NMR Spectrometer Hardware

Transcription

1 NMR Spectrometer Hardware Manoj Naik NMR Facility, TIFR Workshop on NMR & its applications TIFR, Mumbai November 23, 2009

2 Principle of NMR ω = γ B Nuclear Magnetic Resonance

3 A Simplified 6 0 MHz NMR Spectrometer RF (60 MHz) Oscillator hν Transmitter absorption signal RF Detector Recorder Receiver MAGNET MAGNET N S Probe ~ 1.41 Tesla (+ / - ) a few ppm

4 CONTINUOUS WAVE (CW) METHOD THE OLDER, CLASSICAL METHOD The m agnetic field is scanned from a low field strength to a higher field st rength while a constant beam of radiofrequency (continuous wave) is supplied at a fix ed frequency (say 100 MHz). Using this m ethod, it requires several minutes to plot an NMR spectrum. SLOW, HIGH NOISE LEVEL

5 PULSED FOURIER TRANSFORM (FT) METHOD THE NEWER COMPUTER- BASED METHOD FAST LOW NOISE Most protons relax (decay) from their ex cited states very quickly (within a second). The ex citation pulse, t he data collection (FID), and the com puter- driven Fourier Transform (FT) take only a few seconds. The pulse and data collection cycles may be repeated every few seconds. Many repetitions can be perform ed in a very short time, leading to im proved signal..

6 First NMR Signal In1946 Bloch s Laboratory in Stanford 1949

7 NMR in 1964

8 NMR Spectrometer in 1970 HFX90

9 NMR Spectrometer in 1985 AM console

10 Routine NMR Spectrometer in 2000

11 NMR Spectrometer Schematic

12 Spectrometer Layout

13 NMR Spectrometer hardware components 1. Magnet 2. RT Shims 3. Field Frquency Lock 4. Probes 5. Transmitters 6. Preamplifiers 7. Receiver 8. Digitiser 9. Host Computer

14 Type of Magnets 1: Permanent Magnet: Weak 2: Electromagnet: Bulky & Power Consuming 3: Superconducting Magnet: Compact

15 Superconducting Magnet Compact & Efficient Ability to support high current density Niobium-titanium for low field < 9 Tesla Niobium tin for high field : expensive,brittle Multifilamentary embedded in cu matrix For better stability & less diamagnetism Operated at 4.20K upto 700MHz Operated at 20K above 700MHz

16 Magnet Coil

17 Cryostat Cutview

18 800 MAGNET CRYOSTAT Adjustment Screw for Needle Valve Quench Valves

19 Pump Cryostat arrangement

20 Supercon magnet SC material Proton frequency Central field He capacity Hold time Liq. N capacity Al5 + NbTi MHz Tesla 1200 litres ( ) > 56 days 450 litres

21 Cryogenics data for the NMR magnets System Liquid Helium Boil off Refill Volume rate Liquid Nitrogen Total Volume Boil off rate Refill volume Total Volume 500 MHz 35 ml/hour 100 Lit /3month 286 Lit 400 ml/hour 200 Lit/21days 286 Lit 600 MHz 35 ml/hour 80Lit /2month 195 Lit 250 ml/hour 135 Lit/21days 200 Lit 700 MHz 45ml/hour 162Lit /5month 257 Lit 455ml/hour 229Lit/21days 286 Lit 800 MHz 220 ml/hour 200Lit. /month 1200 Lit 750 ml/hour 250 Lit/15days 400 Lit

22 RT Shims Homogenous field essential for good lineshape Variation in Magnetic field with Position NMR frequency is proportional to field results in distorted lineshape To create homogenous field Shim coils are placed in vicinity of sample Shim currents strength adjusted by looking at spectra to nullify field gradient

23 Field Homogeneity

24 Classification of Shims Axial or spinning shims Z1,Z2,Z3,Z4 Non-axial or non spinning shims X,Y, XY

25 NMR Probes

26 NMR Probe

27 NMR probes HR Probes with ATM High Resolution Probes with Automatic Tuning and Matching(ATM) 5mm TXI 1H/13C/15N/2H 5mm BBI 1H/2H/ BB

28 Cryoprobe The CryoProbe is a high-performance cryogenically cooled probe developed for high-resolution applications. It has improved signal/noise (S/N) ratios obtained by reducing the operating temperature of the coil and the pre-amplifier. The dramatic increase in the S/N ratio by a factor of 3-4, as compared to conventional probes, leads to a possible reduction in experiment time of up to 16 or a reduction in required sample concentration by a factor of up to 4. Linear behavior in power response Gradient capability CryoProbes are available as Triple Resonance, Enhanced Dual, Quad Nucleus Probes, or Dual Inverse configurations at 400 MHz and higher All high resolution probes have a 2H lock circuit

29 Cryoplatform The CryoPlatform's main purpose is to provide the cooling infrastructure needed for the operation of cryogenic probes. Components of the CryoPlatform consists of a CryoCooling Unit and a Helium Compressor. The Cryocooling Unit, the heart of the cooling system is a Gifford McMahon cryocooler which is driven by pressurized helium supplied by the helium compressor. Cooling CryoProbes is accomplished with a closed-looped helium gas flow via a flexible transferline. The CryoCooling unit has dedicated electronics control and monitors the entire system during all phases of operation. It has an automatic error handling and safety shutdown functions.

30 Cryoplatform

31 Variable temperature In general stable temperature (during experiment) Controlled air flow over sample Cooler and Heater are used to control the sample temperature - Thermocouple are used to sense sample temp.

32 Low temperature Moderately low: BCU-05 cooling unit HR- and MAS-Version (pressure) cools gas stream typically by 65 K temperature range: approx. -15 C to +50 C really low: heat exchanger MAS version for VTN, WVT, DVT HR version for DVT cooling down of heat exchanger only under dry N2 gas

33 VT gas N2 from boil-off device: dry, oil free low pressure fluctuation inert! Caution!! Look after fresh air!! O2 warning system where required! Compressed gas from compressor: pay attention to dew point and oil content (0.0 %) condensation of O2 at low temperature risc of oxidation at high temperature pressure fluctuation

34 Probe and magnet protection transfer tube shimsystem NO ice nor heat at O-rings! frame cooling

35 Probe and magnet protection flush gas (ambient temperature) for shimsystem probehead ( frame cooling ) transfertube critical parameters: high temperature: shim foils: T 70 C low temperature: in case of icing of O-rings risc of leakage of vacuum and quench!

36 Slow heating and cooling! For heating and cooling consider thermal burden on probe using a ramp of 6 to 10 degrees per minute is recommended

37 AQS REF-22 Block Diag.

38 Simple Pulse Sequence PW FID d1 d1 Selective Excitation NCO0 NCO2 No Frequency F1+ F NCO1 F1 NCO3 Lo = F1 + 22MHz NCO0 No Frequency

39 DDS NCO0 SHAPE Zero Amplitude Zero Frequency W1ϕ 1 W1ϕ 2 W1ϕ 3 W2ϕ 1 W2ϕ 2 W2ϕ 3 DIGITAL CONTROL FROM FCU3 W3ϕ 1 W3ϕ 2 W3ϕ 3 Σ NCO1 MOD W1 Amplitude W1 Frequency 16x16 bit Σ NCO2 Σ NCO3 MULTIPLIER DAC UP CONVERTER 13-23MHz W2 Amplitude W2 Frequency W3 Amplitude W3 Frequency Wn Frequency MULT ϕ Information NCO = Numeric Controlled Oscillator. DDS = Direct Digital Synthesis. X MHz RF OUT

40 Example of a 10-bit ROM table

41 ROM table for 16 bit 2 U 1 6 = P 6 4 K D 0 O W N ( b it 1 5 ) 1 6 K ( 1 4 b i t ) U P I N V E R TD E O D W S I G N ( b it 1 6 ) N O n ly t h e f ir s t 1 /4 o f t h e T h e o t h e r q u a r t e r s a r e c o m p le t e o b t a in e d s in e p b y s y m

42 Flexible Frequency Generation NCO1 NCO2 NCO1 Phase Coherent: F1 NCO1 F1+ F NCO2 NCO1 NCO2 F1 NCO1 Phase Continuous: NCO1 NCO2 Phase Resetted: F1+ F F1 Reset F1+ F F1 Reset

43 Blockdiagram of the SGU

44 Timing and Frequency Specifications TIMING Pulse Duration: Minimum 50ns Delay Duration: Minimum 50ns Resolution 12.5ns FREQUENCY Range MHz Bandwidth of each Channel MHz Resolution <0.005Hz or 34bit Switching Time <+-2.5MHz Steps < 300ns

45 Phase and Amplitude Specifications PHASE Phase resolution < or 16bit Phase switching time < 300ns AMPLITUDE Modulation (MOD) Range Resolution 96dB or 16bit < 0.01dB up to 54dB < 0.05dB up to 60dB Level (MULT) Range 90dB Resolution 0.1dB Amplitude Switching Time 50ns Next Event: Phase + Amplitude 100ns

46 Amplifier

47 Amplifier

48 Characterisation of Pulse Shapes overshoot measured in % of pp voltage only little for short pulses ripple measured in % of pp voltage only little for short pulses low frequency ripple:» comes from insufficient power supply high frequency ripple:» comes from RF impurities droop measured in % of pp voltage important for long pulses and pulse trains

49 Characterisation of Pulse Shapes on/off ratio measured in db important for noise reasons rise and fall time from 10 % to 90 % ca. 100 ns not much of a concern, because it is determined by the narrowest component (which is the probe)

50 Transmitter Specification

51 Preamplifier design Controller Amplifier Multiplexer Probe Preamplifier

52 Passive RF switching Top: wiring of amplifier, probe, and receiver (preamplifier) bottom left: switching under influence of a pulse (> 700 mv) bottom right: switching under influence of an NMR signal

53 Linear Pre-amp Specification

54 Receiver RX22 RX22Receiver: Receiver: Intermediate Intermediate Frequency FrequencyIF IF22MHz. 22MHz. FID FIDinput inputfrom from Preamplifier. Preamplifier. Receiver Receiveroutput outputisis audio audiofrequency frequency Channel ChannelAAand andbb (quadrature) (quadrature)into into digitizer. digitizer.

55 Quadrature Detection

56 Digital to Analogue Conversion (ADC)

57 RXAD Does four main function To amplify signal from HPPR Demodulation of RF signal to AF range To match the input range of ADC To digitize quadrature signal Specification Frequency range: MHz Gain range: 93dB in 1dB steps ADC resolution. 200KHz. 20Bit 5MHz.16Bit Data rate.560mbits/s

58 Digital Filtering SWH Oversampling 200 khz SAMPLING WINDOW ANTI-ALIASING ANALOG FILTER 125 khz DIGITAL FILTER 20 khz SWH 2-100kHz -62.5kHz -10kHz 0 F khz 62.5kHz 10kHz CONVENTIONAL ANALOG FILTER SW=20 khz Image Signal fully suppressed by Digital Filter SWH 2 (After Decimation) SWH (Before Decimation) 100kHz

59 DRU Digital Receiver Unit does following fucntion Interface to two ADC channel Capture peak values for receiver gain adjust (rga) Automatic DC offset calibration (AutoZeroCompensation), digital phase control, Digital quadrature detection, Digital filtering & Decimation of NMR signals, accumulation and acquisition & Data transfer to workstation via ethernet Diagnosis & access to the DRU relies on HTTP & HTML enabling service access just by any web browser

60 AQS Avance AV OneBay

61 AQS CCU10

62 AQS CCU10 Block Diag.

63 CCU10 Specification RISC based cpu 100MHz Ethernet 100MHz half duplex 10 RS232 CHANNEL 2 RS485 CHANEL 64Mb DRAM

64 TCU3 Timing Control Unit TCU3 The TCU3 is used. to synchronize and control the timing of the RCU, FCU s and GCU to send frequency information to the FCUs via the F bus to generate various front panel switching signals used in external amplifiers, QNP Pneumatic Unit, BSMS, etc. to provide the RCU and GCU with a 40MHz TTL clock signal to generate the RCUGO signal for the RCU and the AQS signal for the GCU The timing of the TCU3 is implemented using an 80MHz internal clock (this signal is generated on board from the 20MHz of the REF22 board)

65 FCU3 Frequency Generation Unit Digital control of all aspects of frequency, phase and amplitude Real-time control of all gating signals generated on the SGU LVDS (low voltage differential signal) link between the FCU3 and SGU The FCU3 can be viewed as the digital section of the SGU One FCU3 contains four independent channels No jumper-settings required due to auto configuration No on-board DDS generation - this is performed by the SGU

66 FCU / SGU LVDS Interface Low Voltage Differential Signaling 20MHz (50ns) FCU3 SGU Twisted Pair 4x7 bit at 20MHz 560Mbit/s

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68 NMR Consol

69 The RF Part

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