Well, according to the specs the official bandwidth is limited to 60Mhz but that is in order to exploit the first Nyquist zone (0 - 62.5 MHz) with the max sampling rate of 125 MHz.
I assume (I do not own a red-pitaya yet ) that there must be a low pass anti -alias filter with a steep cut of frequency well below 60 MHz (but how much below ?- probably around 40-50 MHz ) so the effective -3dB bandwidth exploitable might be lower than 60 MHz (but how much lower I have no Idea - and since the hardware is closed source - it is impossible to find out the characteristics of the low pass filter by looking at the schematic diagram )
Regarding UHF and VHF, the Nyquist Theorem states that it is feasible to under sample (with caviats in terms of amplitude and sound to noise ratio) a signal as long as the sampling rate (so 125 MHz in the case of the RP) is twice as much as the bandwith of the the signal to be under sampled. So, for instance receiving the UHF sigbal of 432MHz with a bandwidth of 2Mhz with the red pitaya would imply to bypass the low-pass anti-alias filter / amplifier on the red pitaya board and to replace it by an external bandpass saw filter centered on 432 MHz and of a 2 MHz -3dB Bandwidth. Thanks to the aliasing effect of the undersampling, the signal would actually be down converted between 56 and 58 MHz (with spectral inversion - i.e., 432 - 3* 125 = 57 MHz)
See the interesting paper from TI : http://www.ti.com/lit/an/slaa594a/slaa594a.pdf
This could possibly work if the analogue spec of the ADC chip used in the red pitaya allow handling such an UHF signal (not all ADC chips are analoguely designed to perform well with under sampling - so to this would have to be checked as a pre requisite ). In this example the 431 - 433 MHz signal would be theoritically downconverted between 56 and 58 Mhz as an indirect result of the under sampling done but with which amplitude ? and with which sound to noise ratio ? The analogue input signal of 431-433 MHz would probably have to be amplified considerably before being under sampled and of course, the sound to noise ratio of the down converted signal would probably not be as good as if using a good oldfashion analogue down converter ?
I wonder however if sombody had tried playing around with under sampling and the red pitaya to down convert signals above 60 MHz accessing directly the DAC/ADC using alternate filters to the bandpass anti-aliasing one on the board ?
Does the redpitaya hardware architecture provide a way to easily and smoothly bypass the anti-aliasing filter to allow to experiment with under sampling ? Some kind of jumper to disable the onboard bypass filter and a connector to access the input of the ADC and the output of the DAC directly ?
If anybody has tried this or something similar, it would be nice to know !