# A Temperature-Robust Envelope Detector Receiving OOK-Modulated Signals for Low-Power Applications
## 1 Introduction  
Envelope Detectors (EDs) are used in several applications involving an incoming high-frequency amplitude modulated signal whose envelope needs to be extracted.  
A usage example for EDs is that of Wake-Up Receivers (WuRXs), widely employed in the Internet of Things (IoT). IoT nodes are organized in subnetworks, the Wireless Sensor and Actuator Networks (WSAN), typically composed of a central node; the gateway, which manages all communication within the network; and several end nodes communicating wireless.  
In such nodes, the transceiver is typically the most power-hungry section, and a popular power reduction method is the use of WuRXs. The WuRX is an always-on ultra-low-power additional receiver, which can be integrated in end nodes to continuously monitor the channel instead of the main transceiver and activate the rest of the node only when a Wake-Up packet is received from the gateway. This approach allows event-driven, asynchronous communication [1, 2, 3, 4, 5, 6, 7].  
Despite its noise sensitivity, low-power applications mostly use OOK modulation, which is the simplest kind of Amplitude Shift Keying modulation, the digital form of Amplitude Modulation [1, 8]. The modulating signal is a square wave: the carrier is present during the transmission of a logic 1, whereas there is no carrier during the transmission of alogic 0. It is frequently employed in communication, such as RF and optical communication, thanks to its simplicity and spectral efficiency.  
OOK demodulation is easily carried out by leveraging the second-order non-linearities of MOSFETs in subthreshold, which is also beneficial for low-power operation. Two approaches are available [9], the active [10; 11] and the passive one [12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Unlike in active EDs, in passive ones, demodulating MOSFETs or off-the-shelf diodes (in the case of discrete-component solutions) are biased with zero current and the RF signal is coupled to MOSFET terminals by capacitances. No static current results in an inherent absence of flicker noise. This directly translates into an enhanced sensitivity, i.e., the minimum input power that can be correctly detected. Yet, a trade-off exists between the propagation delay and the rectification gain [1]. Then, the demodulation mechanism of passive EDs is heavily dependent on the input RF frequency due to the presence of the coupling and filtering capacitances. Finally, more constraints exist in terms of the maximum input power that can be correctly received than for active EDs (Section 3.4) [11].  
For optimal communication, ED operation needs to be reliable in all settings, where environmental conditions, such as temperature, may vary widely. Therefore, it is advisable to implement some sort of compensation in order for the ED to be fully functional in every condition. This paper accomplishes this task by adding a nanowatt temperature compensation block to a passive ED for robust operation in case of significant temperature variations. Two different versions are implemented, the first targeting minimal mismatch and the second targeting minimal area, for comparison in terms of room temperature sensitivity and sensitivity spread due to mismatch.  
This paper is organized as follows: Section 2 presents the architectural choices for the passive ED with the Proportional-to-Absolute Temperature (PTAT) compensation block, comparing them with other EDs in the literature. In Section 3, the passive ED is thoroughly described, from the basic structure presented in [12] to the proposed temperature compensation mechanism and the reasons and consequences for choosing a differential approach. Section 4 shows the validation of the proposed temperature compensation mechanism as well as the comparison between the two implementations. Finally, Section 5 concludes the paper.