Protocol: Tunerp  Sequence: tunerp.c  Apptype: solidseq1d

Description: 

A pulse sequence for pulse tuning through the directional couplers in 
the VNMRJ display. Tunerp is used for high-power pulsed tuning and for 
characterization of phase transient. It provides a phase-detected 
output of the pulse that can be displayed in phased or absolute value 
mode. Launch Tunerp with the Fidscan button on the Shims page in Setup 
tab. 

To use Tunerp it is preferable to be able to measure the ratio of 
forward to reflected power.  The standard directional couplers are 
wired to measure reflected power only.  The arrow on the side of the 
coupler should point back toward the Front End to measure reflected 
power. To measure forward power reverse the coupler so that the arrow 
points toward the probe.

A second optional bidirectional coupler is available on some systems.  
With this coupler reverse the direction of the arrow by turning the 
knob on the top. 

Setup: 

Load a calibrated data set or load Settancpx into a workspace that will 
not be used to acquire data.  Convert the data set with Tunerp. Set 
Tunerp to obs the desired channel with the procedure below. It is 
helpful to set up Tunerp for each of 1-4 channels in the first 1-4 
workspaces and join each of them when tuning is needed.  

To tune a particular channel, enter its number (1-4) in Channel entry 
box on the Sequence page. Also set the particular channel as observe on 
the channels page and choose the nucleus.  

For two-channel experiments, where channels 1 and 2 are used as obs and 
dec, it is simply necessary to enter the desired nucleus in the observe 
nucleus entry box. The correct channel will be selected automatically 
and the channel number will be displayed. 

For three-channel experiments or any time channels 3 and 4 are 
involved, it is necessary to configure probeConnect and preAmpConfig 
before tuning. See the instructions below for configuration of these 
parameters. Enter one of the nuclei designated in probeConnect in the 
observe transmitter-nucleus entry box.  The correct channel will be 
selected automatically and the channel number will be displayed

Set aTune and tpwr to appropriate values.  The amplitude of the tuning 
pulse is determined by aTune (not aX90) and tpwr. One should tune 
routinely with about 25 to 50 Watts of power or less.  Sometimes it is 
necessary to retune with the precise amplitude to be used in the 
experiment. 

Pulse Tuning:

Before pulse-tuning always rough-tune the probe with the mtune 
function. 

Press the Tune button to set pwTune at 300 us and select a full FID 
display.  Note that Tune sets a 5.0 MHz spectral width and the 
appropriate acquisition time. Enter Fidscan on the Shims page of the 
Setup tab and select magnitude mode only. Adjust the repetition rate 
with d1 as desired. Alternatively type av and collect one-scan displays 
with acquire. 

Switch to forward power and measure the pulse shape. It may be 
necessary to reduce reciever gain to avoid receiver overload.  For 
high-power tuning it may be necessary to put attenuation between the 
coupler and the Front End. 

Switch to reflected power and tune the probe to minimize the central 
component of the pulse. Characterize the forward/reflected ratio by 
recording two traces with the same value of vertical fid scale vf.  
Good tuning is a ratio of > 30/1. 

Phase Transient: 

Minimization of phase transient on the proton channel is needed for 
multiple-pulse proton experiments such as Hetcorlgcp2d. Minimization of 
phase transient on the X channel is needed for Pisema2d and is desired 
for multiple-pulse X experiments such as C7inad2d.

Be sure the probe is tuned before measuring phase transient. 

To characterize phase transient press the button labeled Transient to 
set a 10 us pulse whose rise and fall are clearly visible. Note that 
the Transient button sets a 5.0 MHz spectral width and the appropriate 
acquisition time. Enter Fidscan on the Shims page of the Setup tab and 
select real and imaginary modes. Adjust the repetition rate with d1 as 
desired. Alternatively type av and collect one-scan displays with 
acquire. 

Set the coupler for forward power. Collect a trace and phase it so that 
the real channel is 90 degrees out of phase and the imaginary channel 
is in phase. In this mode one will see zero amplitude with two 
transients of opposite phase at the beginning and ends of the pulse. 
These transients are the phase transient and represent pulse amplitudes 
during the rise and fall times that are 90 degrees out of phase from 
the pulse. 

Adjust probe tuning or cable lengths so as to minimize the amplitude of 
the two transients. Note that removal of phase transient with the probe 
tuning alone will detune the probe and increase reflected power. 
Generally one cannot achieve both good tuning and no transient by 
changing only the probe. 

To remove phase transient by probe tuning adjust the Tune knob on the 
probe to move the tuning dip either up or down in frequency. Rephase 
the display and note whether the transients have gained or lost 
amplitude.  Choose a value that minimizes the transient. 

Phase transient can be removed permanently by adjusting the cable 
length between the probe and the directional coupler. Phase transient 
is a minimum for cable lengths that are multiples of 1/2 wavelength 
plus a constant. To find the correct length it is helpful to have a set 
of short cables and connectors and experiment with different lengths. 
The high-band channel can be adjusted with a set of elbow connectors.  
Once the correct length is found it is desirable to have a single 
permanent length made. Be sure that the probe remains tuned during this 
process.  Note that one must have a different cable length for each 
different nucleus. 

Three and Four Channel Experiments: 

To tune on channels 3 and 4 one must set probeConnect and preAmpConfig. 
These two parameters are "Global" strings than must be created manually 
by the system manager or user. As global parameters these strings apply 
to all workspaces in a user and do not affect other users. Note that 
these parameters are NOT created in the "Update User" function of the 
VNMRJ administrator interface or by the "makeuser" function. 

probeConnect is a global string whose entries are the nuclei to be 
assigned to each channel. Create it with the command create 
('probeConnect','string','global'). Type 
display('probeConnect','global') to verify its exisitence. Type 
display('probeConnect') alone to verify that a "current" version of 
probeConnect does not exist. The result should be negative. 

Set [rpbeConnect equal to the nuclei for channels 1 to the number of 
channels, numrfch, in order, separated by spaces.  For example: 

probeConnect = 'H1 C13 F19 N15' 

sets up a four channel spectrometer with an HFXY probe tuned to the 
indicated nuclei. Note that the first entry is always highband and the 
second always low band. On three-channel spectrometers the third entry 
must match the band of channel-three amp. On four channel spectrometers 
a second highband amp is always placed on channel 3 if it is present. 

preAmpConfig is a global string whose entries indicate the receiver 
function attached to each channel. Create it with the command 
create('preAmpConfig','string','global'). Type 
display('preAmpConfig','global') to verify its exisitence. Type 
display('preAmpConfig') alone to verify that a "current" version of 
preAmpConfig does not exist. The result should be negative. 

The characters of preAmpConfig can be "H" for highband, "L" for lowband 
and "X" for no preamp.  The band of the preamp on a channel must match 
the band of the amplifier. A channel must have a preamp to be selected 
as the observe function. 

An example for preAmpConfig is: 

preAmpConfig = 'HLHL' 
         
for the four-channel machine above.

probeConnect and preAmpConfig are present on the Channels page of all 
sequences. An output of "---" means that parameter does not exist. An 
Output of "  " means that the parameter exists but has null value. 
 
Parameter Groups: 

tune:  Module:  no
Sequence:  tunerp.c
Description:  Implements a directional-coupler pulse on a selected 
              hardware channel for pulse tuning.  
Parameters:  Sequence Page
       atune -  the amplitude of the tune pulse.
       chtune -  the hardware channel to be tuned. 
       pwtune -  the length of the tune pulse. 
