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LimeSDR output: updated documentation

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plugins/samplesink/limesdroutput/readme.md
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@ -83,6 +83,30 @@ This is the center frequency of the mix of LO and NCO combined and is the sink p
☞ In the LMS7002M TSP block the NCO sits after the interpolator (see Fig.14 of the [datasheet](http://www.limemicro.com/wp-content/uploads/2015/09/LMS7002M-Data-Sheet-v2.8.0.pdf) p.7) so it runs at the actual DAC rate. Hence the NCO limits are calculated as +/- half the device to host sample rate multiplied by the hardware interpolation factor. For example with a 4 MS/s device to host sample rate (10) and a hadrware interpolation of 16 (8) you have +/- 32 MHz span around the LO for the NCO. In this example you can tune all HF frequencies with the center frequency set at its lowest (30 MHz). ☞ In the LMS7002M TSP block the NCO sits after the interpolator (see Fig.14 of the [datasheet](http://www.limemicro.com/wp-content/uploads/2015/09/LMS7002M-Data-Sheet-v2.8.0.pdf) p.7) so it runs at the actual DAC rate. Hence the NCO limits are calculated as +/- half the device to host sample rate multiplied by the hardware interpolation factor. For example with a 4 MS/s device to host sample rate (10) and a hadrware interpolation of 16 (8) you have +/- 32 MHz span around the LO for the NCO. In this example you can tune all HF frequencies with the center frequency set at its lowest (30 MHz).
<h3>7A: External clock control</h3>
Use this button to open a dialog that lets you choose the external clock frequency and enable or disable it. When disabled the internal 30.72 MHz VCTCXO is used.
![LimeSDR input plugin gain GUI](../../../doc/img/LimeSDR_plugin_extclock.png)
<h4>7A.1: Exrernal clock frequency</h4>
Can be varied from 5 to 300 MHz
Use the thumbwheels to adjust frequency as done with the LO (1.1). Pressing shift simultanoeusly moves digit by 5 and pressing control moves it by 2. The boundaries are dynamically calculated from the LO center frequency, sample rate and hardware decimation factor.
<h4>7A.2: Enable/disable external clock input</h7A>
Use this checkbox to enable or disable the external clock input
<h4>7A.3: Confirm changes</h4>
Use the "OK" button to confirm your changes
<h4>7A.4: Dismiss changes</h4>
Use the "Cancel" button to dismiss your changes
<h3>8: LMS7002M hardware interpolation factor</h3> <h3>8: LMS7002M hardware interpolation factor</h3>
The TSP block in the LMS7002M hardware has an interpolation chain that acts on both Tx channels. It is composed of 5 halfband interpolation stages and therefore can achieve interpolation between 1 (no interpolation) and 32 in increasing powers of 2: 1, 2, 4, 8, 16, 32. The TSP block in the LMS7002M hardware has an interpolation chain that acts on both Tx channels. It is composed of 5 halfband interpolation stages and therefore can achieve interpolation between 1 (no interpolation) and 32 in increasing powers of 2: 1, 2, 4, 8, 16, 32.