but the suppl-ier [2] was very cooperative and istherefore recommended.

The Board size is 30 x 50mm with thebottom surface as a continuous groundplane (a matching milled aluminium caseis used). Input and output use SMAconnectors and the +5v power supplyuses an SMB female as usual.

The central ground connection on theunderside of the MMIC package requiresits own 0.6mm wide ground island with 6vias. All other ground planes on the PCBare carefully separated having enoughvias. Actually, that's old hat, but it ispurely and simply to give "neutral pointgrounding” that is recommended at lowfrequencies to avoid a tendency to oscill-ate. By the way: all vias have a diameterof only 0.3mm.

More on the Board follows in Chapter 3.

3.

Improvement of outputreflection S22

First start a new Ansoft simulation forthe frequency range from 200 up to600MHz and display result of the S22 Sparameters as a Smith chart (Fig 6).There is something very pleasing:

The 32.5mm long 50Ω line at the outputmoves the phase of S22 just so far thatthe S22 point for 440MHz is almostexactly on the reactance circle that runsthrough the centre of the chart. Thatmeans that S22 can be improved with asmall additional inductance between theend of the line and the SMA outputconnector to compensate the capacitivereactance of S22 at this location.

Thus the chart centre (perfect match)comes close.

A small interim is calculation is required:

According to Fig 6 for the point "f =440MHz", Ansoft designer SV gives anormalised impedance of:

Z = 1.101-j0. 892

This results in a capacitive reactance of

Fig 6: The diagramfor S22 at therequired frequencyshows thatcorrection is needed(see text).


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XC = (50Ω) x (0.892) = 44.6ΩThe required inductive reactance gives aninductance of

This is something that cannot be bought"off the shelf", the closest standard valuechosen was 15nH and the simulationdiagram corrected accordingly is shownin Fig 7. The connection point for theSMD inductor is as close as possible to

the output SMA connector. A 30mm longsection of 50Ω line from the MMIC isconnected to the 15nH inductor followedby the remaining 2.5mm of line up to theSMA connector.

It is worthwhile to analyse the result (Fig8) and looking at the the Smith chart youcan see that it has landed not far from thechart centre for a perfect match.

You should not miss the result for S22 asa Cartesian chart together with S21 (Fig9). The comparison with Fig 5 is veryinteresting showing the matching. But,

Fig 7: These modestchanges will do thetrick.

Fig 8: The changesgive almost theideal matchingpoint.


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S21 will now decrease much faster withincreasing frequency because of the indu-ctance inserted in the output line since itsreactance is also increasing. Therefore

the stability factor “k” should be checkedvery closely up to 10GHz so that thereare no unpleasant surprises, however Fig10 is reassuring.

Fig 9: Suchbeautiful simulationresults look reallygood.

Fig 10: Themodified circuit hasa positive effect onthe stability.


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4.

Measuring the S parameters ofthe prototype

Starting with the circuit diagram (Fig.11) showing the changes from Fig 1 that

are minimal but extremely effective. Thisis followed by an almost identical boardwith a break in the output line after adistance of 30mm to insert the additional15nH inductance. The revised layout isno problem (Fig 12), but the existingboard can be used. A gap can be made onthe original 1.7GHz version. A smallpiece of the conductor can be cut out

Fig 11: Who can spot the few changes from the 1.7GHz version?

Fig 12: This is theonly change to theprevious PCB: asmall gap for theadditional inductor.


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with a scalpel but this requires a warning:

The copper layer of the Rogers RO4350Bmaterial does not adhere as well to thesubstrate as the previous RO4003 substr-ate so it is easy to remove the wholetrack with such an operation!

It is best done using a "Dremel" (smalluniversal electric hand drill) with a smalldiamond cutting disc.

The finished board fitted into the alumin-ium housing, marked as a test Board andwaiting for the Vector Network Analyseris shown in Fig 13.

The measurements were performed withan HP8410 true vector analyser and theassociated S parameter test set (HP87-45A). A 20dB attenuator was used infront of the input for the S21 measurem-ent to avoid clipping.

The graph of S21 can be seen in Fig 14and it leaves no wish unfulfilled.

Convincingly, Fig 15 demonstrates thesuccess of the measures to improve theoutput reflection S22, giving satisfaction.

It is advisable to disconnect the input ofthe DUT for the measurement of theoutput "Port 1" of the Network Analyser

Fig 13: When youfinally see this sightyou know that youhave done a lot ofwork to produce thefinished prototype.

Fig 14: Withoutmeasurements wedo not know howthe prototypeworks. S21 is asexpected.


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and to fit a 50Ω terminating resistor onthe SMA socket instead. This results in asignificantly more accurate result; beca-use the transmitter output has more refle-ctions than "little blue" (this is not only ajoke, but a note on the blue paint of SMAtermination made by Watkins Johnson. Itis proven that Watkins Johnson togetherwith the "Huber and Suhner" companyare the front runners for making gooddevices with very low reflections over awide frequency range).

One strange result is the shift of the"resonance peak" at low frequencies des-pite the use of the next standard value forthe additional inductance (15nH insteadof 16.6nH). This resonance frequency islower by 20MHz.

For the measurement of S11 the amplifieroutput was separated from the analyserand connected to a "little blue" terminat-ion. Fig 16 shows the measurement ofS11 at the amplifier input together withsimulation. The result of 2.5 dB is notgreat but at least shows that theory andpractice match.

Measuring S12 is more difficult due tothe low amplitude but the measurementfor a frequency of 440MHz is around thesimulated value of approximately 45dB.

5.

The noise

The precise measurement of small noisefigures (NF about 0.4dB) for the firstamplifier version last year (see [1])proved to be a major hurdle for thebasement workshop and it was complet-ely dependent on the help of friends withappropriately high quality measuringequipment. In response to the article,there were some important tips andemails from readers who themselveshave already struggled with the sameproblems. Therefore some experimentat-ion and additional brooding suddenly

produced a solution to solve this tasksuccessfully.

Fig 17 shows an overview of the schema-tic for the slightly modified measurementsetup from last year that suddenly delive-red the desired results with sufficientaccuracy using changed settings of instr-uments. The crucial difference is the 6dBprecision attenuator inserted betweengenerator output and input of the amplif-ier being measured. As a result, 6dB isadded to the expected noise figure andthat fits perfectly with the lowest outputlevel range of the SKTU noise transmit-ter from Rohde and Schwarz with NF = 0to 8dB.

The measurement is as follows using thepractical setup shown in Fig 18:

The HP8555 analyser is used as a measu-ring receiver to measure the power at f =440MHz (scanning function switchedOFF to get a selective level meter). Oncethe whole chain is in operation check theinfluence of the noise floor of the 20dBpost amplifier in this manner:

Set the output level of the SKTU to Zeroand remove the supply voltage from theamplifier being measured and determinethat this decreases the indicated noiselevel on the spectrum analyser screen bymore than 20dB. That should be enoughto not distort the result.

Here we go with the measurement:

The amplifier to be measured is switchedon again and the noise transmitter turnedon. It is very nice to see how increasingthe noise is displayed as a "marchingpoint" on the screen. But to determine anincrease by 3dB (then the LNA noise isexactly the same as the external input anddisplayed on the instrument of the noisetransmitter), the following steps are requ-ired: • Start with the noise transmitter output

power set to “Null”. • The bandwidth of the spectrum anal-

yser must be set to 30kHz to get atiny and little "wriggling" point on


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the screen (more decreasing bandwi-dth always gives a restless display).The video bandwidth is set on "b =10Hz".

• Move this dot (with the help of amagnifying glass!) to be exactly inthe centre of the screen.

• Increase the noise output level withone hand until the dot moves byexactly 3dB upwards (using a magni-fying glass in the other hand to watchthe analyser's screen!).

• Read off the indicated noise level onthe SKTU instrument and subtractthe 6dB of the attenuator.

The exciting result as the average of 10measurements:

NF approximately 0.38dB

That does not agree with the simulationbut is exactly the noise figure of the1.7GHz version that had a NF = approxi-mately 0.3dB at 1GHz.

Fig 15: S22 ishopefullysatisfactory despitethe small differencecompared to thesimulation.

Fig 16: S11 theoryand practice are thesame but byfavouring noiseadjustment meansthe values not quitethe same.


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6.

What does a professionalquality measuring instrumentgive?

I was really happy with my own measur-

ement results but because the elderlymeter results should be verified with thelatest equipment with greater accuracy.Uli Kafka (from the electronics companyEisch, Ulm) was able to help again.

The following results were as followsand the comparison with Figs 14 to 16 isreally worth while (again: thank you,

Fig 17: This is the improved DIY noise figure measurement setup (see text).

Fig 18: This is what the noise measurement system looks like: however it isimportant to understand the operation (see text).


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Uli):

With f = 438MHz S11 lies between -2.7dB (for an input level -20dBm) and -2.5dB (with an input level of zero dBm).

S22 = -21dB with the minimum at f =438MHz (the best fit to my measurementis not reached with -25dB at 430MHz).

The exact graph of S21 (approximately28dB) and the noise figure NF between400 and 500MHz is shown in Fig 19. Thevalue of NF = 0.375dB at 438MHz isvery reassuring and convincingly confir-med the accuracy and reliability my ownnoise measurement described above. Butit does have a considerably greater opera-tion and calibration time.

S12 is where the simulation predicts(much less than -40dB) and my ownmeasurements gave the same result.

7.

Summary

The electronics developer’s life is full ofsurprises and there is no opportunity tobe comfortable and relax. You could

write that as a conclusion - but it doesshow the very fine line between simulat-ion and prototype designs and encoura-ges new projects.

Since this worked out so well is a144MHz version of the amplifier possi-ble?

8.

Literature

[1] Development of a preamplifier from 1to 1.7GHz with a noise figure of 0.4dB,Gunthard Kraus, DG8GB:. VHF Comm-unications Magazine, issue 2/2013 pp 90- 101

[2] Board manufacturer in Munich:www.aetzwerk.de

Fig 19: Thecrowning glory arethe results from theprofessionalmeasurementequipment. Thereward for a lot ofwork is a noisefigure of 0.375dBand a gain of 28dBat 438MHz.


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a low noise preamplifier for the 70cm band with a gain of ... · this design was presented at the...

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