Using high frequency signals (825 MHz and up) two types of information are transmitted: digital data containing system level instructions, and audio data, the voice portion of the transmission. When the high frequency signals from a cellular base station are received at the portable phone's antenna they are filtered and down-converted to a much lower 455KHz intermediate frequency (IF) that is easier to manipulate. Once the signal has been converted to audio, it is conditioned to reduce noise and then outp ut through an audio amplifier to the speaker of the telephone. Going in the opposite direction, voice input from the phone is conditioned, modulated and upconverted to RF frequency, amplified and transmitted back to the base station.
Sounds simple, right? In theory perhaps, but in real-life it gets very complicated. Juggling high frequency signals, eliminating stray noise, accurately up-converting and down-converting frequencies, maintaining accurate data when switching from cell to c ell, sensing and compensating for varying signal strengths, and putting all of this into a design that can be cost-effectively manufactured is not easy. Philips does make it a lot simpler with a wide array of IC solutions that, despite their small size an d low power consumption, meet the stringent standards of the telecommunications industries in major markets around the world. SO and SSOP (Shrink Small Outline Package) surface mount packaging simplify manufacturing, too.
Let's take a closer look at the main RF IC product groups, again using the cellular phone as an example.
Figure 1
Philips has chosen to take a building block approach with most of its RF device families. The front-end family starts with the NE/SA600, a front-end device that includes a low noise amplifier and RF mixer. It can support input frequencies to 1 GHz. The SA 620 integrates the functionality of the SA600 with a Voltage Controlled Oscillator (VCO). Plus, it supports 3.0 volt operation and is packaged in a SSOP20. The system diagram (above) shows several alternate signal paths in the RF section. The path taken will depend upon the designer's choice of circuit implementation. Some designers will choose to use separate mixers and amplifiers and others will choose the fully integrated SA620 low noise amplifier/ mixer/VCO. Either way, the input signal is in the 900 MHz range, and the output after mixing will be a signal in the 90 MHz range for input to the IF section.
IF Systems The next building block is the IF system. When combined with the NE602A, it provides a high performance, low-power mixer/FM IF system. The NE614A is a relaxed spec version of the NE604A. For improved integration, Philips also offers the NE605. This 20-pin device is the functional equivalent of the NE602A/NE604A set and is available in the SSOP package. The NE615 is a relaxed spec version of the NE605.
Figure 2
This figure shows the family relationship between many of the Philips RF components. Note that the trend is to higher integration, smaller SSOP packages and lower voltage operation.
Why does Philips offer several versions of only slightly similar devices? It's because OEMs will design systems with different price considerations, space constraints, and performance tradeoffs. Components with similar functionality but different specific ations help them make the optimum selection.
The requirements at the IF section are to capture the high frequency signals and mix them with lower frequency signals to output a lower frequency, intermediate frequency (IF) signal that can be more easily processed. Naturally, the key parameters here wi ll be the ability to pick up very low input signals (sensitivity and lower their frequency without reducing accuracy. This specification, essentially, "how well does the device listen?") is noted as the Sensitivity Input Pin.
Another important feature is the ability to quickly detect whether a signal is present (Received Signal Strength Indicator or RSSI). This ability is very useful for the cellular phone developer who must be concerned with switching from one cellular base s tation to another. As the mobile user moves from cell to cell, the portable phone must detect when the signal is getting weaker and then quickly switch to the next cell in order to maintain seamless communications. A fast RSSI specification gives the desi gner more time to decode the incoming data and audio.
In the future, this specification will be even more important because of the switch from analog to digital transmission and the narrowing of channel bandwidths. Digital systems will pack more users into the same number of channels, making fast signal dete ct and processing a must. Further, the existing AMPS cellular protocol standard used in the U.S, calls for a channel bandwidth of 30 KHz. The proposed Narrow AMPS or NAMPS protocol uses a 10 KHz channel bandwidth with the phone starting up in AMPS mode an d then switching over to a NAMPS mode.
To support these needs, Philips offers the NE25/27 family of high performance, low-power mixer/FM IF system with high-speed RSSI. These devices are also packaged in the popular SSOP format.
Finally, there is an ever growing demand for low voltage devices. Today this means operating at as little as 2.7 volts (the voltage available from two NiCAD batteries at less than full charge). The SA606/616/ 607/617/608 family supports this need. With a little less sensitivity and an input frequency of 150 MHz versus the 500 MHz of the 605/625 families, these devices are designed specifically for the low operating power requirements of portable telecommunications systems.
Combining the performance of the NE625 with 3 volt operation is the SA626. This high performance/low voltage device is ideally suited for future portable digital systems.
Figure 3
Philips offers a large family of compandors. Of particular interest are the NE577 and NE578 low-power compandors. These draw less than 50 percent of the power of other compandors, 1.4 mA @3.6 volts. In addition, the NE578 provides a power down/mute functi on that allows the designer to mute the audio while powering down the device. In addition both devices include the ability to set unity gain (0 dB) levels. For other compandors, if the designer wanted a different 0 dB level, external op amps would be need ed. In addition, other external parts needed in previous family members are now incorporated into the device.
All of these improvements make the NE577/57X compandors useful for a wide range of applications, including cellular phones, cordless telephones, consumer audio products, wireless microphones, modems, electronic keyboards and hearing aids.
The two devices provide all of the functions required to process audio signals with the minimum of noise and without a lot of external devices. This figure shows the path taken by an audio signal through the two devices.
Figure 4
The filters screen out unwanted noise, the signal is compressed and expanded to maintain signal integrity and the pre-emphasis and de-emphasis function is used to overcome the noise present in all FM receivers, generated by the FM demodulator. This noise worsens at the upper voice band and without pruned de-emphasis, a person with a high-pitched voice will not be heard as well as a person with a low, deep voice.
Philips has upgraded the 5750/ 51 combination with the SA5752/53. Both offer similar features as the 5750/51 combination, except they operate with a 3 volt supply and are packaged in the SSOP package. In addition, the SA5753 is even more highly integrated , eliminating several external level adjustment components.
Philips frequency synthesizers include the bipolar UMA1014T and UMA1016xT single chip synthesizers and the CMOS UMA1005 synthesizer designed for very low power applications. The narrowband UMA1014 has a frequency range of 100 MHz to 1100 MHz with a channe l spacing of 5 to 100 kHz, and has a power down mode for extending battery life. The UMA1014 contains an on-chip prescaler, an RF programmable divider, a reference oscillator, and an I2C bus interface. It is targeted for analog communications such as AMPS cellular phones, amateur radios, and VHF and UHF landmobile communications equipment.
The wideband UMA1016 is similar to the UMA1014, but is targeted for digital communications applications. It has a frequency range of 500 to 1000 MHz and supports a channel spacing of 70 to 1000 kHz. It also cuts supply current in power down mode from 2.5 to 0.8 mA and features very fast (100µs) switching times.TheUMA1016 is well suited for 915 MHz industrial, scientific, and medical (ISM) band spread spectrum data links requiring low-cost, small size, low power and low external component counts.
Both the UMA1014T and UMA1016T are highly integrated and permit complete designs to be implemented using as few as three to five external passive components, a significant reduction compared to competing devices.
Figure 6
The CMOS UMA1005 is designed using Philips SACMOS process and supports a supply voltage range of 2.9 to 5.5 volts, and uses a very low current (5 mA max.). It is the first commercially available synthesizer to have fully integrated fractional-N operation and it includes a second, nonfractional-N synthesizer that is useful in many applications. The significance of fractional-N operation is that it allows designers to build circuits that switch much faster for a given cumulative noise level, are much quieter for a given switching speed, or are both faster and quieter.
Manufactured by using Philips proprietary QUBiC process, the NE630 draws only 170 µA of current when operating. It can be of use to develop low power cellular radios, digital transceivers, antenna switches and video switches. For example, in digital cordl ess telephones it acts as a duplexer, performing the rapid switching function between transmit and receive channels that allow the user to listen and talk at the same time.
The UAA20XOT is a high performance low power radio receiver circuit primarily intended for VHF and UHF (25 to 512 MHz) pager receivers for wide area digital paging systems that employ direct FM non-return-to-zero (NRZ) frequency shift keying (FSK). Its wi de frequency range and high sensitivity makes the UAA20XOT attractive to pager designers.
The PCF5001T paging decoder with EEPROM storage is a very low power Decoder and Pager Controller specially designed for radio pagers. It interfaces directly to the UAA20XOT digital paging receiver.
Both the UAA2080T and PCFSOOIT are fully compatible with the CCIR Radiopaging Code Number 1 standard, also known as the POCSAG code. This standard supports a 512 bps data rate and a 1200 bps data rate. In addition, the PCF5001T offers extended features an d an on-chip 114-bit EEPROM capable of holding up to four user addresses, two frame numbers and the programmed decoder configuration.
DECT - Digital European Cordless Telephone
DTMF- Dual Tone MultiFrequency generator. Used in telephony to provide dial tones.
IF - Intermediate Frequency. Radio receivers convert the incoming high-speed radio frequency signal to this lower intermediate frequency before extracting the audio information and performing other signal conditioning.
GSM - Global System for Mobile Communications, the European digital cellular radio system protocol.
TACS - Total Access Communication System, a cellular radio protocol used in the United Kingdom.
NADC - North American Digital Cellular
NAMPS - Narrow Advanced Mobil Phone Service, proposed narrow band cellular protocol.
PHP - Personal Handy Phone - This is the emerging Japanese cordless telephone standard.
TDMA - Time Division, Multiple Access