HPA L1 radio

A quarter of the wavelength

• One chooses a local oscillator whose frequency is equal to the RF carrier frequency
• This means that IF = 0 
• No bandpass filtering is required as the signal is converted to baseband and all filtering can be done on chip

• The drawback is that: it is susceptible to LO re-radiation picked up by the antenna
• This leads to DC offsets.

To remove spurious

• A band pass signal centred on f₀ is sampled at a rate f₀/m. 
• The RF signal is sampled at a frequency lower than twice the maximum input frequency but larger than two times the signal bandwidth.
• This guarantees aliasing will not occur
• The 'replicas' of the signal resulting from the sampling process contain the baseband signal 
• One of the low frequency ones is then digitized directly

Drawbacks:The aliasing of noise Because subsampling by a factor m effectively multiplies the  downconverted noise power of the sampling circuit by a factor of 2m since this noise is not band-limited