BPSK调制器基带滤波器仿真与实践：空间通信的关键技术

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本文档深入探讨了基带脉冲成型滤波器在BPSK调制器中的仿真与实现，针对二相移键(BPSK)调制的特殊需求，该技术旨在降低信号的副瓣级和频谱尖峰问题。在日益拥挤的频率资源背景下，空间通信领域中，这种技术显得尤为重要，尤其是在未来的地球静止轨道卫星（GEO satellite）应用中。作者D.Venkata Ramana、Surendra Paland A.P. Shiva Prasad阐述了一种方法，通过在信号处理的不同阶段实施滤波器来优化频谱利用率。他们关注的三种关键位置包括： 1. 功率放大器后滤波器：放置在功率放大器之后的滤波器可以有效地消除超出其通带范围的所有无用发射，理论上实现了理想的谱效率。这种方式对于频谱管理者来说极具吸引力，因为它能够减少对无线电磁环境的干扰。 2. 中频(IF)滤波器：在信号从射频转换到中频之前或之后进行滤波，有助于保护中频信号的纯净度，减少传输过程中的噪声和失真。 3. 基带滤波器：在信号转换成数字形式之前，基带滤波器对于BPSK信号的形状和质量控制至关重要，它可以改善信号的码间干扰抑制性能，从而提高接收端的信号解码准确率。仿真部分，文章详细介绍了如何通过模拟工具，如Matlab或SPICE等，设计和分析不同滤波器参数对BPSK调制效果的影响。实际实现部分则着重于滤波器的硬件选择、设计步骤以及可能遇到的挑战，如线性度、带宽效率和温度稳定性等问题。这篇文章提供了一个实用且理论相结合的方法，帮助工程师们在BPSK调制系统中优化基带脉冲成型滤波器的设计，以应对日益增长的通信需求和频谱紧张的问题。通过有效的滤波器设计，系统不仅能够提升通信质量，还能满足严格的电磁兼容性标准，对于现代无线通信技术的发展具有重要的参考价值。

42 www.rfdesign.com April 2004

Air & Space Electronics

Simulation and realization of baseband pulse

shaping filter for BPSK modulator

This pulse shaping technique reduces side lobe levels of bi-phase shift

keying (BPSK) modulation and spectral spike elimination. Useful for space

communications, the technique can be implemented in a practical way.

This technique will be used in GEO satellites in the near future.

By D.Venkata Ramana, Surendra Pal and A.P.Shiva Prasad

ith the continuing growth of

Wcommunications and the in-

creasing number of users, frequency

bands are becoming more and more

congested. To cope with this fre-

quency congestion, many authors have

studied methods to increase bandwidth

use [1,2].

Significant RF spectrum limiting

can be obtained in three ways. Here,

the location of filter plays a key role.

The various locations include:

● filter after power amplification.

● filter at intermediate frequency

(IF).

● filter at baseband.

Post power amplifier (PA) filtering

is attractive to spectrum managers because

all unwanted emissions, which are outside

the filter’s passband, are eliminated. Theo-

retically this filter location provides maxi-

mum control over emissions.

However, it would be difficult for post PA

filtering to improve RF spectrum use in most

space missions. Filters can be either stripline

or waveguide bandpass filters, which are gen-

erally used in microwave applications. For

reasonable insertion losses, such filters

are constrained to bandwidths ranging from

1.5% - 2% of the transmitted frequency. It

should be noted that the filters that

have somewhat lower insertion loss

tend to be large and heavy.

Filtering at IF is attrac-

tive because the filter

operates at low power lev-

els and does not reduce

transmitted RF power. It

is also small and light-

weight and does not in-

troduce the spectral spikes

inherent in baseband fil-

tering. For effective spec-

trum management, the IF

filter’s bandwidth needs

to be adjusted to each

mission’s maximum telem-

etry data rate.

Baseband filtering is advantageous

because the filters operate at low

power levels, are lightweight, do not

reduce transmitted RF power and are

small and simple (lowpass rather than

a bandpass). Baseband filtering of

phase-modulated signals suffers from

the disadvantage of introducing spikes

into the RF spectrum [2]. Despite this

limitation, baseband filtering is the only

practical method to limit the transmit-

ted RF spectrum for the purpose of

improving bandwidth efficiency.

Pulse shaping

In general, the MPSK (M’ary phase

shift keying) spectrum consists of a

main lobe representing the middle of

the spectrum and various side lobes located

on either side of the main lobe. Shaping the

spectrum should satisfy two criteria: The main

lobe should be as narrow as possible, and the

maximum side lobe level should be as small

as possible relative to the main lobe [3].

In recent years, studies have shown that

PSK modulation is particularly suited

to digital satellite communications. The power

spectra of a PSK signal has a

char-

acteristic that may interfere with adjacent

Figure 1. Various filter location configurations for BPSK

spectrum .

Figure 3. Unfiltered BPSK spectrum.

For effective spectrum management, the

IF filter’s bandwidth needs to be adjusted to

each mission’s maximum telemetry data rate.

Figure 2. Simulated PSD of BPSK spectrum using new

pulse shaping filter.

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