Digital Signal Processing in RF Applications

1 06 June 2007 CAS, Sigtuna, SwedenDSP Digital Signal ProcessingDigital Signal Processing in RF ApplicationsPart IThomas SchilcherRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June 20072 What are RF Applications ?any application which measures properties of an RF field (amplitude, phase, frequency, ..);typical frequencies in accelerators: MHz tens of GHzapplications which process the measured quantities to control and regulate RF fields(feedback and feedforward)RF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT.

2 Schilcher06 June 20073 Typical RF applicationscavity field loops(amplitude and phase)klystron loops(amplitude and phase)tuner loops(cavity tuning)radial and phase loops(circular machines) RF gymnastics (bunch splitting and merging)Accelerators:CW / pulsed machineslinear / circular machinesApplication areas (examples):electron/hadron/ion acceleratorsnormal-/superconducting RF systemsRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June 20074 Why Digital RF Applications ?RF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT.

3 Schilcher06 June 20075 Key components of Digital RF applicationssignal conditioning/ down detectiondigital Signal Processing /algorithmsvector modulator/ up conversion/ amplificationRF signalmonitoring /control systemRF signalaccelerator / plantLLRF looks very similar to many other Applications , diagnostics (bunch-by-bunch feedback, position monitoring, ..)(feedback Applications )for feedback systems: ultimate error is dominated by the measurement process(systematic error, accuracy, linearity, repeatability, stability, resolution, noise)RF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT.

4 Schilcher06 June conditioning / down of by Digital I/Q sampling I/Q sampling non I/Q sampling Digital down conversion (DDC) in RF Applications feedback systems adaptive feed forward system identificationRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June conditioning / down of by Digital I/Q sampling I/Q sampling non I/Q sampling Digital down conversion (DDC) in RF Applications feedback systems adaptive feed forward system identificationRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT.

5 Schilcher06 June 20078 Signal conditioning / down conversionWhy down conversion of the RF Signal ?ADC speeds are limited. It is not reasonable/possible today to digitize high-frequency carriers directly. (f>500 MHz)ADC dynamic range is bit 60 dB12 bit 72 dB14 bit 84 dBADC clock and aperture jitter become critical at high frequencies(especially for undersampling schemes)RF mixers are essential for Digital high frequency applicationsoften better: use analogue circuits in conjunctionwith the ADC to implement automated gain control (AGC)functions to ensure that this range is best usedRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT.

6 Schilcher06 June 20079RF mixer (ideal)mixer: linear time varying circuit, non-linear circuit ( )lower sidebandupper sidebandeven ideal mixers produce two sidebandsRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June 200710fLOf+fLORFf-fLORFfRFfrequencysigna lamplitudefLOf+fLORFf-fLORFfRFfIMAGE frequencysignalamplitudeideal mixer: output is the multiplication of the two input signalsdown conversion:RF, LO are high frequency inputsIF: lower intermediate frequency outputup conversion:IF is input, RF is outputRF mixer (ideal)fLOf+fLOIFf-fLOIFfIFfrequencysign alamplitudelowersidebanduppersidebandRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT.

7 Schilcher06 June 200711RF mixer (ideal)low pass filtering the upper sideband:important properties:phase changes/jitter are conserved during down conversion, 1 @ fRF= GHz 1 @ fIF=50 MHz comparison: sampling IF or RF (direct sampling)?timing jitter results in different phases!( 10 ps @ 500 MHz ; 10 ps @ 50 MHz )basic properties of RF Signal are conserved ( )tougher requirements for direct RF sampling !down conversion:RF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June 200712RF mixer (real)real mixers = non linear devicesmany undesired harmonics in frequency spectrum non-linearities in IF signalfiltering the output of a mixer might be necessarytake care about the introduced group delay by the filtertrade off!

8 I-V curve of a diodeRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June conditioning / down of by Digital I/Q sampling I/Q sampling non I/Q sampling Digital down conversion (DDC) in RF Applications feedback systems adaptive feed forward system identificationRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June 200714direct amplitude phase detectorsanalogue IQ detectiondigital IQ sampling / Digital Down Conversion (DDC)Amplitude and phase detectionRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT.

9 Schilcher06 June 200715RF vector representationrepresentation of any sinusoidal RF Signal : phasorI: in-phase componentQ: quadrature-phase componentdefinition: positive frequencies counterclockwise rotating phasor(assumption: we measure the vertical component with ADC)(sometimes I/Q aredefined vice versa!)RF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June 200716IQ sampling (1)goal:monitor amplitude/phase (A/ 0) variations of incoming RF/IF signalpossible also to monitor I/Q at a reference time (reference phase) process sampled I/Q values for comparison, rotate phasor back to reference phasor if phase advancebetween sampling is well knownRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT.

10 Schilcher06 June 200717IQ sampling (2)sampling of RF/IF freq.:( 90 phase advancebetween two samples)RF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June 200718IQ sampling (3)rotate corresponding I/Q vector by -90 / -180 / -270 in order to compare to initial I/Q valuesrotation matrix with angle :build up I/Q vector based on two successive samplesI/Q Processing with sampling frequency fSRF applicationsCAS, Sigtuna, SwedenDSP Digital Signal ProcessingT. Schilcher06 June 200719IQ sampling (4)general:phase advance between consecutive samples:1.