THz multipliers, mixers, and amplifiers

Transcription

1 THz multipliers, mixers, and amplifiers Juha Mallat Aalto University Department of Radio Science and Engineering ELEC-E4760 THz techniques, Spring 2016, Agenda THz multipliers Some small recap from course day 1 (by AR) but also more THz mixers THz amplifiers 2 1

2 Part 1: THz multipliers 3 THz multipliers Frequency multipliers provide THz signal power Solid-state oscillators do not quite exist yet or do not deliver suitable power at THz frequencies (other than in the low end) Nonlinear devices for harmonics production Typically voltage-variable C and/or R Schottky diodes and heterobarrier varactors (HBV) Schottky diodes: non-symmetrical => various harmonics HBV: symmetrical => odd harmonics Typical diode I-V response Typical diode C-V response 4 2

3 Applications of frequency multipliers CW (continuous/carrier wave) or modulated power is necessary for many purposes, e.g., measurements and communication applied in transmitters, receivers, etc. Multipliers also enable modulation (or some level of control) at input pump frequency PM and FM in principle quite straightforward N.B.: Effects in phase are times n after multiplication Also oscillator sideband phase noise level increases by 20*log(n) db AM simple at least in ON/OFF keying (likely to be nonlinear otherwise) Phase locking (by PLL) of pump oscillator is a kind of modulation method also 5 Frequency multiplication fundamentals Simple frequency multiplication may be thought as breaking of sinusoidal waveform by some method Frequency components appear at harmonic multiples of the pump frequency of the multiplier Mathematical view: for example, well known Fourier transform or series => amplitudes at frequencies n x f pump This is relevant in thinking about frequency multiplication although with THz multipliers somewhat simplistic for, e.g., circuit design Spectrum produced 6 3

4 Square wave harmonics - basic example: below in linear and db scales Obviously a trivial solution to multiplication is not very effective for conversion even in an ideal case 7 Multiplier characteristics Low order of multiplication provides best efficiency (e.g., up to 30 % at THz frequencies) Narrow-band operation typically accompanies best and P out Multipliers are mostly doublers or triplers or chains of them with possibly amplifiers in between: f n 1 G n 2 Use of capacitive component (varactor) => less power loss in device than in resistive component (varistor) Small diodes & power handling? => more diodes, e.g., balanced doubler or quasioptical combining 8 4

5 Multiplier theory (and practice) Resistive multiplication => losses & less than 100 % efficiency Proven theoretically, limit 100 % / N 2 Reactive multiplication => ideally no loss, efficiency could be up to 100 % Proven theoretically (Manley & Rowe equations) but in practice there is some loss always Efficiencies of ~ 100 % / N already in the 1980s in HUT (TKK) Radio Laboratory, using Schottky varactors 9 Circuit view on multipliers & calculations Ze (w p ) Ze (2w p) Z P s in fp Ze (3w p ) V B C j (V) g(v) Ze( n w p ) Circuit view on multipliers (was already briefly introduced on the 1 st course day by AR) In the equivalent circuit, the active element is surrounded by embedding impedances, with also relevant sources connected Harmonic balance method tackles non-linearity, enabling simulation calculations V V Z B 1 s (w) i Z e 0 d i Z e (w) v e v v R L (x,t) (x,t) v d i g g(v) i c C (V) j 0 L x 10 5

6 Another view on frequency multipliers A frequency multiplier can also be considered as a kind of mixer mixing its input with its own outputs This may sometimes provide a way to get intuition into the multiplier operation when there are many relevant harmonics to consider Ze (w p ) Ze (2w p) Z P s in fp Ze (3w p ) C (V) g(v) j V B Z e( n w p ) For example, actual output frequency port in a tripler Other harmonic frequency ports than the wanted output can be considered also as output ports but they are terminated with reactances reflecting the power back to the circuit 11 Let s explore: HBV As mentioned already: capacitive, non-linear, symmetric structure and C-V response, suitable for producing odd harmonics Thus: 3 rd, 5 th Even harmonics not produced (if no bias) => no concern for respective harmonic currents or embedding impedances Lowest order harmonics the most useful ones also in HBV Single barrier CAD picture Single barrier conduction band with bias V 3 barrier HBV C-V (Chalmers) 2x2 HBV,SEM picture (Chalmers) 12 6

7 HBV matching & efficiency Somewhat difficult matching task (C HBV => L needed) Efficiency improves with power increase Tripler to about 102 GHz, input and output match as realised and optimally (circles in 1 db efficiencysteps) In WG mount measured efficiency 13 Some I & V waveforms Simulated result for a 100 GHz => 300 GHz tripler Features: C HBV larger near 0 V => wants to hold there voltage for a while, then lets loose => makes now quite triangular waveform with harmonics Other observations?: For example, current leads voltage as can be expected (From a UVA study) 14 7

8 Possible way of realization Nice to need no bias! Waveguide tuners used Typical quartz substrate (for almost any discrete THz diode) 15 Examples of commercial devices A few were introduced already on the 1 st course day (AR) => some relevant examples here only (data: Virginia Diodes, Inc.) High-power multiplier performance data: doublers Doubler chain used in a 330 GHz transmitter 16 8

9 Part 2: THz mixers 17 THz mixers Frequency mixing is highly useful at THz frequencies for frequency conversion A possibility to change the frequency of a signal quite freely is used, e.g., in radio receiver front-ends => conversion to intermediate frequency (IF) At lower frequencies, AD or DA conversion and digital techniques have already conquered some areas because frequency conversion can be performed by sampling (and switching), too A mixer can typically perform both down and up conversion in frequency, leading to various possible applications In principle, any suitable non-linear element can be used as a mixer In THz electronics, the familiar Schottky diode is a work horse but in special applications some other components are appearing, too 18 9

10 THz receiver mixers In a (super)heterodyne receiver, a mixer provides signal frequency down-conversion in the front-end This enables subsequently simple and efficient signal amplification, filtering and detection at a low intermediate frequency (IF) THz low-noise amplifiers are appearing but still the Schottky diode mixer is a good choice for the first solid-state component in a front-end This kind of mixer can be readily used at room temperature - but can also be cooled for somewhat improved sensitivity (lower noise) Especially for radio astronomy some other mixers may be used for best sensitivity Such mixers can be, e.g., bolometers or junctions in superconductors but usually with somewhat problematic cooling needed to near absolute zero (=> out of scope for the course book) 19 Briefly on mixing principle v v ( )v ( ) Mixer operation (in receiver): Controlled by a relatively high-power local oscillator (LO) Received RF signal (RF), to be down-converted to IF, is typically of some varying low power level already by its nature v ( ) cos cos ( + ) Mixing produces the up-conversion and downconversion results but the down-conversion result is typically chosen for use in a receiver Mixing products can actually be many more in practice mostly unwanted spurs 20 10

11 Mixing from optical range to THz Photomixers can be applied for down-converting optical power to RF power THz power generation by mixing of power from two optical lasers => optical heterodyning Data signal conversion from optical range to RF and electronics P-I-N diodes and photoconductive mixers => optical photons produce electron-hole pairs, i.e., photocurrents usable in applications Radio engineering and measurement needs typically require narrow spectral lines and a possibility for tuning Good optical lasers can provide these via mixing although their base frequencies are high high up As lasers have (at least had) a tendency to be rather large etc., this has implied restrictions in practical use 21 Photomixing a.k.a. optical heterodyning illustrated Lasers are here depicted as diode lasers Mixing phenomenon is in principle the same as in electronics Non-linear element (e.g. photodiode) This specialised optoelectronic field of coming down from light to THz electronics range is detailed much more in the course book Non-linear element mixes this sum waveform and only the difference frequency component is filtered for use 22 11

12 Using electronic mixing for power generation Frequency multipliers are important in producing THz signal power - but mixing of signals at two RF frequencies can also produce some THz power For example, in transmitters or measurement applications a fast tunable signal may be needed With filtering and/or amplification included according to need, mixing can provide an easily controllable signal source For relatively high THz frequencies, a THz laser output can be mixed in a diode with a microwave signal => a sideband generator with frequency tuning capability Microwave signal generator => accurate and fast tuning Two sidebands (upper and lower) produced relatively closely in frequency In an application, a selective receiver possible for picking only wanted sideband 23 Mixer types in THz electronics At THz frequencies, it is often beneficial to be able to use a local oscillator at some lower frequency than in a fundamental frequency mixer => subharmonic mixers serve quite well & harmonic mixers are useful, too The operation can be readily understood also by mathematics, approximating mixer response by a simple polynomial with higher order terms included Subharmonic = LO s 2 nd harmonic mixer, harmonic mixer = LO s n th harmonic mixer Conversion loss (and noise figure) are (much) less optimal in harmonic mixers - but only mildly degraded in a subharmonic mixer 24 12

13 SSB and/or DSB At THz frequencies, both single sideband (SSB) and double sideband (DSB) operation cases of mixers are relevant At THz, full or some DSB-type operation often occurs or is even intentional The course book presents T SSB and T DSB mixer noise temperature models briefly - but it is good to study some more extensive reference book if you encounter these cases in practise Note: These cases are likely to come up if you are characterising a THz receiver with well-known noiserelated measurement methods (brief descriptions available in the course book, pp ) 25 THz mixers in practice As a separate device, a THz mixer is typically a waveguide block with diode(s) on carrier substrate inside Inner details and outer appearance are very much like already seen in multipliers Mixer features: Usually separate waveguide ports for RF and LO (but can be same => diplexer needed) IF (and possibly bias) via coaxial port Tuners can be somewhat useful for controlling embedding impedance & matching - but in practical designs one wants to avoid all mechanical tuning (if not absolutely necessary) Typical low-thzmixer High-THzmixer Horn antenna 26 13

14 THz mixers in practice Some example data from commercial mixers Features: Single or two anti-parallel Schottky diodes used Operation over full waveguide band & at high IF No tuners Conversion loss typically well below 10 db Best IF amplifiers are so low-noise that exact (but anyway low) mixer conversion loss is not very critical in a receiver 27 Example data of commercial sub-harmonic mixers Noise temperature is the relevant parameter in sensitive receiver use At THz frequencies, e.g., noise figure is not as natural way of reporting noise performance Here T N shown as: T M, mixer noise temperature (now T M,DSB ) Other relevant mixer parameters are seen in the table (f RF, f LO, B IF, L M ) Device gets RF and LO via separate waveguide ports 28 14

15 Example data of commercial harmonic mixers Conversion loss is large by default and dominates mixer operation (thus noise is not specified separately at all, see table) Anyway, harmonic mixers are highly useful in THz radio instruments Only SSB conversion loss is relevant here as shown Reason: Other intended use than most sensitive receiving applications In device: A coaxial port for LO and IF; waveguide for RF 29 Part 3: THz amplifiers 30 15

16 THz amplifiers Low-noise and power amplification at THz with solid-state devices has started to be realizable at steadily increasing frequencies, approaching 1 THz High-electron-mobility transistor (HEMT) in indium phosphide (InP) technology is an enabler here (additionally also SiGe HBT) Electrons can move in HEMTs at high speed, without collisions, in a 2- dimensional electron gas region Of course, also very short gate lengths in the field effect transistor realization are needed (down to some tens of nanometers) A previously unknown abbreviation, TMIC, meaning THz monolithic integrated circuit, has appeared, too ELEC MNT recently reported also a CMOS amplifier operating above 300 GHz => on the way to more mass producible devices 31 THz - a challenge to amplifiers THz design features => costly technologies, extremely small device dimensions but also high losses, noise, unwanted modes and resonances, signal leakage, lack of good models for devices and components, mismatch, on-wafer probe station measurements, VNA extension units, calibration & simulation & measurement uncertainties, etc. However, promising progress in filling the THz gap with solid-state devices is going on also in this field! Satisfactory overall gain in amplifiers requires still many transistor stages Small operation voltages (1 V or so) and currents problematic for power amplifiers (PA) => transistors to be combined suitably in parallel As already common at lower frequencies, low-noise amplifiers (LNA) are highly desirable in order to facilitate systems and partly supersede use of mixers and heterodyne principle 32 16

17 THz being conquered also by amplifiers Typical TMIC LNA building block: cascode stage (or simpler common source) In its HEMTs, field-effect control needs gates with a length of only some tens of nanometers Standard extra challenge of LNA design: modelling or measurement of (all 4) noise parameters Cascode amplifierstage HEMT layout 33 TMIC amplifiers THz beauty in microphotographs Amplifierwith microstripon quartz transitionsto waveguides 0.85 THz LNA (year 2015): G = 15 db, T N = 4300 K (F = 12 db), P OUT approaching 1 mw 9 amplifier stages Some of the HEMTs etc. in SEM photo (Note: air bridges in coplanar wg) 34 17

18 In addition to the course book, some references (for some figures shown etc.) 1. Stake et al., High Efficiency HBV Multipliers, Proc. 1st EuMIC, Manchester, UK, 2006, pp M. Ingvarson, Modelling and Design of High Power HBV Multipliers, PhD thesis, Chalmers, Sweden, Jones et al., GaAs/InGaAs/AlGaAs Heterostructure Barrier Varactors For Frequency Tripling, Fifth Int. Symposium on Space THz Technology, 1994, pp A. Lehto and A. Räisänen, Millimetriaaltotekniikka, Otatieto Oy, (visited ) 6. (visited ) 7. K. M. K. H. Leong et al., "A 0.85 THz Low Noise Amplifier Using InP HEMT Transistors," IEEE Microwave and Wireless Components Letters, vol. 25, no. 6, pp , June