The Measurement of (1/f) AM noise of Oscillators

Transcription

1 The Measurement of (1/f) AM noise of Oscillators Enrico Rubiola FEMTO-ST Institute, Besançon, France (CNRS and Université de Franche Comté) Outline Introduction Power detectors Experimental method Results Perspectives and conclusions

2 introduction 1 Motivations for AM noise metrology 2 Emerging need, after the progress of oscillators and sources phase noise metrology (bridge/interferometric) method Impacts on frequency synthesis AM/PM conversion oscillators power effects on the resonator microwave photonic systems laser RIN... Measurement the AM noise of a source relies on instruments

3 introduction 2 AM noise 3 polar coordinates v(t) = V 0 [1 + α(t)] cos [ω 0 t + ϕ(t)] Cartesian coordinates In low noise conditions Relates to power fluctuations v(t) = V 0 cos ω 0 t + n c (t) cos ω 0 t n s (t) sin ω 0 t α(t) = n c(t) V 0 and ϕ(t) = n s(t) V 0 α(t) = 1 2 δp P 0 Same formulae as for frequency noise 0 Power-law S α (f) = h i f i h 2/f 2 random walk i= 2 α(t) y(t) h 1/f h0 flicker white Allan variance σ 2 α(τ) = h 0 2τ + 2 ln(2) h 1 + 4π2 6 h 2 τ white flicker random walk

4 detectors 1 The diode power detector 4 law: v = kd P same form as in optical quantum detectors differential resistance R d = V T I 0 V T = kt/q 25 mv thermal voltage rf in video out rf in video out ~60 Ω pf external 50 Ω to 100 kω ~60 Ω pf external 50 Ω to 100 kω two-diode detector output voltage, mv kω 1 kω 100 Ω linear region (power detector) linear region (envelope detector) input power, dbm

5 detectors 2 Tunnel and Schottky power detectors 5 parameter Schottky tunnel input bandwidth up to 4 decades 1 3 octaves 10 MHz to 20 GHz up to 40 GHz vsvr max. 1.5:1 3.5:1 max. input power (spec.) 15 dbm 15 dbm absolute max. input power 20 dbm or more 20 dbm output resistance 1 10 kω Ω output capacitance pf pf gain 300 V/W 1000 V/W cryogenic temperature no yes electrically fragile no yes The tunnel diode is actually a backward diode. The negative resistance region is absent. Measured detector gain, A 1 load resistance, Ω DZR124AA DT8012 (Schottky) (tunnel) conditions: power 50 to 20 dbm output voltage, dbv Herotek DZR124AA s.no Schottky 3.2 kω 10 kω 100 Ω 320 Ω 1 kω input power, dbm ampli dc offset output voltage, dbv Herotek DT8012 s.no Tunnel 10 kω 3.2 kω 1 kω 320 Ω 100 Ω ampli dc offset input power, dbm

6 detectors 3 Noise mechanisms 6 Shot noise SI (f ) = 2qI0 Rothe-Dahlke model of the amplifier detector rf in video out amplifier in v n noise free out ~60 Ω pf external 50 Ω to 100 kω i n Thermal noise SV (f ) = 4kBT0R Flicker (1/f ) noise is also present Never say that it s not fundamental, unless you know how to remove it In practice the amplifier white noise turns out to be higher than the detector noise and the amplifier flicker noise is even higher

7 method 1 Cross-spectrum method 7 v a (t) = 2k a P a α(t) + noise source under test power meter monitor P a P b v a v b dual channel FFT analyzer v b (t) = 2k a P b α(t) + noise The cross spectrum Sba(f ) rejects the single-channel noise because the two channels are independent. S ba (f) = 1 4k a k b P a P b S α (f) Sα (f) log/log scale 1 2m single channel cross spectrum meas. limit Averaging on m spectra, the singlechannel noise is rejected by 1/2m A cross-spectrum higher than the averaging limit validates the measure The knowledge of the single-channel noise is not necessary f

8 method 2 Calibration 8 source under test atten 0.1 db step P a P b power meter v a v b voltm. Set a reference P/Pa (0.1 db) with a by-step attenuator Measure va at the output k a P a = v a P/P a Repeat interchanging the channels Note that only the kp product is needed because 1 S ba (f) = S α (f) 4k a k b P a P b source under test ν 0 atten P a P b power meter v a v b voltm. Alternate (and complex) calibration method. It exploits the sensitivity and the accuracy of a lock-in amplifier. As before, it requires a reference power-ratio ν b = ν 0 ν s reference ν s atten ref in lock in amplifier input Im Re out

9 results 1 Example of AM noise spectrum Wenzel E 100 MHz OCXO P0 = 10.2 dbm avg 2100 spectra Sα ( f ) db/hz Fourier frequency, Hz flicker: h 1 = Hz 1 ( db) σ α = Single-arm 1/f noise is that of the dc amplifier (the amplifier is still not optimized)

10 results 2 AM noise of some sources 10 source h 1 (flicker) (σ α ) floor Anritsu MG3690A synthesizer (10 GHz) db Marconi synthesizer (5 GHz) db Macom PLX GHz multipl db Omega DRV9R F 9.2 GHz DRO db Narda DBP-0812N733 amplifier (9.9 GHz) db HP 8662A no. 1 synthesizer (100 MHz) db HP 8662A no. 2 synthesizer (100 MHz) db Fluke 6160B synthesizer db Racal Dana 9087B synthesizer (100 MHz) db Wenzel D 100 MHz OCXO db Wenzel E no MHz OCXO db Wenzel E no MHz OCXO db worst best

11 persp. & concl. 1 Measurement of the detector noise 11 In progress P a A v a adj. gain diff. ampli g(p c P a ) R a low noise source P c C v c JFET input dual channel FFT analyzer AM input osc. out lock in amplifier monitor power meter input Im Re out P b B R c R b v b adjust the gain for the Re output to be zero adj. gain Basic ideas diff. ampli g(p c P b ) Remove the noise of the source by balancing C A and C B Use a lock-in amplifier to get a sharp null measurement Channels A and B are independent > noise is averaged out Two separate JFET amplifiers are needed in the C channel JFETs have virtually no bias-current noise Only the noise of the detector C remains In all previous experiments, the amplifier noise was higher than the detector noise

12 persp. & concl Conclusions Method for the measurement of AM noise in oscillators High sensitivity and accurate calibration Suitable to optics and to microwave photonics Measurement of some RF/microwave sources Single-channel sensitivity still limited by the dc amplifier Measurement of the detector noise in progress Free downloads (text and slides) (text only)