原创 调频调相区别、预加重去加重限幅器泼溅滤波器

调频调相区别、预加重去加重限幅器泼溅滤波器The issue of pre-emphasis, de-emphasis, clipping and repeater audio quality翻译<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

要点：

带预加重的直接FM发射器的输出和不带预加重电路的PM发射器的输出是没有分别的。

泼溅滤波器只是一个低通滤波器，它设计用来去除削波过程中产生的谐波。

对英文的翻译：

原文网址：http://www.repeater-builder.com/tech-info/fmtheorydiscussion.html

The issue of pre-emphasis, de-emphasis, clipping and repeater audio quality.

By Paul Sexauer K3VIX

Preface and Introduction: The issue of pre-emphasis, de-emphasis, clipping and repeater audio quality have been debated in the ham arena since two meter FM became popular. The following are my thoughts based on 26 years in engineering with Motorola Inc.

I still think that the Micor is probably the best repeater for ham use since you can do just about anything with it using wire cutters and diodes (no software needed!).    I did work at Motorola for 26 years before taking early retirement in 1998.     The first half of my career there was primarily in RF design and the last half was involved primarily in systems engineering.     I'm presently the RF Systems Engineering manager at TX RX Systems out here in New York (near Buffalo).     It was kind of nice to escape the hustle and bustle of the Chicago area plus now I get to work on the "other" end of the business.

I did maintain a Micor ham repeater for about 10 years out in Aurora, IL and it had a reputation for excellent audio.    I ran that one "by the book" as far as audio adjustments and that is described later on in this article along with background info on FM, PM, pre-emphasis, de-emphasis and other fun stuff...

The goods:  The first question which needs to be addressed is why pre-emphasis and de-emphasis is used.    The answer can be found when the properties of FM demodulators are analyzed.    If the output of an FM demodulator (discriminator, quadrature detector or whatever) is monitored on an audio spectrum analyzer with no RF carrier input,  it will be noted that the response displayed is one which rises at a 6 dB per octave slope.     {The mathematics for the FM demodulation process predict this effect}    In other words, with a flat RF noise spectrum entering the demodulator,  the output will exhibit a noise characteristic which rises with frequency.   If an FM signal were applied to the input of the demodulator and the modulating frequency were swept from low to high frequencies,  maintaining constant deviation,  it would be noted that the signal to noise ratio at the output of the demodulator would degrade as the modulating frequency increased. 为什么要使用预加重、去加重？分析FM解调的特性就知道答案了。用音频频谱分析仪监测FM解调输出，并且没有RF载波输入时，会发现显示的响应是6dB/OCT上升。 {FM解调过程的数学分析也预见了这个现象}  就是说，当一个平的RF噪音频谱进入解调器后，输出端将表现出随频率而上升的噪音特征。  如果FM信号加到解调器，并且调制频率从低频扫到高频， 保持相同的频偏， 可发现解调器的输出端的信噪比将随着调制频率的增大而下降。

To compensate for this, a de-emphasis circuit is used.  In it's simplest form, this would consist of an R-C network which would roll off at a rate of 6 dB per octave,  canceling out the rising noise characteristic of the demodulator.   This eliminates the problem of S/N ratio degrading as the modulating frequency rises, but now results in a rolling off of the audio response if a "flat" FM signal is received.   To have a net "transparency" in audio response, it becomes necessary to pre-emphasize the transmit audio at a corresponding 6 dB per octave rate.    From this, you can see that de-emphasis came first and created the requirement for pre-emphasis at the transmitting end. 为补偿这个，使用了去加重电路。 最简单的形式是使用RC网络，产生6dB/OCT的下降， 这样来去除解调器的噪音上升特性。 这消除了随解制频率增大而下降的S/N比，但是造成音频频率响应的下降，当收到一个“平”的FM信号时。  为得到一个净“透明”的音频频率响应， 需要对传送的音频进行预加重，速率是6dB/OCT。  可见，先有了去加重，后来出现了预加重的需要。

Going to the transmitting side, we have two seemingly different modulation schemes available: FM and PM.      发射端有FM调频和PM调相二种方式。

The major difference between these two schemes is that a phase modulator has a 6 dB per octave rising audio response (i.e. pre-emphasis is inherent in PM systems with no added circuitry).    调频和调相的主要区别是：相位调制有一个6dB/OCT的上升音频响应（也就是，预加重是调相的内在特性，不需要加另外的电路）

In a phase modulator, the total deviation is a function of both the modulating signal amplitude as well as frequency.    相位调制中，总频偏是调制信号幅度和调制信号频率的函数。

In a direct FM modulator (i.e. in one where the modulating signal is applied across a varactor to vary the oscillator frequency) , the deviation produced is a function of the modulating signal amplitude only.     To achieve pre-emphasis, a series R-C network needs to be inserted into the audio path. 在直接FM调制器中（也就是，调制信号加到变容二极管上来改变振荡频率）， 产生的频偏只是调制信号幅度的函数。

A sidelight here is that if you look at the mathematical representations of a PM signal versus an FM signal,  the difference is that the modulation component in a PM signal is the mathematical derivative of the modulating signal in the FM signal.     If you add the series R-C circuit ahead of the FM modulator, the signal at the output of the R-C network is the mathematical derivative of the applied signal.    In other words, by adding the series R-C (pre-emphasis) circuit, the output of the FM modulator is now identical to the output of the phase modulator.     The output of a direct FM transmitter with pre-emphasis can not be distinguished from the output of a PM transmitter with no added pre-emphasis circuit.    The pre-emphasis circuit effectively "makes" an FM transmitter into a PM transmitter.  如果你看调相PM信号和调频FM信号的数字表达式的对比，它们的不同点是调相PM信号调制向量是调频FM信号调制向量的数学派生。如果你在FM调制器之前加RC电路，则RC网络的输出信号是输入信号的数学派生。  换种说法，通过增加RC（预加重）电路，FM调制器的输出就和PM调制器的输出完全一样了。 带预加重的直接FM发射器的输出和不带预加重电路的PM发射器的输出是没有分别的。 预加重电话有效地使一个FM调频发射器“变成了”一个PM调相发射器。

In the early days of FM, there were no varactor diodes so it was difficult, at best, to produce a direct FM modulator.     PM modulation was, however, easy to achieve which is the reason that it was the "standard". 在早期进行调频FM时，还没有变容二极管，那时很难生产出一个直接FM调制器。 但是调相PM调制器容易生产，由于这个原因它成了“标准”。

The rising audio response of the phase modulator did cause some problems for manufacturer's.    Since the audio response fell off at the low end,  phase modulators had a hard time when it came to modulating them with PL tones. Direct FM modulators, being flat, had no such problem.   With the advent of data communications, direct FM was the only way to modulate a carrier with baseband data.   调相PM调制器的上升的音频响应确实给生产厂商带来了一些问题。因为在低端音频响应下降，当调制PL同步音调时，调相PM调制器就困难了。直接FM调制器的频率响应是平的，就没有这个问题。  当数据通讯的时代到来时，直接FM调频是调制一个带基带数据的载波的唯一方法。

Now, since the FCC (and the similar authorities in other countries) mandate that FM transmitters must not exceed their assigned bandwidth allocations, some means of limiting the deviation was needed.    Along came the clipper circuit. The purpose here was to ensure that the modulating signal amplitude never exceeded a certain specific value (that which produces rated deviation).    Of course, clipping an audio signal produces major distortion in the signal and this must be minimized.    Therefore a "splatter filter" is inserted after the clipper. This splatter filter is simply a low pass filter and is designed to roll off the harmonics created during the clipping process.     Note that the splatter filter does not "undo" the effect of the pre-emphasis circuit.    The splatter filter does not "kick in" until the upper end of the audio range. 现在，因为FCC规定FM发射器必须不能超过他们划出的带宽分配范围，这样就需要一些手段来限制频偏。   削波限幅电路就产生了。 目的是保证调制信号的幅度永远不超过一定的指定数值。 当然，对音频信号削波将产生主要的失真，这个要最小化。  因此在削波后插入一个泼溅滤波器。泼溅滤波器只是一个低通滤波器，它设计用来去除削波过程中产生的谐波。 注意，泼溅滤波器并不会“撤消”预加重电路的效果。直到音频达到频率范围的高端，泼溅滤波器才会“介入”。

At this point, let's put some numbers on the pre-emphasis circuit along with the splatter filter.      For normal voice communications, the FCC specifies (in commercial radios) that the pre-emphasis be a 6 dB per octave rising response beginning at 300 Hz.     In actuality, most commercial transmitters will begin the rising response between 200 and 300 Hz.    The FCC further specifies that the pre-emphasis be within +1 and –2 dB of the 6 dB per octave response.      This effectively defines a "box" which the TX audio must fit in. Above about 3000 Hz (I don't have the spec in front of me) , the FCC requires the audio response roll off at a minimum of 12 dB per octave.     This is the response of the splatter filter.    Generally, the splatter filter begins to have an effect at around 2500 Hz.     At that point, the rising audio response will begin to flatten out and then begin the 12 dB per octave roll-off at around 3000 Hz. 让我们用数字说明预加重电路和泼溅滤波器。  对于普通的语言通讯，FCC规定（在商业无线电中）预加重是从300HZ开始6dB/OCT上升。  实际上，多数商业发射器在200－300HZ之间上升。  FCC又规定预加重要在6dB/OCT 的+1DB和-2DB之间。    在大约高于3KHZ（大概是这个数），FCC规定音频响应以最少12dB/OCT下降，这个就是泼溅滤波器的频率响应。一般地，泼溅滤波器大约在2500HZ左右发挥作用。  在这个点上，上升的音频响应将开始变平，然后开始在3KHZ左右以12dB/OCT下降。

{wxleasyland我附注：

泼溅滤波器只是一个低通，-12dB/OCT的低通，用来去除谐波。

引用别的文章《对讲机电路的理解》：限幅器会将超高的信号波形拦腰截断，这样变成方波，带出了个BUG，输出信号的高次谐波变得严重了。为了补这个窟窿，人们又发明了泼溅滤波器。图中，滤波器左边阴影画的6dB/OCT是预加重的曲线。右边阴影是泼溅滤波器的-12dB/OCT，FCC要求泼溅滤波器在3KHz开始至少要能提供-12dB/十倍频的衰减。事实上，由预加重抬起来的曲线在2.5KHz时就被展平了，得到的频率响应曲线大致就图中这个样子：

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The way the transmitter audio response is measured is to apply a fixed modulating signal level which produces 60% of rated system deviation (3 kHz in our case) at 1000 Hz.     Holding this level constant, the frequency is then swept from 300 Hz to about 10 kHz and the output of a calibrated receiver discriminator is plotted.    The pre-emphasis and splatter filter responses are then clearly visible.  要测量发射音频的频率响应，方法是加上一个固定调制信号电平，这个电平在1000HZ下产生系统设计频偏的60%（这里是3KHZ）。  固定这个电平，频率再从300HZ扫到10KHZ， 校准后的接收机鉴频的输出就会画出图来。  预加重和泼溅滤波器的响应就可以清楚看出来。

On a side note, the received audio quality is largely controlled by how "hard" the clipping in the transmitter is.      If the clipping is "soft"  i.e., it only occasionally goes into significant clipping, the audio will sound very clean. If the clipping is "hard", the intelligibility suffers greatly.   Some manufacturers allow for this by providing a deviation set pot (IDC in Motorola) which is set to the rated deviation with the modulating signal in full clip, and then also provide a mic level or line level control to adjust how hard the modulating signal hits the clipper.     Proper setting of this control is imperative if good audio is to be maintained.   We will see the importance of this in setting repeater levels shortly.

In the case of most amateur transceivers I've seen, the clipping circuits are quite poor which means that deviation may not be limited as much as might be desired, but it does result in very clean sounding audio.   The ham market is not subject to the same technical requirements as the commercial folks.   Clean audio is, however, achievable with good commercial clipping circuits.

Now we get to the question of how to obtain good quality repeater audio. To do that really requires that we know what produces "bad’ audio.     As was discussed above, in a mobile or base it is important that we control how hard we go into clip on audio peaks.   Harder clipping yields degraded intelligibility. With a repeater, the problem is compounded.    This is because we have a clipper in the mobile and another in the repeater.   It all comes down to properly setting repeat audio levels.    I'm going to stick with the Micor here since I know that one really well.

In the Micor, the first step in setting up the audio path is to set the IDC for 5 kHz system deviation.   This is done by injecting a 1 volt rms.1000 Hz tone into the Exciter audio input.    This runs the audio into full clipping.   The IDC pot is then adjusted for 5 kHz (this should be 5 kHz including any PL which may be generated).   The alignment procedure then asks that you reduce the audio level from the generator until the deviation drops to 3 kHz.    Write down the audio level into the exciter at this point.     Don't rely on the factory value since that was probably done a long time ago.     This is referred to as the modulation sensitivity "mod sense" level and is written on the exciter at the factory. At 3 kHz deviation, you are out of clip.   At this point, the exciter audio adjustments are done and should not be changed.

The next step is to adjust the Repeat Level (this applies whether you are using a standard Micor or have an external controller supplying audio to the exciter).     Here, you inject a signal into the repeater receiver with 1000 Hz modulation and 5 kHz deviation.      You measure the voltage at the exciter audio input point and adjust the Repeat Level pot for a reading equal to twice the mod sense value.    Since the mod sense level is the level to produce 3 kHz deviation, setting the input to twice that you should get 6 kHz deviation out with 5 Khz deviation input.       Now, the clipper will not allow you to exceed 5 kHz deviation, so this adjustment puts you into a "soft" clip situation. It actually provides a bit of an audio boost for signals which are coming in at less than 5 kHz deviation which is probably the norm.   Adjusting repeat audio in this way will produce "near simplex" audio quality.    I maintained a repeater out in Aurora, IL for about 10 years and used this procedure exclusively. We had a reputation for one of the best sounding repeater audio systems in the area.      I am not a believer in pulling receiver audio off the discriminator and directly into a "flat" transmitter.     That originated in the ham circles and I don't believe that it will produce audio any better than "doing it right the first time"!    Let the pre-emphasis and de-emphasis circuits do their job (it really results in a fairly flat response) and pay attention to the clipping levels.    You'll have great audio and you will keep your deviation within spec, which is mandatory in today's FM bands.    Actually, if we could get everyone to adjust their repeater deviation to 4 kHz, a lot of adjacent channel problems would be reduced to acceptable levels.  Just a thought..

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