II. Operating at Low Frequencies
Acquisition Parameters
The filter pipeline of the RadioProcessor contains a CIC filter with a cut-off frequency of
approximately 1 kHz. At low frequencies when using base-band conversion, the result of the
conversion leads to spurious peaks in the pass-band of the CIC filter. To avoid these spurious
peaks, it is necessary to set the spectrometer frequency to 0 Hz.
DC Offset
By removing the capacitor at C680, a DC offset will be introduced into the receiver.
This offset can be eliminated in software by first calculating the DC offset at the receiver when
loaded by a 50 Ohm capacitor, and adjusting the values of data acquired. To remove the DC
offset:
1. Terminate the RadioProcessor receiver using a 50-Ohms terminator.
2. Acquire data using at least 1024 points and a spectrometer frequency of 100
Hz.
3. In software, calculate the average value of the data captured by the
RadioProcessor.
4. Modify the acquisition program to apply offset correction by subtracting the
value calculated in step 3 from each point of data downloaded from the board
before storing it to the FID.
www.spincore.com
2010-01-29
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