SESSION X: Avalanche Circuits

THPM 10.2: Excess Noise in Stable and Injection-Locked IMPATT Diode at High-Power Levels

H-J. Thaler of the LF noise currentand ihe oscillator FM noisecan be Siemens AG very strong. If the external bias impedance is compensated by theRF induced negative diode resistance, bias oscillations Munich, Germany occur. The internal feedback structure of an IMPATT oscillator, as described by Figure I, allowsa partial FM noisecancellation for a proper bias impedance.This mechanism is similar to THE NOISE PERFORMANCE of power amplifiers noisesuppression by externalfeedback4. Figure 2 shows for FM radio links is best characterized by the FM noise experimental results for the Flll noise of IlllPATT oscillators at of the amplified output signal. For high power IMPATT diode twopower levels, obtained with state-of-the-art siliconmulti- amplifiers the FM noise does not decrease continuouslywith mesa IMPATT-, with a power capability of over 2 Rr at increasing RFinput power as expected. For boththe stable 4 GHz. The FM noise increases stronglywith RF power level andthe injection-locked oscillator mode FM noise and hasa flat minimum forexternal bias impedances of minima have been foundat medium input power levels. At approximately 100 a. For high bias impedances (>1k8) it high power levels the noise increases strongly. It is therefore is up to 3 dB higher. not possible topredict the noisebehavior of IMPATT diode For the design of practical IMPATT poweramplifiers it is power amplifiers from small signal noise data’. convenient to characterize thediode by its large signal The excess noise of IMPATT modes at high power levels impedance and effectivenoise figure as a function of the RF was, untilnow, ascribed to an increase of the primary RF current magnitude at the diode terminals. Typical data for the noise of the diode at high RF current levels as described earlier diodes investigated are shown in Figure 3. theoretically’. There is, however, an additional contribution to Under small signal conditionsthe equivalentnoise figures the large signal excess noise of IMPATT diodesresulting from areequal forthe stableamplifier andthe injection-locked signal modulation by enhanced low frequency noise currents in oscillatormodes. Inthe large signal case, however,the values thediode3. This mechanism is bestdescribed by the signal of the effectivenoise figure Feff calculated from FM noise flow graph shownin Figure 1 which is anextension of a measurementsdiffer considerably, as canbe seen from Figure diagram originally developed forlow power IMPATT diode 4, where the noise figure is plotted asa function of output oscillators4. At high power levels an internal down conversion power both for amplifiers withdifferent small signal gain Go path becomes effective, resulting from the influence of the RF and for injection-locked oscillatorswith different free running excitationon the mean diode voltage;nonlinear rectification. output powers. The stableamplifier always haslowera This down conversion mechanism increases the LF diode noise effective noise figure than aninjection-locked oscillator with voltage andshifts the LF diode resistance to lowerand even the same output power. negative values; large signal induced . The An analysis by means of simpleequivalent circuits shows decrease of the LF diode resistance with increasing RF power that for a given output power the diode RF current is always level is muchmore pronounced for the oscillator modethan lower forthe stableamplifier mode thanfor the injection- for the amplifier mode. locked oscillator;Figure 5. As the excess noise is astrongly Themodulating LF noise currentthrough the diode is increasing function of the RF current magnitudewithin the determined by the magnitude of the total bias loop impedance diode, the differencebetween the effectivenoise figures is a given by thesum of theexternal LF bias impedanceand the direct consequence of thedifferent RF current excitations in LF diode impedance. At low bias impedances the enhancement both modes.

~ For a given output power the FM noise of the amplified signal is determinedby the effectivenoise figure andthe Tatsumchi, I., Dietrich, N. R. and Burke Swan, C., amplifier gain. For low-level input signals the injection-locked “Power Noise Characterization of Phase Locked IMPATT Oscillators”, IEEE J. Solid State Circuits, p. 2-10; Jan., 1972. mode has high gain and therefore shows large FM noise. In the 2Kuvas, R. L., “Noisein IMPATT Diodes: Intrinsic high power range where both modes have similar gain, the Properties”, IEEETrans. Electron Devices, p. 220-233;Feb., larger effectivenoise figure of theinjection-locked mode 1972. results in higher FM noise, making thestable amplifier mode 3M~~thaa*,K. and Rijpe?, H. P. MI, “Nonlinearity and clearly superior withrespect to noise performance.This is Noisein the Avalanche Translt-Time Oscdlator”, PhilipsRes. Repts., p. 391-413; Oct., 1971. confirmed by theexperimental data of Figure 6, representing 4Thaler, H. J., Wch, G. and Weidmann, G., “Noisein the FM noise of both stable amplifiers with different values of IMPATT DiodeAmplifiers and Oscillators”, IEEETrans. small signal gain, and ofinjection-locked oscillators with Microwave Theory and Techniques, p. 692-705: Aug., 1971. different values of freerunning output power. For a given ’Thaler, H. J. and Weidmann, G., “IMPATT Oscillator upper limit of the FM noise the stableamplifier has the NoiseSuppression by ModulationFeedback”, Proceedings 1971 EuropeanMicrowave Conference. Stockholm; paper advantages of higher output power and higherefficiency over A7/2. the injection-locked oscillator. RF NOISE AMPLITUDE 1000

N I w CONVERSION cn UPCC,NVEHSION - CURRENT 0 Z 1.80 W x RF NOISE LL 100 OUA3HATURE”-- COMP FREOUENCY unstableI 1X3W PRIMARY NOISE OSCILLATOR NOISE < -- FIGURE 1-Signal flowgraph describing noise in high power IMPATT diode oscillators.

fo=L.l GHz fm=2 MHz B= 100 HZ 104 10 100 1000 LF BIAS IMPEDANCE, R

FIGURE2-Influence of externalbias impedance on FM noise of highpower IMPATT diodeoscillators; parameter RF output power.

0 0.5 1 .o 1.5 PEAK DIODE RFCURRENT , A FIGURE 3-Largesignal impedanceand effective noise figure of an IMPATT diode as a functionof RF diode current.

i 2 3 8.9 dB RF OUTPUTPOWER , W FIGURE 5-Calculated RF diodecurrent for both the - stable amplifier and the injection-locked oscillator mode as 3 32 a function of output power. RF OUTPUT POWER, dBm

FIGURE&Noise figure versus output power of stable [seep. 206 for Figure 6.1 amplifiers (parameter small signal gain) and injection-locked oscillators (parameter free running RF output power). U

u =o.131 x IO-

FIGURE 6-Distribution of sourcedrain cell current (A) - punch-through bias.

-- T 100- 0 ,. f, f, 1 MHz B = 100Hz N I

200 300 500

0 i 2 RF OUTPUTPOWER W FIGURE 6-FM noise of the output signal of high-power IMPATT amplifiersversus output power;parameter RF input power in mW.