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Due to the crowding of licensed and unlicensed

bandwidths by an increasing number of wirelessapplications, including cellular telephones, wirelesslocal-area networks (WLANs), and the coming ofBluetooth devices (see the supplement on Blue-tooth packaged with this issue), Rxs are faced witha growing number of undesired carriers along withthe wanted signal carriers. When the unwanted car-riers are close in fre-quency to the desiredcarr ier, front-end Rxfilters cannot removethe interference sig-nals. The unwantedcarriers then mix with

each other and withthe desired signals andgenerate intermodula-tion distortion (IMD).

E very systemdesigner wants to min-imize IM products. Innarrowband Rxs, oneof the most trouble-some IM products isthe third-order prod-uct, which is measured

in relation to the I P3 of the Rx. The higher the Rxs

IP3, the better suited it is to capture the desiredsignal in the presence of unwanted IMD.

Conventional double-balanced and triple-bal-anced mixers are produced using Schottky barrier-diode quads. An I P3 of +25 to +31 dBm can beachieved by using this technique. However, toachieve such high IP3 performance in a diode-based

mixer requires a greatdeal of tuning, whichthen increases themanufacturing com-plexity and cost. Well-designed FET mixers,on the other hand, can

provide extremely lin-ear performance, andcan be used as mixingelements, with highI P3 performancereadily possible. Thisarticle highlights aseries of double-bal-anced and dual-dou-ble-balanced (patent-pending) FET mixersdesigned for I P3 per-

Engineering Department

Mini-Circuits, P.O. Box 350166, Brooklyn, NY 11235; (718) 934-4500, FAX: (718) 332-4661, Internet:

http://www.minicircuits.com.

Novel Passive FETMixers ProvideSuperior Dynamic Range

RECEIVERS (Rxs) in support of the continually expanding wireless market-place rely heavily on improved performance from mixer components. Whatwas once considered adequate performance in terms of dynamic rangemixer noise figure at the low-level end of the dynamic range and third-order

intercept point (IP3) at the high-level endis no longer good enough. Since mod-ern mixers must provide third-order intercept performance in excess of +35dBm, the engineers at Mini-Circuits (Brooklyn, NY) set out to beat that numberby at least 3 dB, with a new line of field-effect-transistor (FET) mixers designedfor third-order intercept performance to +38 dBm.

The dual -double-bal anced con f i gu r at i on of these FET mi xer s

helps to i mpr ove dynam i c r an ge wi th r educed conver si on loss

and noi se fi gu r e wi thou t expendi ng DC bi as.

1. This schematic diagram shows the basic

configuration for a double-balanced FET mixer.

LO

RF

IF

AN-00-003 Rev OR (M79277)This document and its contents are the property of Mini-Circuits .

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formance to +38 dBm.Modern mixers with I P3 on the

order of +30 dBm and higher typical-ly use F E Ts as mixing elements.Some of the disadvantages of com-mercially available FET mixers are a

higher conversion loss/noise figure ofapproximately 9 dB, and the need forDC bias current.

Mixers are usually specified fortheir input I P3 performance. Thecorresponding output I P3 perfor-mance is the difference between theinput I P3 and the conversion loss.

Thus, higher conversion loss resultsin lower output I P3. Higher mixernoise figure results in higher Rxnoise figure. To reduce the Rx noisefigure, high-gain low-noise amplifiers(LNAs) must be used in the front

end. However, this increases the sig-nal level (wanted and unwanted)reaching the mixer, and limits sys-tem performance due to the IP3 limi-tation of the amplifier.

The need for DC current increasesthe complexity of a circuit design.

The current supply should have lownoise, or it may introduce spurioussignals at the output. I t requires ad-ditional real estate on a printed-cir-cuit board (PCB) to transfer the cur-rent from its source to the mixer. Theneed for mixer current increases the

DC power budget for an Rx design.Fortunately, the traditional short-

comings of FET-based mixers havebeen addressed by a line of dual-dou-ble-balanced F ET mixers and dou-ble-balanced FET mixers from Mini-Circuits. The patent-pendingdual-double-balanced and double-balanced F E T mixers feature lowconversion loss, low noise figure, andhigh I P3 performance, even thoughthey do not require DC bias current

and use patented1FET quads.F igure 1 shows a schematic dia-

gram of a double-balanced mixerbased on FET devices. I t consists of

four FETs in a quad configuration,along with RF , local-oscillator (L O),and intermediate-frequency (I F ) bal-anced-unbalanced (balun) transform-ers. The operation of this type ofmixer is similar to that of a conven-tional diode-based double-balancedmixer. The main difference is that theFET mixer has six terminals, com-pared to the four terminals of thedouble-balanced diode mixer. Duringthe positive half-cycle of the LO sig-nal to the FE T mixer, two of theF E Ts are in conduction while the

other two are turned off. As a result,the secondary winding of the RFbalun is connected to the secondarywinding of the I F balun through theFETs that are switched on. Duringthe LO signals negative half-cycle,the FETs which were on during thepositive half-cycle are turned off andvice versa. This results in a reversalof the polarity of the RF signal reach-ing the IF balun. The frequency atwhich the FETs are turned on and off

is determined by the frequency of theL O signal. This is mathematicallyequivalent to a multiplication of theRF and LO signals, resulting in thegeneration of sum and difference fre-quencies at the IF port.

To obtain high IP3, all the switch-ing elements in the mixer circuitshould be linear and well balanced.

To achieve even higher I P3 and isola-tion compared to a standard double-balanced FET mixer, a dual double-balanced mixer configuration (patentpending) is used in the new series ofF E T mixers from Mini-Cir cuits.

These high-IP3 mixers are availablein numerous cellular and personal-communications-services (PCS) RFranges through 2 GHz (see table).

An example of the new FET mixer

line is the model HJ K-21H (F ig. 2),which operates with RF signals from1850 to 1910 MHz and LO signalsfrom 2090 to 2150 MHz. I t yields IFsignals from 180 to 300 MHz and isdesigned for LO power levels of +17dBm. The mixer exhibits maximumconversion loss of 8.9 dB, with typicalperformance of 7.6 dB. The HJ K-21Hachieves typical input I P3 perfor-mance of +36 dBm (Fig. 3). The LO-to-RF isolation is typically 28 dB,while the LO-to-IF isolation is typi-cally 25 dB. The mixer, with return

loss of typically better than 9 dB,exhibits minimal change in return-loss and isolation-performance levelsas a function of LO power. The effectof L O power on RF and LO returnloss is minimal for the F ET-basedmixer, although the effect of varyingthe L O power on IF return loss iscomparable to the effects seen withdiode-based mixers.

A significant advantage of a F ETmixer over diode is in its compression

Model Frequency LO IP3 Conversion L-R L-I Caseno. (MHz) level input loss isolation isolation style

(dBm) (dBm) (dB) (dB) (dB)typ.

RF LO IF Typ. Max. Typ. Min. Typ. Min.

HJK-21H 1850 to 1910 2090 to 2150 180 to 300 17 36 7.6 0.2 8.9 28 20 25 18 TTT167

HUD-19SH 1819 to 1910 1710 to 1769 50 to 200 19 38 7.5 0.2 8.9 38 27 36 25 BK377

HJK-19H 1850 to 1910 1780 to1840 70 to 130 17 36 7.5 0.2 8.9 28 20 22 16 TTT167

HJK-9H 818 to 853 753 to 778 40 to 100 17 33 6.7 0.2 8.0 35 24 31 23 TTT167

Electrical specifications (FET mixers)

2. The model HJK-21H is a double-

balanced FET-based mixer

designed for use with RF signals

from 1850 to 1910 MHz and LO

signals from 2090 to 2150 MHz.


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characteristics. The RF power levelat which the conversion loss of themixer increases by 1 dB, when com-pared to the conversion loss at lowRF power, is known as the 1-dB com-pression point. The 1-dB compres-

sion point of a diode mixer is general-ly 4 to 6 dB lower than the powerlevel of the LO signal, whereas the 1-dB compression point of a FET mixersuch as the HJ K-21H is typically +20dBm, or 3 dB higher than the powerlevel of the LO signal.

Dual double-balanced mixersemploy two FET quads and a novelbalun arrangement (patent pending)to achieve high IP3 and high isola-tion. An example of such a mixer isthe model HUD-19SH, which has atypical I P3 of +38 dBm (Fig. 4). This

I P3 performance is high enough tomeet the most demanding require-ments of todays communicationequipment. The FET-based mixer isdesigned for RF signals from 1819 to1910 MHz and LO signals from 1710to 1769 MH z. I t yields I F signalsfrom 50 to 200 MHz. I ts conversionloss, which is typically 7.5 dB for anLO power level of +19 dBm, suffersvariations within 0.2 dB for changesin LO power level. The LO-to-RF iso-lation is also tightly controlled. Theisolation performance of this mixer is

approximately 8 to 10 dB better thanthat of a double-balanced mixer, withreturn loss that is comparable to theperformance of a diode-based mixer.

Two other mixers are listed in thetable, models HJ K-19H and HJ K-9H.

The former is designed for RF signalsfrom 1850 to 1910 and LO signals from1780 to 1840, and yields I F signalsfrom 70 to 130 MHz. The latter isdesigned for RF signals from 818 to853 MHz and LO signals from 753 to778 MHz and yields IF signals from 40to 100 MHz. Both mixers are suitablefor use with an LO power level of +17dBm. The HJ K-19H achieves IP3 per-formance of typically +36 dBm withtypical conversion loss of 7.5 dB, LO-to-RF isolation of typically 28 dB, andLO-to-IF isolation of typically 22 dB.

The HJ K -9H achieves I P3 perfor-mance of +33 dBm with typical con-version loss of 6.7 dB, typical L O-to-RF isolation of 35 dB, and typicalLO-to-IF isolation of 31 dB.

Future designs will address appli-

cations beyond 2 GHz. These FET-based mixers have been developed tosatisfy the demanding requirementsof modern communications applica-tions. I n these mixers, the IP3 per-formance is predictably sensitivity(with a shift of approximately 2 dB inIP3 performance for every 1-dB shiftin L O power) over a 2-dB level ofnominal L O power requirements.

This is useful for system designerswhen budgeting for power consump-tion. In addition to superior IP3 per-formance, these mixers provide con-

version loss and isolation per-formance that is comparable with thestate of the art of present day mixers.

A series of F E T mixers, whichoffer an IP3 of +15 dBm in excess ofL O power (i.e., +7-dBm LO powermixer offering +22-dBm IP3), is alsobeing offered to complement thisseries of mixers.

The HJ K and HU D series of FETmixers have been designed by usingstandardized fixtures and internaldesign layouts, enabling automatedmanufacturing in Mini-Circuits man-

ufacturing plants. Consequently,these mixers are offered at a lowercost compared to market offerings.Mini-Circuits, P.O. Box 350166,Brooklyn, NY 11235; (718) 934-4500, FAX: (718) 332-4661, Inter-net: http://www.minicircuits.com.

Reference1. Uni ted States Patents, N os. US5416043 and

US5600169.

4. The IP3 of the HUD-19SH FET mixer was measured from 1800 to 1940 MHz

for LO power levels of +18, +19, and +20 dBm.

41

40

39

38

37

36

35

RF frequency MHz

34

IP3

dB

m

1800 1820 1840 1860 1880 1900 1920 1940

LO18 (dBm)

LO19 (dBm)

LO20 (dBm)

3. The IP3 of the HJK-21H FET mixer was measured from 1790 to 1970 MHz

for LO power levels of +16, +17, and +18 dBm.

LO16 (dBm)

LO18 (dBm) LO19 (dBm)

LO17 (dBm)

42

40

38

36

34

32

30

RF frequency M Hz

28

IP3

dBm

1790 1820 1850 1880 1910 1940 1970


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