Receiver phase cycle

For example, a final result of Ix (first column) will be received as Iy if the receiver phase is set to —y (fourth row). Now we can convert our results for the final 90° pulse, taking into account the effect of the receiver phase cycle x, —y, —x, y ... 

Where X and Y are the real and imaginary FIDs coming out of the ADC. Thus, we can speak of the receiver phase as the point of view from which we view the FID signal in the rotating frame of reference. If the receiver phase is shifted by 90° (from x to y axis), we mean that the real channel has been shifted counterclockwise by 90° from the x axis to the y axis, and the imaginary channel has been shifted counterclockwise by 90° from the y axis to the -x axis. With this 90° shift, for example, a coherence of Iv would be observed as Ix, and in a four-scan phase cycle with Ar = 90°, we would observe a four-scan sequence of coherences lx, y, —Ix, —Iy as 4IX in the sum-to-memory. To keep track of these phase shifts, a shorthand notation is often used where 0 stands for a 0° phase shift (real part of receiver on the x axis), 1 stands for a 90° phase shift (receiver on the y axis), 2 stands for a 180° phase shift (receiver on the —x axis), and 3 stands for a 270° phase shift (receiver on the —y axis). In this notation, the four-scan receiver phase cycle with A
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The effect of setting the receiver phase cycle is to position each A/-fold mask so that it passes the desired coherence order change Ap resulting from that pulse. 

The first two pulses give the same result pulse phase of 0 2 and receiver phase of 0 2, selecting Ap = +1, -1, -3, and so on. For the third pulse, Ap is -1 and A p is 90°, so A r = -(-1) x 90° = 90°. So every time we advance the phase of the third pulse by 90°, we must also advance the phase of the receiver by 90°. Note that we have done exactly the same experiment as the DQF-COS Y, but we have changed the receiver phase cycle slightly from 0321210321030321 to 012323012301012 3, thus letting the NOESY signals accumulate in the sum-to-memory whereas the DQF-COSY signals cancel out. 

Table 2.2.2 The CYCLOPS sequence for transmitter and receiver phase cycling...

With respect to the pulse sequence layout, the HMBC experiment is essentially a HMQC experiment incorporating a low-pass filter to suppress the one-bond correlation peaks. The low-pass filter, consists of a delay d2 = 1/(2 U(C, H)) and a 90° pulse, which transfers the U(C, H) coherence into a multiple quantum state. In a second period coherences which are generated by JCC, H) evolution are also transferred to a multiple quantum state by a 90° l C pulse but with a different phase in relation to the first 90° pulse of the low-pass filter. A combination of appropriate receiver phase cycling and pulse phase cycling enables the exclusive detection of J(C, H) correlation peaks in the 2D experiment. 

Spurious signals due to imperfect carbon-13 180 pulses are suppressed if receiver phase imbalance causes zero frequency artifacts, these may be removed by adding receiver phase cycling, extending the sequence to include 32 different sets of phases. 

R) = receiver phase. When the sums of coherence phase changes in different scans match the receiver phase cycle, successive scans add, while when the relative phases change 0, 2 successive scans cancel. Thus in each case, the signals from the N path add while signals from the P path cancel. 

To suppress other interference effects, the phase of the transmitter pulse is also shifted by 180° and the signals subtracted from sections A and B, leading to the CYCLOPS phase cycling scheme shown in Table 1.4, in which the two different receiver channels differing in phase by 90° are designated as 1 and 2 and the four different receiver pulses (90°, 90°, 90° and 90f,) are called x, y, — x, and —y, respectively. 

Figure 1.43 The first two steps of the CYCLOPS phase cycling scheme. Any imbalance in receiver channels is removed by switching them so they contribute equally to the regions A and B of the computer memory.

Figure 13 Timing diagram for the clean HMBC experiment with an initial second-order and terminal adiabatic low-pass 7-filter.42,43 The recommended delays for the filters are the same than for a third-order low-pass J filter. <5 and 8 are gradient delays, where 8 — <5 + accounts for the delay of the first point in the 13C dimension. The integral over each gradient pulse G, is H/2yc times the integral over gradient G2 in order to achieve coherence selection. The recommended phase cycle is c/)n = x, x, x, x 3 — 4(x), 4(y), 4( x), 4(—y) with the receiver phase c/)REC = x, x.

Figure 22 Pulse sequence of the HMBC-RELAY experiment. Filled and open bars represent 90° and 180° pulses, respectively. All other phases are set as x, excepted otherwise stated. A two-phase cycle x, —x is used for the pulse phases (j>, and

The spectrometer supports phase cycling, asynchronous sequence implementation, and parameter-array experiments. Thus, most standard solid-state NMR experiments are feasible, including CPMAS, multiple-pulse H decoupling such as TPPM, 2D experiments, multiple-quantum NMR, and so on. In addition, the focus of development is on its extension of, or modification to, the hardware and/or the software, in the spirit of enabling the users to put their own new ideas into practice. In this paper, several examples of such have been described. They include the compact NMR and MRI systems, active compensation of RF pulse transients, implementation of a network analyzer, dynamic receiver-gain increment,31 and so on. 

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

In practice, a particular phase cycle is defined by means of an array of RF pulse settings (to be used cyclically during consecutive scans) and an associated array of receiver phases . The receiver phase , however, does not correspond to any hardware device setting. Rather, it is an interlocution for the various modes of how each single-scan signal should be handled by the data accumulation procedure (add, subtract, quad add, quad subtract, etc.). 

Vanderbilt University Medical Center has recently completed accruing patients to a Phase II study of neoadjuvant chemoradiation, which consists of preoperative paclitaxel (175 mg/m2,3-h infusion) followed by cisplatin 75 mg/m2 d 1 and 21. Concurrent radiation was given to a total dose of 3000 cGy, in 200 cGy/fraction. Patients who are resectable go on to surgery 4 wk after completion of chemoradiation, whereas those who are unresectable (i.e., cervical esophageal cancer) continue to a total dose of 60 Gy without treatment interruptions. One month following surgery, patients receive two cycles (q 21-28 d) of postoperative chemotherapy, which consists of paclitaxel 175 mg/m2 over 3 h d 1,5-FU 350 mg/m2, d 1-3, and leucovorin 300 mg d 1-3. Preliminary analysis of this... 

A minimum two-step phase cycle was used by inverting the selective 90° pulse and the receiver phases on alternate scans. Exorcycle could also be applied on all 180° pulses. 

Compared to other multidimensional experiments the exchange experiments are fairly simple and, thus, easy to optimize. Experiments are robust with regard to the pulse imperfections and miscalibration. All artifacts except coherence transfer can be removed with standard phase cycling of RF pulses and receiver. The coherence transfer can be removed by appropriate pulse sequences, preferably with T-ROESY. 

Create a Hahn-echo sequence with 16/32 ns in one channel and optimize the echo intensity. Do the same in a second channel and adjust its phase 180° to the first cannel. Both channels are used for the two-step phase cycle. Running the experiment with the two-step phase cycle eliminates receiver offsets and allows reading off the real echo signal intensity, which is important for accurately determining Vx in spin counting experiments (Section 5.1). 

Active systems add an additional dimension to the imaging system because of their ability to resolve the depth of the imaging target. Similar to lateral resolution, two objects at two different ranges can be distinguished if their received phase variations differ by at least 2tt radians or one cycle as the frequency is swept over a bandwidth, B. This leads to a depth resolution of... 

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