Linrad-02.58 used as a resampler.
(Dec 30 2008)
The new network mode BASEBAND in Linrad-02.58 sends I/Q data over the network in the same format as is used for 16 bit RAW data, but another port address is used.

By selecting CW or SSB without AGC or coherent processing one can obtain a frequency shifted signal at a bandwidth of slightly less than the Nyquist frequency of the output sample rate. For 96 kHz sampling rate the bandwidth is thus a bit below 48 kHz. When I and Q are sent out over the network it would be possible to send twice as much bandwidth, but the necessary code modifications are not done in Linrad-02.58.

Figure 1 shows the screen when running two instances of Linrad under X11 in Linux. The flat bandwidth is about 30 kHz.


Fig 1.The left side is the master running at 128 kHz while sending data to the slave at 96 kHz


There are two signals and a noise floor generated by the builtin signal generator in Linrad. The weak signal is close to the noise floor and on-off keyed. The strong signal is 40 dB stronger and sweeps from the weak signal to a frequency far out into the stopband. The dynamic range is limited by a false signal that appears near the left side of the frequency range when the strong signal passes across the filter edge on the right side.

Linrad was originally not designed for wideband output and distortion as well as phase errors at high frequencies are of no concern at all when the signal is sent to a loudspeaker. Approximations to save CPU time were well motivated some years ago when computers generally were slower.

Oner obvious usage of a setup like figure 1 would be to send fft2 data at 96 kHz from the slave for usage with MAP65 to decode JT65 signals. For that purpose the dynamic range should be sufficient. There could be more demanding applications, but for the moment there is no reason to put priority on improving the dynamic range of the baseband output.

Figure 2 shows a similar screen under Microsoft Windows.


Fig 2. Here the master runs at 1 MHz and the slave at 96 kHz.


The bandwidth in figure 2 is about 40 kHz and the dynamic range is about 40 dB. Similar to figure 2. There is no noise floor added in figure 2 because Linrad does not have a random number generator under Windows.


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