Strange behavoir of the mux, and signal limt =2mhz!

Hi,
we have this code below, and we want to get two signals from one BNC
connector.
with this code we can get two signals out of one output but not higher
then 2mhz. the mux is really strange if the value of the mux 8, 9, A, C
we get an output. Why is 2mhz the limit the interp is low, are some
other variables that have effect on the output frq.

can someone give us some advice?
thank you,
Wim & anmar

def main():

    interp = 32
    waveform_type = gr.GR_SIN_WAVE
    waveform_ampl = 32000
    waveform_freq1 = 1e6

waveform_freq2 = 2e6

    waveform_offset = 1
    fg = gr.flow_graph ()
    sink = usrp.sink_c (0, interp)

sink.set_nchannels(2)
sink.set_mux(0x00a9)

    usb_freq = sink.dac_freq() / interp

    siggen1 = gr.sig_source_c (usb_freq,
                                   gr.GR_SIN_WAVE,
                                    waveform_freq1,
                                   waveform_ampl,
                                   waveform_offset)

    siggen2 = gr.sig_source_c (usb_freq,
                                   gr.GR_SIN_WAVE,
                                   waveform_freq2,
                                   waveform_ampl,
                                   waveform_offset)

inter = gr.interleave(gr.sizeof_gr_complex)

fg.connect (siggen1, (inter, 0))
fg.connect (siggen2, (inter, 1)))
fg.connect (inter, sink)

fg.start()
raw_input(‘Press enter to stop\n’)
fg.stop()

if name == ‘main’:
main()