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Posted on 01 February 2019

Wireless Gets Tough On Power Management, Cost

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New modulation enables high output power

As integration and features add complexity and reduce battery lifetime, the need to add wireless connectivity to a product has also grown. Since the introduction of short range radios (SRD), a wide range of applications has added wireless connectivity creating increased demand for cheaper and more integrated solutions.

By Staale Pettersen, Product Marketing Manager, Micrel, Inc.

 

Today’s wireless ICs require only a handful of external components and have become an inconsequential percentage of the total solution cost. Looking into the very near future and considering what existing wireless applications currently have to offer, the need for additional features and performance has placed considerable limitations on battery life. This has driven the need for more advanced power management solutions. In today’s applications, the battery and power management cost is close to 50 percent of the total bill of material. To address these new challenges, companies like Micrel, Inc., have begun rolling out solutions such as the MICRF405, a 290-980MHz transmitter including a new type of modulation called Spread Spectrum ASK/OOK. This new modulation enables high output power via its spread spectrum technology with just 50 percent of the power consumption. This article will focus on how solutions like the MICRF405 radically reduce both current consumption and cost.

The Federal Communication Commission (FCC) in North America specifies frequency bands covering 902–928 MHz, 2400–2483.5 MHz, and 5725–5850 MHz. Today, the frequency band is crowded with a wide range of applications ranging from RFID to WiMAX. Unfortunately, some applications, including the widely adopted in residential, building and industrial environments, have created a fairly high noise floor which makes the technology unsuitable for industrial, low power and long range applications. For these applications, the 902-928MHz frequency band is considered to be the better suited ISM band. To operate in this frequency band, some sort of frequency spread spectrum is required. Historically, there have been two types of spread spectrum. Frequency Hopping Spread Spectrum (FHSS), a method where the data signal is modulated with a narrowband carrier signal that “hops” in a random but predictable sequence from frequency to frequency, as a function of time and over a wide band of frequencies. The most significant disadvantage of frequency hopping spread spectrum transmissions is the required frequency synchronization between the transmitter and the receiver. The frequency synchronization requirement results in a slow access time and higher power consumption as the system needs to transmit on all channels to synchronize with the receiver.

The second popular form of spread spectrum transmission is referred to as digital modulation or Direct-sequence spread spectrum (DSSS). DSSS, is where a data signal at the sending station is combined with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. The issue with a DSSS radio lies in its fairly complex demodulation scheme since the signal that is received requires de-spreading and synchronization.

Spread Spectrum ASK/OOK

As more and more wireless applications on the market require ever higher output, power becomes more and more important. With Digital Modulated Systems,systems using digital modulation techniques may operate in the 902–928MHz, 2400–2483.5 MHz, and 5725–5850MHz bands, the maximum radiated output power is defined as 1W. However, a modification to this rating was made and now, maximum output power is defined as: the peak power spectral density conducted from the intentional radiator to the antenna shall not be greater than 8 dBm in any 3 kHz band during any time interval of continuous transmission. This modification opens up new and interesting modulation possiblities that enable long range wireless links using extremely low power. Micrel, Inc. has recently patented one such new modulation solution referred to as Spread Spectrum On Off Keying (SSOOK ) or Spread Spectrum Amplitude Shift Keying (SSASK).

Amplitude-shift keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave. The advantage of ASK radio systems is the simplicity of the transmitter and receiver topology and the low current consumption. ASK/OOK is a simple, yet powerful modulation scheme and is cost effective to implement both for the transmitter as well as the receiver using silicon technology. Unfortunately, an ASK/OOK modulation system occupies bandwidth less than 500 kHz or has a peak density that does not fall under “Digital Modulation Systems”. This means that for an ASK/OOK modulation system, the output power of a transmitter is limited to 50mV/m or some form of FHSS technology has to be implemented to fall within the FCC part 15.247.

Spread Spectrum Amplitude Shift Keying is a method of combining the traditional known ASK/OOK with a digital modulated signal. A typical block diagram of a transmitter, shown in Figure 1, illustrates how Micrel’s MICRF405 solution operates in SSASK/OOK mode. The SSASK/OOK modulation is created by adding “user data” to an AM modulator and creating an amplitude Shift or turning “on” and “off” a Frequency Shift Keying (FSK) modulated carrier. The FSK signal is generated by adding a PN sequence to the FSK modulator that is programmed to give an occupied bandwidth >500kHz as specified by FCC. The FSK data rate and the PN sequence are selected in a ratio giving as equal peak density within the 6dB bandwidth as possible. The result is a Spread Spectrum ASK/OOK spectrum as shown in Figure 2.

SSASK-OOK Block Diagram

The radiated spectrum and the peak density of a SSASK/OOK modulated spectrum are “equal” to a “Digital Modulated System” and therefore, are considered by FCC a “Digital Modulated System”. The main benefits of this new modulation type lies in its low power consumption since it only transmits when sending a “1” and the ability to increase the output power without the need of a FHSS.

SSASK-OOK

SSASK/OOK Application Circuit

In the 902-928MHz band, ASK/OOK intentional radiators is required to implement Frequency Hopping Spread Spectrum when the application requires higher output power then 50mV/m. By using Micrel’s MICRF405 in Spread Spectrum ASK/OOK mode, transmission with an output power of 10dBm is achieved without the need of FHSS. The application circuit, Figure 3, consists of a matching circuit, crystal and decoupling capacitors. The maximum allowed output power allowed by FCC when using the MICRF405 as a Spread Spectrum ASK/OOK device and an external power amplifier is +20dBm. Another application circuit, shown in Figure 4, has a power consumption of 83mA when using SSOOK and 50 percent duty cycle. The maximum peak density of 8dBm/3kHz specified by the FCC limits the maximum output power.

SSOOK Application Circuit 10dBm

Micrel’s MICRF405, Figure 5, is a 290MHz-980MHz RF transmitter IC intended for unlicensed ISM band operations, Table 1. It is designed to work in the North American 315MHz and 915MHz bands as well as the European 433MHz and 868MHz bands. The device is fully FCC Part 15.247 and EN300-220-compliant.

SSOK Application Circuit +20dBm

Micrel’s MICRF405 Reference

The transmitter consists of a FSK/ASKS modulator, PLL frequency synthesizer and a power amplifier. The frequency synthesizer consists of a voltage-controlled oscillator (VCO), a crystal oscillator, dual modulus pre-scaler, programmable frequency dividers and a phasedetector. The loop-filter can be internal or external. The output power of the power amplifier can be programmed to eight levels. A lock detect circuit detects when the PLL is in lock. In FSK mode, the user can select between three different modulation types thereby allowing a data rate up to 200kbps. When selecting FSK modulation is applied with dividers, the MICRF405 then switches between two sets of register values (M0,N0,A0:”0” and M1,N1 and A1:”1”). The second modulation type is closed loop VCO modulation using the internal modulator that applies the modulated data to the VCO. The third FSK modulation type is Open loop VCO modulation. In ASK modulation, the user can select between two modulation types, with or without spreading. In both modes, the modulation depth is programmable.

MICRF405 Block Diagram

Note: MFL is a registered trademark of Amkor Technology.

 

 

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