RadioProcessor
Appendix I: Using the Undersampling
Capabilities of the RadioProcessor
- 70 MHz
IF Application
The standard A/D sampling frequency of the RadioProcessor board is 75 MHz. At this 75 Ms/s sampling
frequency, input signals up to 37.5 MHz (the Nyquist frequency, i.e., ½ of the sampling frequency) can be
sampled without any aliasing effects occurring. In addition to this, it is also possible to use the
RadioProcessor with input signals higher than 37.5 MHz by taking advantage of the high bandwidth of the
input amplifier and A/D (Analog-to-Digital) converter.
In general, when input signals are above the Nyquist frequency, the signal is folded back to a lower
apparent frequency when it is sampled by the A/D. The folded frequency can be calculated with the following
formula (1):
(1)
f
folded
: Frequency that appears to the RadioProcessor system
f
AD
: Sampling frequency of the A/D (75 MHz in this case)
f
input
: Input frequency applied to the RadioProcessor
n
: Integer which results in
f
folded
f
AD
2
The rest of this document demonstrates the use of the RadioProcessor Model TRX-I-50-75-300 with a 70
MHz IF (intermediate frequency) signal in a high-field NMR spectrometer. This particular models uses a 50
MHz reference frequency, 75 MHz A/D sampling frequency, and 300 MHz D/A frequency. Transmitting (Tx)
and Receiving (Rx) performance at and around 70 MHz will be presented.
Excitation Section
In order to be usable for 70 MHz applications, the RadioProcessor must output short pulses with a carrier
frequency of 70 MHz and agile phase and frequency modulation. This can be accomplished with
RadioProcessor's Tx excitation section which is equipped with the on-board 300 MHz 14 bit DAC converter
followed by a wide-band output amplifier. The figure on the next page (Figure A1.1) shows an example RF
output pulse
that was generated by the RadioProcessor. This data was captured using a Tektronix TDS224
oscilloscope. Notice the time base of 25 ns/division.
2020-10-07
31
f
folded
=
∣
n
⋅
f
AD
2
−
f
input
∣
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