PCS1900手机基带与射频设计详解

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"手机基带和射频设计是移动通信领域中的关键技术，涉及到手机通信系统的信号处理和无线传输。本文详细介绍了在PCS1900频段（即1900MHz）的GSM（全球系统移动通信）手机中，射频到基带部分的设计。" 手机基带和射频设计是移动通信设备的核心组成部分，它们共同确保了手机能够正确接收和发送无线信号。基带负责处理数字信号，包括编码、解码、调制和解调，而射频部分则涉及信号的无线发射和接收，包括频率转换和放大。在本文中，作者Liam Devlin, Steve Fitz和Geoff Smithson来自Plextek Ltd，他们提供了PCS1900手持设备的RF到基带设计的详细内容。设计涵盖了详细的模块图、电路描述以及实际测量性能。手机的射频部分被设计成与基带评估板（EVB）协同工作，以实现完整的手机功能。基带评估板是测试和验证基带处理性能的重要工具。论文中还展示了手机在与基带配合时的整体性能，这包括了信号处理、链路预算、误码率等关键指标。此外，为了便于批量生产，研究团队还开发了一种案例工作设计，它包含了一个集成的贴片天线，并允许在Z轴方向进行组装，这大大简化了制造流程，提高了效率。第一代模拟蜂窝电话系统在20世纪80年代由欧洲多个国家独立开发，但随后意识到统一的标准对于日益融合的欧洲市场至关重要。GSM作为第一个广泛采用的数字标准，解决了兼容性问题，并在全球范围内推广。随着技术的发展，基带和射频设计的重要性日益凸显，特别是在满足更高数据速率、更低功耗和更小体积的需求方面。 GSM系统在PCS1900频段的实现，使得手机可以在这一频段进行通信，扩展了网络覆盖范围，提高了服务质量。射频到基带设计的优化不仅影响到通信质量和稳定性，还直接影响到手机的电池寿命和整体用户体验。这篇论文为手机基带和射频设计提供了一个实践案例，对基带工程师来说是一个宝贵的参考资料，有助于他们在设计和优化移动通信设备时更好地理解和应用相关技术。

The receiver uses three stages of

downconversion to provide analogue baseband

quadrature outputs. Analogue to Digital

Converters (ADCs) convert the baseband

outputs to digital and the baseband processor

carries out timing synchronisation,

equalisation and error correction. An

alternative topology to baseband sampling is

to sample an IF signal directly. In this case the

final mix to baseband is performed digitally.

This is often achieved by “sub-sampling”,

which allows the sampling rate to be below the

IF frequency [5]. One disadvantage with this

technique is that the ADC must work at a

higher frequency making it more difficult to

achieve the required performance.

The main disadvantage with a baseband

sampling approach, as used in this design, is

DC offset signals caused by leakage of the LO

signal used for the mix to baseband. Any LO

signal leaking into the IF circuits is mixed

down to zero frequency giving a DC offset. A

common way of significantly reducing this

problem is to generate the IF LO signal at a

multiple of the required frequency and to

divide it down on-chip. A divide by 2

approach can also be used as a means of

generating accurate quadrature LO signals.

The signal drives two dividers with the flip-

flops in the dividers being triggered on either

the negative or positive edges, to give output

signals at half the input frequency with 90° of

phase difference.

A treble conversion superhet design is a

popular receive architecture for GSM

handsets. It allows a first IF at a high enough

frequency (in this case 225MHz) to allow good

filtering of the image signal. A second

conversion to a lower IF allows the AGC to be

realised at a lower frequency, with a final

conversion to baseband or to a third IF in the

case of IF sampling designs. Double

conversion architectures are also possible but

the AGC must then be realised at a higher

frequency. Another alternative, which is

possible, is to use a direct conversion to

baseband architecture [8]. This is attractive

because it reduces the number of RF

components used and removes the need for

image filtering. However, the practical

difficulties of implementing a direct

conversion GSM receiver should not be

underestimated [5].

In transmit mode the inputs to the transceiver

IC are differential I and Q baseband signals.

These are used to modulate a relatively low

frequency IF signal, which is then used to

modulate a VCO operating at the transmit

frequency in a translational loop architecture.

The translational loop works by mixing the

transmit VCO signal with the channel

oscillator to generate an IF signal. A phase

comparator is used to compare this IF to the

modulated IF with the difference being passed

back to the transmit VCO, via a loop filter, as

an error signal. This closes a phase locked

loop around the transmit VCO which locks it

to the modulated IF signal. Translational loop

architectures are only possible with constant

envelope modulation schemes such as

Gaussian Minimum Shift Keying (GMSK).

One disadvantage with a translational loop

architecture is that an extra VCO is required

as compared to a direct modulation or

superhet. architecture. However, it has become

increasingly popular for the following reasons:

• The input to the PA is not the direct output

of a mixer with all of it’s associated

spurious mixing products. This eases the

filtering requirements of the transmitter.

• The oscillator driving the PA can have a

relatively high output level. Less PA gain

is therefore required, which reduces the

thermal noise generated at the output of

the PA, making it easier to pass the receive

band noise emissions requirement. This

means a switch can be used at the antenna

port instead of a diplexer filter, which is

more expensive and has higher loss.

• Susceptibility to pulling of the transmit

VCO is reduced because the wide loop

bandwidth makes it very agile and able to

track and correct any sudden pulling

effects

Synthesiser Design and

Measured Performance

Three synthesiser designs are used in the

handset: a channel VCO that must select

frequencies from 1705.2MHz to 1774.8MHz

in 200kHz steps. An IF VCO which is set to

540MHz and the translational loop oscillator

which must tune over the transmit band from

1850.2MHz to 1909.8MHz [9]. The

synthesiser for the translational loop is on the

transceiver IC, the other two oscillators make

use a single dual synthesiser IC. VCO

modules were used for the channel and

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