RE: [Discuss-gnuradio] Up Conversion Weirdness

[Tom Rondeau]

> -----Original Message-----
> From: address@hidden [mailto:discuss-
> address@hidden On Behalf Of Thomas Schmid
> Sent: Monday, October 23, 2006 6:22 PM
> To: gnuradio mailing list
> Subject: [Discuss-gnuradio] Up Conversion Weirdness
> 
> Hi Everyone,
> 
> I have some strange behavior of gnuradio while trying to do an
> upconversion and multi-channel transmitter. I am currently trying to
> have two packet transmitters send packets simultaniously on two
> different radio channels. For this, I upconvert one of the
> transmitters and add the two signals together. Following is the code
> to connect the different blocks:
> 
> # interpolate the two transmitters
> self.connect(self.packet_transmitter1, self.interpolator1)
> self.connect(self.packet_transmitter2, self.interpolator2)
> # upconvert the first transmitter)
> self.connect(self.interpolator1, (self.multiplicator, 0))
> self.connect(self.sin, (self.multiplicator, 1))
> # add the two signals
> self.connect(self.multiplicator, (self.adder, 0))
> self.connect(self.interpolator2, (self.adder, 1))
> # send the signal to the USRP
> self.connect(self.adder, self.amp, self.u)
> #self.connect(self.adder, self.filesink)
> 
> The code compiles, but I don't receive the messages on the other side.
> Therefore, I did some tests to check the different parts. The
> following connection works, i.e., the packet transmitter and its
> interpolation can be received at the other side:
> 
> self.connect(self.packet_transmitter1, self.interpolator1)
> self.connect(self.interpolator1, self.amp, self.u)
> 
> Now, if I add the following, it suddenly doesn't work anymore, even
> though the added connections should not affect the path to the USRP:
> 
> self.connect(self.packet_transmitter1, self.interpolator1)
> self.connect(self.interpolator1, (self.multiplicator, 0))
> self.connect(self.sin, (self.multiplicator, 1))
> self.connect(self.interpolator1, self.amp, self.u)
> self.connect(self.multiplicator, self.filesink)

First, I assume you meant the second to last line to read:
> self.connect(self.multiplicator, self.amp, self.u)

What is the sampling rate at this point in the flow graph, and what is the
frequency of the sine wave? I'm just trying to make sure you're not
upconverting beyond the first Nyquist zone. For this app, you're still
looking at the upconverted signal as baseband, so your sampling rate must be
at least 2x(f_sin + signal BW). I assume that's what the interpolator is
doing, but just making sure.

What's the file look like when you plot it locally?

Tom