Reducing Current Consumption and Cost with 902MHz to 928MHz Spread Spectrum ASK Modulation

By: BY STAALE PETTERSEN
( 1 Mar 2007 )

As integration and features add complexity and reduce battery lifetime, the challenge to add wireless connectivity to a product have been many. 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. Today's wireless ICs require only a handful of external components and have become a minor percentage of the total solution cost. Looking into the future and with respect to what existing wireless applications have to offer, the need for additional features and performance has placed considerable constraints on battery capacity. This, in turn, has created the need for more advanced power management solutions. In today's applications, the battery and power management cost is close to 50% of the total bill of materials. To address these new challenges, Micrel Inc. began rolling out solutions such as the MICRF405, a 290MHz to 980MHz transmitter, which includes a new type of modulation called Spread Spectrum ASK/OOK. This modulation enables high output power via its spread spectrum technology with just 50% of the power consumption. This article will focus on how solutions such as the MICRF405 dramaticallyreduce both current consumption and cost.

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A number of licensing-free frequency bands have been set aside for “Industrial, Scientific and Medical” use (ISM bands) by the Federal Communications Commission (FCC) in North America. The operation of these frequency bands is specified in FCC part 15.247. They cover the 902MHz to 928MHz, 2.4GHz to 2.4835GHz, and5.725GHz to 5.85GHz spectrum.

Since the introduction of Wireless Local Area Networks (WLANs), the 2.4GHz frequency band has experienced considerable success. At the same time, computer/cellular wireless accessories continue to drive down the cost of the wireless connection. 2.4GHz wireless connectivity has primarily been related to wireless applications requiring both high data rates and rechargeable battery solutions. 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.

Since the introduction of Wireless Local Area Networks (WLANs), the 2.4GHz frequency band has experienced considerable success. At the same time, computer/cellular wireless accessories continue to drive down the cost of the wireless connection. 2.4GHz wireless connectivity has primarily been related to wireless applications requiring both high data rates and rechargeable battery solutions. 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.

To operate in the 902MHz to 928MHz ISM band, FCC regulations require manufacturers use 25 or more frequencies with a maximum dwell time (the time spent at a particular frequency during any single hop) of 400ms. 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 synchronizewith the receiver.

Another form of spread spectrum transmission is referred to as digital modulation or Directsequence spread spectrum (DSSS). DSSS, Figure 2, is a transmission method 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 chipping code is a redundant bit pattern where, as bit is transmitted, increases the signal's resistance to interference. If one or more bits in the pattern are damaged during transmission, then the original data can be recovered due to the redundancy of the transmission. DSSS radios have a short access time since the channel is stationary. The disadvantage of a DSSS radio lies in its fairly complex demodulation scheme since the signal that is received requires despreadingand synchronization.

SPREAD SPECTRUM ASK/OOK
Operating within the 902MHz to 928MHz, 2.4GHz to 2.4835GHz, and 5.725GHz to 5.85GHz ISM bands— with high output power—has historically only been allowed by using either FHSS or DSSS technology. A few years ago, the FCC changed this regulation as more and more applications were electing to use the IEEE 802.11x specification. This is when the demand for higher data rates started to escalate. To further increase the data rate, the industry proposed to remove the requirement for the digital processing gain of a receiving unit. This was accepted and the IEEE specification was then incorporated into the protocol description, thereby allowing automatic adjustments of the spreading code depending upon the signal to noise (S/N) ratio. These events have now been implemented in the new frequency regulations 15.247 and are now referred to as “Frequency Hopping Systems “and“Digital Modulated Systems”.

The FCC classifies “Digital modulated System” with the following definition: “Systems using digital modulation techniques may operate in the 902MHz to 928MHz, 2.4GHz to 2.4835GHz, and 5.725GHz to 5.85GHz bands. The minimum 6dB bandwidthshall be at least 500kHz.”

As more and more wireless applications on the market require ever higher output, power becomes more important. With “Digital Modulated Systems,” the maximum radiated output power is defined as 1W. However, the maximum output power is more affected by the definition 15.247 d: “(d). For digitally modulated systems, the peak power spectral density conducted from the intentional radiator to the antenna shall not be greater than 8dBm in any 3kHz band during any time interval ofcontinuous transmission.“

This modification to the 15.247 ruling opens up new and interesting modulation possibilities that enable long range wireless links using extremely low power. Micrel has recently patented one such new modulation solution referred to as SpreadSpectrum 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 simplest and most common form of ASK technology operates as a switch, using the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero. This type of modulation is called on-off keying (OOK) and is very power efficient method as it only transmits when sending “1”. Amplitudeshift keying requires a high S/N for recovery, as by its very nature, much of the signal is transmitted atreduced power.

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 bandwidths less than 500kHz 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 to50mV/m or some formof FHSS technology hasto be implemented tofall within the FCC part15.247.

SSASK combines the traditional known ASK/OOK with a digital modulated signal. A typical block diagram of a transmitter is shown in Figure 3 and illustrates how the MICRF405 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” (Figure 4) an FSK modulated carrier (Figure 5). 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 Figure6.

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

With SSASK/OOK, the “User data” information is present in the variations of the amplitude of the signal and enables the use of traditional ASK/OOK super heterodyne receivers such as the MICRF005 (Figure 7). The SSASK/OOK signal received by Micrel's MICRF005 will appear as a standard OOK/ ASK modulated signal and will be demodulatedaccordingly.

SSASK/OOK APPLICATION CIRCUIT
In the 902MHz to 928MHz band, ASK/OOK intentional radiators is required to implement FHSS when the application requires higher output power than 50mV/m. By using the MICRF405 in SSASK/ OOK mode, transmission with an output power of 10dBm is achieved without the need of FHSS. The application circuit (Figure 8) consists of a matching circuit, crystal and decoupling capacitors. The maximum allowed output power allowed by FCC when using the MICRF405 as an SSASK/OOKdevice and an external power amplifier is 20dBm.

An application circuit is shown in Figure 9 and has a power consumption of 83mA when using SSOOK and 50% duty cycle. The maximum peak density of 8dBm/3kHz specified by the FCC limitsthe maximum output power.

The MICRF405, see (Figure 10) is a 290MHz to 980MHz RF transmitter IC intended for unlicensed ISM band operations (see 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- andEN300-220-compliant.

The transmitter consists of an 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 phase-detector. The loopfilter 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 VCOmodulation.

In ASK modulation, the user can select between two modulation types, with or without spreading. Inboth modes the modulation depth is programmable.

About the Author Staale Pettersen is the Product Marketing Managerat Micrel Inc.