Re: [Discuss-gnuradio] Multiplexing modulators

[Johnathan Corgan]

> In GRC I would like to combine the outputs of several NBFM modulator
> blocks to drive a USRP sink.  The idea is to output several discrete
> channels within the USRP transmit bandwidth, e.g., three signal
> channels at -30 kHz, 0 kHz, and +30 kHz offset from the USRP tune
> frequency.

As was already mentioned, this can be done by mixing each of of the NBFM 
streams with an appropriate complex carrier offset, then adding them together 
to get the final TX baseband stream.

Close attention must be paid to amplitudes, spectral bandwidth, and sample 
rates to make this work.  The final baseband stream must have a high enough 
sample rate to accommodate the total number of channels, and must also be 
acceptable as an input sample rate to the TX USRP.  The most general way would 
be to upsample each NBFM stream to the final baseband sample rate and then 
multiply it by a complex carrier generated at that sample rate.  If you have a 
lot of channels, this can be CPU intensive, but it allows you to generate your 
final baseband with the individual channels at arbitrary spacings.

Some tricks exist to lower CPU requirements using multi-rate upconversion if 
the channel spacings have fixed offsets.  Also, as the number of channels 
increases, at some point it is more efficient to switch to a synthesis 
filterbank structure.

The output of the frequency modulators will be [-1.0,1.0].  The resampling and 
summing blocks will affect this; the final baseband output must be scaled to be 
appropriate as input to the USRP sink.  This means that the TX power of the 
USRP will be divided among the channels and any individual channel will only 
achieve a fraction of the USRP output power.

Finally, while a single NBFM channel is constant amplitude and can drive the 
USRP in its non-linear output power range, the above mix of channels is *not* 
constant amplitude, and the output power must be backed off from maximum until 
the IMD from cross-channel mixing is reduced to acceptable levels.

Johnathan