Frequency hopping flow graph problems

Hi,

I’m trying to sent X samples on frequency Y then after that on frequency
Z
and so on.

But it looks like I have some problems with the flow graph because my
second
frequency is not beeing transmitted.
It just stops after the first frequency has been transmitted.

I tried different combinations of lock() unlock() and tb.start() but it
didn’t work.

Here is my simple Code.
Thx in advance

#!/usr/bin/env python

from gnuradio import gr, eng_notation, usrp2
from gnuradio.eng_option import eng_option
from optparse import OptionParser
import sys
import math

nsamples=100000000 #samples to
transmit
interface=‘eth0’
mac_addr=’’

print “nsamples:”, nsamples
print “interface:”, interface

usink=usrp2.sink_32fc(interface, mac_addr) #usrp2 as sink

dac_rate=usink.dac_rate() #dac rate

interp=4
usink.set_interp(interp)
eth_rate=dac_rate/interp #eth rate

g=usink.gain_range() #set auto
calculated gain
gain = float(g[0]+g[1])/2
usink.set_gain(gain)

center_freq=24000000000
usink.set_center_freq(center_freq)

amplitude=1 #waveform
params
type=gr.GR_SIN_WAVE
waveform_freq=1000000
offset=0

#signal source
source=gr.sig_source_c(eth_rate, # Sample rate
type, # Waveform
type
waveform_freq, # Waveform
frequency
amplitude, # Waveform
amplitude
offset) # Waveform offset

tb=gr.top_block()

head=gr.head(gr.sizeof_gr_complex, int(nsamples))

tb.connect(source, head)
tb.connect(head, usink)
tb.run()

waveform_freq+=1000000
source.set_frequency(waveform_freq)
tb.run()