Nokia 6225 meets analog RF design challenges

Click to Enlarge Image
Figure 1: The Nokia 6225 CDMA2000 handset

Because of the “quiet nature” of CDMA’s correlated noise spread-spectrum signaling and the lower gain of the 6225’s internal planar antenna design, job one is to crank up the incoming signal level. The TriQuint ACO520 RF front end combines a mixer and low noise amplifier (LNA) to boost received signals and mix the amplified RF with precise frequencies from an oscillator paired to a National Semiconductor LMX2310 frequency synthesizer. The downconverted mixer output, now an intermediate frequency (IF) signal, passes to an STMicroelectronics receive chip (TZA5B) for second-stage downconversion and handoff to the baseband processor. The two-stage downconversion reflects a need for greater care with CDMA signal demodulation, and shortcomings of a simpler one-step downconversion seem to have dictated the more complex receiver design.

For transmission, a second STMicroelectronics component provides for upconversion of baseband signals with the oscillator, but this time to reverse the process and modulate outgoing signals onto RF. Prior to transmission through one of two RF power amplifiers from RF Micro Devices (one for each transmit band), devices from the same company control gain to set appropriate—but not excessive—transmitted power. Note that the transmit stage does not appear to use a two-stage heterodyne radio architecture, suggesting the higher signal levels involved in the transmit path provide relief from radio design issues.

GPS capability is provided by Texas Instruments, implementing the receiver (TRF5101) in a single chip integrating LNA, downconverters, mixing oscillator, and filtering. While the building blocks for GPS are thus similar to the receive path for the CDMA radio, higher integration stems from the less stringent analog radio requirements for GPS.

Callouts in Figure 2 and the previous article point to substantial analog content for power audio, and ESD protection, but analysis of the CDMA RF paths reveals some of the more intriguing engineering challenges.

Click to Enlarge Image
Figure 1: Details of the main board; Side 1 (upper image) and Side 2 (lower image)

About the Author
David Carey is President of Portelligent. The Austin, Texas company produces teardown reports and related industry research on Wireless, Mobile, and Personal Electronics www.teardown.com