Amplificatori a RF. Caratterizzazione di amplificatori e oscillatori a RF. Guadagno e compression point. Compression point

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1 Corso di Laboratorio di misure ad alta frequenza Amplificatori a RF Caratterizzazione di amplificatori e oscillatori a RF Docenti: Andrea Mostacci; Alessandra Paffi andrea.mostacci@uniroma1.it; paffi@die.uniroma1.it A.A Guadagno e compression point Compression point The output power of an amplifier typically exhibits a linear correspondence to the input power as it changes. The gain, i.e. the output power/input power quotient remains constant. The 1 db compression point specifies the input power of an amplifier at which the output signal lags behind the nominal output signal by 1 db. Al variare della frequenza 1

2 Compression point Misura del compression point G (db) P in (dbm) f (Hz) The difference in the level of the output signal to the nominal output signal can be at least qualitatively explained by the overproportional in harmonics with a high driving signal. To prevent the power of the harmonics from corrupting the measurement result, the output power must be selectively measured. The amplifier compression is best measured by using a setup with a signal generator and spectrum analyzer. If you want to use a power meter instead of the spectrum analyzer to measure the power, a suitable lowpass or bandpass must be connected ahead of the power meter to eliminate the effect of the harmonics on the result. Compression measurements can also be performed with network analyzers using the power sweep function. Misura del compression point con analizzatore di spettro Distorsione armonica caratteristica in-out di un sistema debolmente non lineare On the spectrum analyzer, enter the deviation of the analyzer's level display from the level set on the generator as the Ref Level Offset V in = Acosωt V out = a 0 + a 2 A2 2 + (a A + 3a 3 A3 1 )cosωt + ( a 2 A2 4 2 )cos2ωt + ( a 3 A3 4 )cos3ωt 2

3 2nd harmonic intercept (SHI) The SHI in or SHI out point corresponds to the fictitious input or output level at which the 2nd harmonic of the output signal would exhibit the same level as the fundamental at the output. The fundamental is assumed to be linearly transferred. Misura della distorsione armonica Lowpass for suppressing the harmonics of the signal generator. The cutoff frequency and the slope of the filter are selected such that the fundamental is within the filter's passband but the harmonics are sufficiently attenuated. Prodotti di intermodulazione Third order intercept point f mn = nf 1 ± mf 2 The order of a partiular component is the sum of n and m IP3 in (dbm) = Pin(dBm) + a IM 3 2 (db) I prodotti: f 21 f 12 sono i più vicini ai toni originali 3

4 Misura dei prodotti di intermodulazione Power combiner Misura del rumore Noise power is specified either as the total power in the transmission channel or as the power referred to a bandwidth of 1 Hz. The noise at the output of an amplifier is determined by its noise figure and gain. To standardize the measurement result to a bandwidth of 1 Hz, the result is also corrected by -10 log (RBW noise ), with RBW noise being the power bandwidth of the selected resolution filter (RBW) The noise power density is measured in the default setting with the sample detector and using averaging. Other detectors that can be used to perform a measurement giving true results are the average detector or the RMS detector. The Pos Peak, Neg Peak, Auto Peak and Quasi Peak detectors are not suitable for measuring noise power density. Noise factor F Noise figure Figura di rumore F = S i /N i S o /N o =10logF Noise figure represents the degradation in signal/noise ratio as the signal passes through a device. Since all devices add a finite amount of noise to the signal, F is always greater than 1 and is always >0. Thermal noise power Metodi per la misura della figura di rumore Noise source Room temperature impedance Y-FACTOR: It relies on a noise source connected to the input of the amplifier. When the noise source is turned off, it presents a room temperature (cold) source termination. When the noise source is turned on, it creates excess noise, equivalent to a hot source termination. Under these two conditions, noise power is measured at the output of the DUT, and the scalar gain and noise figure of the amplifier is calculated. COLD SOURCE: With this method, only one noise power measurement is made at the output of the DUT, with the input of the amplifier terminated with a room temperature source impedance. The cold source technique requires an independent measurement of the 4

5 Noise factor F F = S i /ktb d S i G d /N o = Noise figure Cold source F = S i /N i S o /N o N o G d ktb d =10logF =10logN o 10logG d ( 174dB m +10log B d ) Cold source The noise figure of amplifiers can be obtained from the noise power display. Based on the known thermal noise power of a 50 Ω resistor at room temperature (-174 dbm (1Hz)) and the gain of DUT in db. The noise at the output of the DUT has two components. One is the amplified thermal noise from the input resistor at 290 K. The second is the noise generated in the DUT itself. Note that the receiver cannot distinguish these two components of noise. To the receiver, the same output noise power density could be measured from a noisefree DUT with its input resistor heated to some higher temperature (290 + Te) equivalent noise temperature. Noise factor in terms of temperature F = 1 + T e /T 0 Y-factor measurement Y-factor measurement ENR (Excess Noise Ratio): a noise source has a precalibrated ENR. This is defined as ENR = (T S ON - T S OFF ) / T 0 T S ON and T S OFF are the noise temperatures of the noise source in its ON and OFF states. T 0 is the reference temperature of 290 K. Y-factor is a ratio of two noise power levels, one measured with the noise source ON and the other with the noise source OFF: Y = N ON /N OFF Because noise power is proportional to noise temperature, it can be stated: Y = T ON /T OFF Sistema multi-stadio F 12 = F 1 + [(F 2-1)/G 1 ] F 1 = F 12 - [(F 2-1)/G 1 ] T 1 = T 12 - T 2 /G 1 5

6 Y-factor measurement Confronto metodi The first step is called calibration and is done without the DUT in place. The noise source is usually connected directly to the input of the instrument (taratura rispetto alla figura di rumore dello strumento) At the end of calibration, the instrument stores the measured values of N 2ON and N 2OFF, and the computed values of Y 2 and T 2. Combined Y-factor Y 12 = N 12ON / N 12OFF T 12 = (T SON - Y 12 T SOFF ) / (Y 12-1) T 1 = T 12 - T 2 /G 1 G 1 = (N 12ON - N 12OFF ) / (N 2ON - N 2OFF ) Sensori: a termocoppia a diodo Power meter Il sensore a diodo è il più sensibile (-70 dbm) e più veloce ma non è indicato per misurare potenze elevate (20 dbm) Due canali per misurare la potenza incidente e la potenza riflessa o l incidente e la trasmessa Segnale di riferimento (50 MHz; 1 mw) per la calibrazione Il segnale RF è assorbito da termocoppie che generano una tensione DC proporzionale alla potenza RF in ingresso Un diodo rettifica la radiofrequenza applicata, producendo una tensione DC che varia con il quadrato della potenza RF dissipata su un carico Incertezza Nelle misure di potenza con i power meter intervengono quattro cause principali di incertezza che sono: la calibrazione del sensore (disadattamento e impedenza) il disadattamento, sorgente carico la sorgente di riferimento l elettronica di elaborazione (risposta in frequenza ed ampiezza del voltmetro digitale). 6

7 Caratteristiche di un oscillatore Potenza di output Oscillatore YIG (Yittrium Iron Garnet) e driver, Micro Lambda MLOM-0704 V tune Oscill. Accoppiatore direzionale Un accoppiatore direzionale è un sistema a quattro porte passivo. Alle porte 3 e 4 è possibile prelevare una porzione, attraverso un fattore di accoppiamento α, della potenza riflessa ed incidente, rispettivamente. Misura del rumore The noise of an oscillator is determined by phase noise near the oscillator frequency and by thermal noise of the active elements far from the oscillator frequency. Phase noise can mask weak signals near the oscillator frequency. In genere α è dato in db (10log α ): α(db)=10 α=1/10 α(db)=20 α=1/100 7

8 Misura del rumore di fase Phase noise is a measure for the short-time stability of oscillators. Phase noise is referred to the carrier level and as a function of the carrier offset. The specified values are stated as relative noise levels within a bandwidth of 1 Hz. Accordingly, the units are dbc (1 Hz) or dbc/hz, where c is referred to the carrier. Internal settings and corrections: PHN dbc in a RBW is normalized to a 1 Hz using the following equation. (PHN dbc - [10*log(10kHz/1Hz)]) = (PNH - [30]) dbc (1 Hz) Modulazioni analogiche Si possono avere modulazioni di alcune caratteristiche di una portante sinusoidale (ampiezza, fase, frequenza). Un analizzatore di spettro è in grado di caratterizzare e misurare tale modulazione, sia nel caso in cui è intenzionale (trasferimento di informazione nelle comunicazioni elettriche), sia nel caso in cui sia indesiderata (rumore di fase, modulazione FM residua di un oscillatore). v(t) = Acos(2πf c t + θ) A: ampiezza della portante f c : frequenza della portante ϑ: fase della portante fase Possiamo considerare modulazione di ampiezza e di angolo frequenza Modulazione di ampiezza v(t) = A c [1+ am(t)]cos(2πf c t + θ) A c : ampiezza della portante f c : frequenza della portante a: indice di modulazione (0 a 1) m(t): segnale modulante normalizzato A c [1+m(t)] definisce l ampiezza dell inviluppo portante Bande laterali v(t) = A c cos(2πf c t) + A c am(t)cos(2πf c t) = v c (t) + v s (t) V ( f ) = A c π δ( f f c ) + δ( f + f c ) V ( f ) = V c ( f ) + V s ( f ) [ ] + A ca [ M( f f c ) + M( f + f c )] La banda occupata dal segnale modulato è doppia di quella del segnale modulante 2 Modulazione di ampiezza Modulazione sinusoidale v(t) = A c cos(2πf c t) + A c acos( 2πf m t)cos(2πf c t) v(t) = A c cos(2πf c t) + A ca 2 [ cos( 2π ( f c + f m )t)cos(2π( f c f m )t)] Oltre alla portante ho due bande laterali centrate sulle frequenze somma e differenza tra quelle della portante e del segnale modulante. Entrambe hanno ampiezza pari a quella della portante per la metà dell indice di modulazione a = 2A SB A c A SB (db) A c (db) = 20log( a 2 ) 8

9 Misura dell indice di modulazione Visualizzazione in time domain fc=100 MHz fm=1 khz SETTING Centr. Freq. 100 MHz SPAN 0 RBW > BW RANGE LINEAR VIDEO TRIGGER 9