To reduce the complexity of direct radio frequency (RF) undersampling real-time spectrum monitoring in wireless Internet of Things (IoT) bands (920MHz, 2.4GHz, and 5 GHz bands), a design method of sampling frequencies is proposed in this paper. The Direct RF Undersampling receiver architecture enables the use of ADC with sampling clock lower frequency than receiving RF signal, but it needs RF signal identification signal processing from folded spectrums with multiple sampling clock frequencies. The proposed design method allows fewer sampling frequencies to be used than the conventional design method for continuous frequency range (D.C. to 5GHz-band). The proposed method reduced 2 sampling frequencies in wireless IoT bands case compared with the continuous range. The design result using the proposed method is verified by measurement.

In this paper, we propose a radio frequency (RF) anti-aliasing filter design method considering the effect of a roll-off characteristic on a noise figure (NF) in the direct RF undersampling receiver. The proposed method is useful for broadband reception that a system bandwidth (BW) has nearly half of the sampling frequency (1/2 fs). When the system BW is extended nearly 1/2 fs, the roll-off band is out of the desired Nyquist zone and it affects NF additionally. The proposed method offers a design target regarding the roll-off characteristic not only the rejection ratio. The target is helpful as a design guide to meet the allowed NF. We design the filter based on the proposed method and it is applied to the direct RF undersampling on-board receiver for Ka-band high throughput satellite (HTS). The measured NF value of the implemented receiver almost matched the designed value. Moreover, the receiver achieved the reception bandwidth which is 90% of 1/2 fs.

RF under sampling is more suitable for Satellite receiver systems in comparison to terrestrial systems. In conventional RF under sampling the minimum sampling frequency (fs) should be atleast twice the system bandwidth; therefore for a system with a wide bandwidth, a relatively high fs is necessary. In this paper we propose a direct RF under sampling reception method that halves fs. The proposed f's is achieved by folding in band noise in half. A method of adapting f's for the reception of signals in different channels is also proposed; this ensures that the SNR is not degraded for any channel. To evaluate the proposed technique's performance and compare it to the conventional case a 3 channel, 1 GHz band test receiver and it's key device (i.e. S/H circuit) are developed. Using SNR and EVM as performance indexes, the performance of the proposed technique has been evaluated and compared to that of the conventional technique. The evaluation results show that the proposed technique can achieve the same performance as conventional RF under sampling for all 3 channels, using only half of the sampling frequency of the conventional technique.