LIMESDR-USB send defined Tx Burst

I have recently bought a LimeSDR-USB which I would like to use for sending a defined length TX RF pulse at 3MHz. I am currently using pyLMS7002Soapy to communicate with the limeSDR and to adjust the NCO Frequency to mix down.

from pyLMS7002Soapy import *
from SoapySDR import * # SOAPY_SDR_ constants
import numpy as np
sdr.sdr.setSampleRate(SOAPY_SDR_TX, 0, 50e6)

sdr.txRfFreq = 120e6
sdr.txNCOFreq = 117e6
sdr.txGain = 60

TxTSP = sdr.LMS7002.TxTSP['A']

txTimeout =	100000


txStream = sdr.sdr.setupStream(SOAPY_SDR_TX, SOAPY_SDR_CF32, [0])
numSamps = 1024
sdr.sdr.activateStream(txStream, SOAPY_SDR_END_BURST, numElems=numSamps)
x = np.linspace(-10, 10, numSamps)
txPulse = np.sinc(x) * 1/np.sinc(x).max()

txTime0 += long(0.1e6) #100ms timestamp to get the signal at specific time 
sr = sdr.sdr.writeStream(txStream, [txPulse], txPulse.size, txFlags)  

How can I define the pulse length of my Tx RF Pulse?

My application is NMR related, much similar to Which software did the cMichal use to generate the RF pulse?
Thank you very much in advance, I have tried for around one week to get a similar output pulse on my LimeSDR-USB without success.

Please also answer - this is definitely not possible - or you could start here … ?!

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