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Mixer Noise FigureConcerns andMeasurements

Ed CantrellOct. 1984

RF &.. MicrowaveMeasurementSymposiumandExhibition

rhO- HEWLETT~~ PACKARD

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Why Is Noise Important?

ReceiverSystem

SmallSignal

ImperfectDevice

orSystem

Signal Noisier

Noise limits the detection of weak signals. As a signal is passed through a device or system, noise is added, thusdegrading the signal to noise ratio. Noise figure is a measure of this degradation.

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F

Noise Figure Definition

Total Output Noise Power

Output Noise Power Due To Input Noise @ 290K

kTsB ~ Nor-----O_---4!Na~>------o=

..> V"

..: Zs @ Ts..-==-

Na + kTsBG

FNa + kTsBG

kTsBG Ts 290K

FdB 10 log F

Noise figure is a measure of signal to noise ratio degradation of any device or system that has at least one inputand one output. It applies to systems as well as components of the system.

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Why Is Mixer Noise Figure Important?

• A low noise preamplifier (to reducethe effect of the mixer noisefigure) is not always appropriatein a system design

• Manufacturers specify it; it needsto be verified

Some receiver systems precede their mixer with a low noise preampifier to reduce system noise figure. Cost,reduced dynamic range, etc. may prohibit the use of preamplifiers in other systems; in these systems, mixer noisefigure is very important.

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Conversion Loss

..-...-. EE mm 'a'a

............ Q)

Q) 'a'a ~...~ .- ___1--... c..-_.

Ec.E <C L

<C ...N~... c.~ ...

0. ....=1 01 1,\

freq. fa freq.

Conversion loss represents the loss to a signal in a frequency-translating (converting) device or system.

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Noise Figure/LossOf An Attenuator

~

~: Zs @ Ts...

kTsB-.

LNo

@Ts -.L...-_......~Attenuator

~... ~ @...: Z EQUIV. Ts...

-==-

NO = kTsB.....

F =Total Output Noise Power

Output Noise Power Due To Input Noise @ 290K

F =

F = l

kTsB

kTs B/l Ts = 290K

The thermal (Johnson) noise power available from a conductor is dependent only on the temperature of thatdevice. Two conductors at the same temperature will therefore have the same available thermal noise power as onedevice at that temperature. In the noise figure equation, the total output noise power, kTs B, is divided by theattenuated input noise power. The result: noise figure equals insertion loss.

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Diodes Are ..... Different

Depending on where on its V-I curve a diode is operatinga diode emits ...

-Shot NoiseAvailable Shot Noise Power = 112 k TAMS. B

(TAMS.: Ambient Temperature)

- Johnson NoiseAvailable Johnson Noise Power = k TAMS. B

Therefore ...112 k TAMS. B < N DIODE < k TAMB. B (can be higher)

Or

112 < TD < 1 (again, can be higher)

(TD: Diode Temperature)

Because the dominant noise mechanism over most of a diode's v-i curve is shot noise (rather than Johnsonnoise) a diode's equivalent temperature, TD , can range down to Yz TAMB . Diode temperature can also be greater thanTAMB , especially with high LO power. What does all this have to do with the noise figure/conversion loss relation­ship?

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Mixer Noise Figurevs.

Conversion Loss

7 To=1

To= 0.75

TAMB TAMB6

Mixer 5

NoiseFigure, 4

ToF (L-1)+1dB 3 TAMB

2 = 0.5 To0.5 < < 1.0

TAMB

1 (possibly higher than 1.0)

1 2 3 4 5 6 7 8 9

Conversion Loss, dB

W.L. Pritchard's· noise figure/conversion loss equation illustrates the difference in noise figure and conversionloss in mixers. (There are other equations that describe the mixer noise figure and conversion loss relationship; theyall illustrate that there is a difference in F and L due to diode temperamre, TD ).

Due to the above mentioned difference, it may be very desirable to measure noise figure and conversion lossseparately- a noise figure meter will do just that.

·W.L. Pritchard, "Notes on a Crystal Mixer Performance," PGMTT, p 39 (January, 1955).

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Noise Is AddedByLO

R

The noise present ± flF from the LO frequency will be downconverted by the mixer, attenuated by the L to I conversionloss and added to the noise generated by the mixer itself.

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LO Noise MakesF#L

-.L-I Conversion Loss (Isolation)

oo::::J

~U1o·::::J

r­oenen-Q.m-

565248444036

32

282420

16

128

4

o18.016.014.012.010.08.0

~OSB Noise Figure

6.0

-.OSB ConversionLoss

4.0

IF Frequency: 10 MHzLO Power: 9.000 dBm

282624222018

16141210

8

64

2o

2.0

-CD"0-

r

In a receiver, the noise from the LO is perceived (by the system) as noise added by the mixer; mixer gain (conversionloss) is not affected. LO noise can, therefore, contribute to a substantial difference between noise figure and conversion loss.

Mixer noise figure can be thought of as an indication of total health - it not only reflects signal degradation due to

internally generated noise, but also indicates the mixer's susceptibility to external noise (i.e. LO noise).

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Image Noise Wi-II DegradeNoise F'igure

- -

--- - --~

[I~ -~~

In addition to noise translated from the signal band, and the noise added by the mixer itself, further degradationof mixer noise figure results from noise translated from the image frequency band. For SSB systems that allow thepassage of the image noise, the noise figure of the system should reflect this degradation in its noise figure.

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,

Noise Figure Of Multiple InputResponse Devices

F• • •

The IEEE definition of noise figure states that the numerator of the noise figure expression should representALL output noise while the denominator should only represent noise translated from frequency bands that containsignal information. Let's see how this affects the noise figure of a mixer in specific applications.

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Single Sideband Noise Figure(Multiple Response System)

Na +

B 1 8 2r-- -

~-

kToB1 + kTOB2L 1 L2

FSSB = ---------­kTOB2

L2

Na

For a mixer used in an SSB system (i.e. a system that processes information found in only one downconvertedfrequency band) the denominator of the noise figure expression represents the input noise found in only one of thetwo sidebands. In other words, the denominator represents the noise that accompanies the signal on its way to the IE

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Double Sideband Noise Figure(Multiple Response System)

Na

FOSB =

In DSB systems (i.e. systems that downconvert useful information from two frequency bands) the denominator ofthe noise figure expression represents input noise from two frequency bands.

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SSB/DSB Noise FigureRelationship

(Multiple Response System)

FSSB =

FSSB = 2FOSB

{FSSB}dB = {FOSB}dB + 3 dB

If L1 =I- L2 {where L2 contains signal}

FSSB = (1 +

For a mixer used in a multiple response system, SSB noise figure will be 3dB higher than DSB noise figure(assuming L1 and L2 are equal). This is valid ONLY for the multiple response systems (i.e. systems that respond to morethan one input for every output).

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HP 8970ANoise Figure Meter Features

- -[].I 119_"0IS«'1OU1\e lIl!T£ll 1-----._-_._--,,- ---, '--'-"--------T - --_._- - - ....~f.~"'*lo , !XI. t { ~~';""~"'" ,,~ ) 'u. ~ ... Q.; ,. 1

",., ,,,,,,,,, ,"" ! B 2 D D - ! I f q. 2 q .! 2. 1 :1 :.""", I . 1 !!.-._...:.__._,,_....:.. ,,__ -J L _,,__" .__~ __,, :--..1

.~~.

•e:~'»"'S'1.;,~,.»:..', .. '<> <:....»·::c:·

• Tunable IF range 10 to 1500MHz

• Sensitivity - No external gain required

• Simultaneous measurement of noise figureand conversion loss

• Controls the external LO

• Built-in digital storage for output of sweptmeasurements to scope

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When DoesFSSB == FDSB + 3dB

Apply?

ONLY When ....1) Converting SSB (non-filtered)

performance from aDSB measurement

And ....2) L 1 = L 2

NOT When ....1) Predicting SSB (filtered)

performance from aDSB measurement(only an approximation)

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Preselected SSB Noise Figure(i.e. Single Response System)

_~SSB ...-._....Filter

FSYSTEM ¥- FDSB + 3dB

The image noise is no longer passed to the IF, therefore ....Mixer noise figure degradation due to image noise is not a factor

How do you determine preselectedSSB noise figure?

ANS: Measure Noise Figure ofMixer/Filter Combination

Although an SSB, single response noise figure can be approximated with FoSB + 3dB, to be accurate one shouldmeasure noise figures of the Mixer/Filter combination.

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MixerNoise Figure Depends On

Appl ication

Inexpensive Radar Radio Astronomy

FSSB 6.5dB

RF

LO

IF

*

FoSB 3.5dB

The mixer noise figure is undefined unless a particular application is also specified. In radio astronomy, thedesired signal is located in both the upper and lower sidebands. In an inexpensive radar, the signal is located in onlyone sideband and there is no preselector to filter out the downconverted image noise. The noise figure of the radarmixer should illustrate its noise figure degradation due to image noise. In the above example, the upper sidebandconversion loss is assumed equal to the lower sideband c.L., therefore, the noise figure is shown to be 3dB worse thanthe radio astronomy (DSB) case.

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Mixer Noise Figure Measurement(Function of RF)

Oscilloscope

_.....-- ," -.

"-r ...,

~

't IA ,.'--~ - '"'-I \.....~v.....

D

HP 8970ANoise Figure

Meter

DHP-IB

HP8672AI c==J1Synthe- I I I

sizer ~t==~

~~ >>----__JHP 346B > ..

Noise Source Calibration

16.0-m"C 12.0-Q)~

j 8.0C)

LLQ)(/) 4.00Z

0.02.0 5.2 8.4 11.6 14.8

Freq. (GHz)

0

-4 -m"C

-8 -cco

-12CJ

-1618.0

Illustrated here is a typical result from a mixer noise figure measurement. The measurement can be summarizedas follows:

1) Set up 8970 (Select appropriate measurement mode, start & stop frequency for sweep, IF frequency, ere.)2) Calibrate (Connect noise source to 8970 & push calibrate key)3) Make Measurement (Connect noise source to mixer & push single sweep key)

Noise figure will be different for different IF's. Measure your mixer at the IF frequency at which it will be used.(The measurement illustrated is DSB.)

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Mixer Noise Figure Measurement(Function 'of IF)

Oscilloscope

D 0

-4 -CD

"--8 c:::tU

CJ-12

-161.5.6 .9 1.2

Freq. (GHz).3

.... -.. --

~ ....... ""'" -I"""" ~

0.00.0

16.0-CD~ 12.0cv~

~C) 8.0u.cv'5 4.0z

Calibration) .

DHP-,B

HP 8672A ••••1••_1Synthe- IIIsizer

HP 346BNoise Source

HP 8970ANoise Figure

Meter

Here is an example of a typical swept IF noise figure measurement of a mixer. The three measurement steps are thesame a the RF measurement (SET UP, CALIBRATE, MEASUREMENT) with a few changes in the SET UP step.

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Noise Figure As A FunctionOf LO Power

"-~...............

... - -- - -- -- -- --~ ...,

"I.'

Gain - - - - Noise Fig. --

Noise Figure, Gain (dB)302010o

-10

-30-8 -6 -4 -2 0 2 4 6 8 10 12

LO Power (dBm)

-20

HP9826

I=~=\=-\-\-\-\1

HP8970A D

HP3468Noise Source

In order to find the LO power required for optimum noise figure or conversion loss, noise figure measurementscan be made at specified LO and IF frequencies, the LO power varied and the results interpreted for optimumperformance.

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SSB Measurement(Single Response Devices)

Oscilloscope

D

HP 8970ANoise Figure

Meter

HP 8672ASynthe­

sizer

HP346BNoise Source

IFAmplifier

flO

RF Measurement(Swept LO, Fixed IF)

For SSB measurements with a preselection filter, a swept RF measurement range of no more than 2 IF can bemade in order to keep the desired sideband in and the unwanted sideband out of the passband of the preselector. Dueto filter skirts, the practical range is usually much less.

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Accuracy

~NF1 (dB) = ~ NF12 (dB) -

F2 - 1F G ) ~M (a N.S. Ys. a N.F.M.)

1 1

NoiseSource

Total Noise Figure Uncertainty Is A Function OfF1, G l' And F2

According to the noise figure uncertainty equation, overall noise figure uncertainty is dependent on Fl , G l' and

F2· (F12 also depends on Fl , G l , and ~).

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Accuracy Example

LlNF1 (dB) = tiNF12 (dB) -

F12 F2 - 1 F2 F2 - 1- (F - F G ) LlM (ON.S. YS. OIN) + ( F G - F G ) ~M (ON.S. YS. 0 N.F.MJ

1 11 11 11

F121:---------,I F1 = 4 dB F2 6 dB IG1 =-4dBI l I

NOISE I I NOISE I ISOURCE""'---I~ I FIGURE I

METER I~20 L__ ---~ I

°IN' - 17I 0 OUT - •

o N.S. = 1.25 f"v 1 7 I°N.F.M. = . I

L- .J

~NF1(dB) = 2.02 dB (RSS)

Mixers, being low gain (G 1) devices, have a high uncertainty in measuring their noise figure (the lower G I is,the higher Li NFl is) but ....

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Accuracy Example(conI.)

~NF1 (dB) = ~ NF12 (dB) -

F12

r------F~;-I

I F1 = 4 dB F2

= 2 dB IIG 1 = -4 dBIF' -= - - - - - l IIFAMP - 2 dB I

NOISE I I I F NOISE I ISOURCEI4----t I AMP FIGURE I

'--..... METER I___ ~I

aN.'. = ~.;5N I "''8 IaJ.. = 1.7 I II

1 7 0 N.F.M. = 1.7I °OUT = . IL- .J

L\NF1(dB)~ (RSS)

~NF1 (dB) = 0.62 dB (RSS) With Preamplifier

... by adding a high gain, low noise preamplifier to the measurement setup (reducing second stage noise figure),uncertainty can be reduced considerably.

If the mixer is to be used with an IF amplifier in the final system, uncertainty can be reduced by measuring themixer/amplifier combination (the higher G 1 is, the lower LlNF1 is).

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