What is the ideal length of an antenna set to receive a signal of frequency f?
A quarter of the wavelength
Explain the basis of the direct conversion receiver including the drawbacks
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.
Why are filters used between the local oscillator and the mixer
To remove spurious
Explain how a subsampling receiver works
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
What are the drawbacks of a subsampling receiver?
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