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Re: [Discuss-gnuradio] Multiplexing modulators
From: |
Johnathan Corgan |
Subject: |
Re: [Discuss-gnuradio] Multiplexing modulators |
Date: |
Tue, 21 Jun 2011 09:04:16 -0700 (PDT) |
> 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
- Re: [Discuss-gnuradio] Multiplexing modulators, (continued)
- Re: [Discuss-gnuradio] Multiplexing modulators, Johnathan Corgan, 2011/06/21
- Re: [Discuss-gnuradio] Multiplexing modulators, Marcus D. Leech, 2011/06/21
- Re: [Discuss-gnuradio] Multiplexing modulators, John Ackermann N8UR, 2011/06/21
- Re: [Discuss-gnuradio] Multiplexing modulators, Nick Foster, 2011/06/21
- Re: [Discuss-gnuradio] Multiplexing modulators, John Ackermann N8UR, 2011/06/21
- Re: [Discuss-gnuradio] Multiplexing modulators, Marcus D. Leech, 2011/06/21
- Re: [Discuss-gnuradio] Multiplexing modulators, John Ackermann N8UR, 2011/06/24
- Re: [Discuss-gnuradio] Multiplexing modulators, Iain Young, G7III, 2011/06/28
Re: [Discuss-gnuradio] Multiplexing modulators, Marcus D. Leech, 2011/06/21
Re: [Discuss-gnuradio] Multiplexing modulators, Matt Ettus, 2011/06/21
Re: [Discuss-gnuradio] Multiplexing modulators,
Johnathan Corgan <=