TECHNICAL FIELD

The present disclosure generally relates to calibrating oscillators of low-power transmitters operable of Internet of things (IoT) devices.

BACKGROUND

The Internet of things (IoT) is the inter-networking of physical devices, vehicles, buildings, and other items embedded with electronics, software, sensors, actuators, and network connectivity that enable these objects to collect and exchange data. IoT is expected to offer advanced connectivity of devices, systems, and services that goes beyond machine-to-machine (M2M) communications and covers a variety of protocols, domains, and applications.

IoT can be encapsulated in a wide variety of devices, such as heart monitoring implants; biochip transponders on farm animals; automobiles with built-in sensors; automation of lighting, heating, ventilation, air conditioning (HVAC) systems; and appliances such as washer/dryers, robotic vacuums, air purifiers, ovens or refrigerators/freezers that use Wi-Fi for remote monitoring. Typically, IoT devices encapsulate wireless sensors or a network of such sensors.

Most IoT devices are wireless devices that collect data and transmit such data to a central controller. There are a few requirements to be met to allow widespread deployment of IoT devices. Such requirements include reliable communication links, low energy consumption, and low maintenance costs.

To this aim, an IoT device and connected wireless sensors are designed to support low power communication protocols, such as Bluetooth low energy (BLE), LoRa, and the like. However, IoT devices utilizing such protocols require a battery, e.g., a coin battery. The reliance on a power source (e.g., a battery) is a limiting factor for electronic devices, due to, for example, cost, size, lack of durability to environmental effects, and frequent replacement. As an alternative to using batteries, power may be harvested from sources such as light, movement, and electromagnetic power such as existing radio frequency transmissions. In order to minimize the power consumption, IoT devices are designed with the minimum required components or implementing low-power consumption oscillators.

FIG. 1schematically illustrates a standard BLE transmitter100including a BLE packetizer110, an oscillator120, a power source130, an amplifier140, and an antenna150. These components allow for transmission of wireless signals from the BLE transmitter100.

The BLE standard defines40communication channels from 2.4000 GHz to 2.4835 GHz. Out of the 40 channels, 37 channels are used for communicating data and the last three channels (37, 38, and 39) are used as advertising channels to set up connections and send broadcast data. The BLE standard defines a frequency hopping spread spectrum technique in which the radio hops between channels on each connection event. A broadcaster device may advertise on any one of the 3 advertisement channels. The modulation scheme defined for the BLE standard is a Gaussin frequency shift keying (GFSK) modulation. To this end, within each channel, a frequency deviation greater than 185 KHz above the carrier frequency corresponds to a bit with a binary value ‘1’ and a frequency deviation less than −185 KHz corresponds to a bit with a binary value ‘0’.

The BLE packetizer110may receive a signal originated from a processor of a host device. Such a signal may include data or control parameters included in the signal transmitted by the BLE transmitter100.

The oscillator120generates a radio frequency (RF) carrier signal that may carry the data signal generated by the BLE packetizer110. The modulated RF signal, carrying the data signal, is amplified by the amplifier140and then broadcast by the antenna150. The power source130may be a battery.

The oscillator120may be a free-running oscillator, which may be used to directly generate an RF carrier signal. Thus, a free-running oscillator may replace a frequency synthesizer to generate an RF carrier signal. Utilization of a free-running oscillator may result in power savings. In the BLE transmitter100, the free-running oscillator generates a RF carrier signal having a frequency within a specific portion of the wireless spectrum, e.g., the 2.4 GHz wireless spectrum.

Typically, the free running oscillator is locked via a phase-locked loop (PLL) to a clock, originating from a crystal oscillator. The crystal oscillator has a resonator121that may be also included on a board hosting the processor of the IoT device. The resonator121is typically a crystal resonator, a quartz resonator, or microelectromechanical systems (MEMS) based resonator which typically provides a sufficiently accurate and stable time/frequency reference. However, for low-cost, ultra low-power, and small form-factor IoT devices, it is desirable to omit such a resonator.

Some devices, such as radio frequency identification (RFID) devices, use an external signal as a reference signal. Typically, an RFID reader demodulates a received signal onto the carrier wave transmitted to RFID tags. An RFID tag receiving the carrier wave reference signal can synchronize its own transmission based on such a signal. The RFID does not time the transmission, but merely transmits data as the reference signal is received. This synchronization solution cannot work in BLE transmitters where transmission occurs in specific sessions. Further, RFID readers and tags do not manipulate signals received over-the-air to serve as an RF carrier signal for BLE transmission.

It would therefore be advantageous to provide solution to design a transmitter without a resonator that would overcome the challenges noted above.

SUMMARY

A summary of several example embodiments of the disclosure follows. This summary is provided for the convenience of the reader to provide a basic understanding of such embodiments and does not wholly define the breadth of the disclosure. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor to delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later. For convenience, the term “certain embodiments” may be used herein to refer to a single embodiment or multiple embodiments of the disclosure.

Certain embodiments disclosed herein include an oscillator calibration circuit oscillator calibration circuit comprising: a first frequency locking circuit (FLC) coupled to a first oscillator, wherein the first FLC calibrates the frequency of the first oscillator using an over-the-air reference signal, wherein the first FLC calibrates the first oscillator prior to a data transmission session and remains free running during the data transmission session; and a second FLC coupled to a second oscillator, wherein the second FLC calibrates the frequency of the second oscillator using the over-the-air reference signal, wherein the second FLC calibrates the second oscillator immediately prior to a data transmission session and remains free running during the data transmission session.

