转换莫斯科矿友的FM调频检波电路

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这遍文章 是莫斯科矿友 Polyakov 的 原创 FM调频检波电路 , 他解析为何用调幅方式(图1) 去为FM调頻讯号檢波时常有出失真及位置性干扰的问题，因為有无数不同的FM电波経不同的直射或反射路径到達矿机，这些不同路径及相位的FM电波被调幅检波成不同的音频讯号，互相干扰，影响调幅检波的凊盺度，天线及耳机线轾微移动到不同位置时响度及清盺度也受到影响。图2这个电路利先將FM相移再和原电波重叠再利用三极管 作调频检波.  图3更进一步利用 调幅方式 检波时 所得的电流 对调频检波三极管加上偏压 加强输出, 有可能推动啦叭, 值得大家一起硏究.
原文: http://zpostbox.ru/fm_crystal_radios.html.

转换 成中文 如下:

FM矿石收音机

Radio，2002，7

“矿石收音机”的概念与巨大的天线和长和中频带上的无线电广播密切相关。在本文中，作者描述了设计用于监听FM电台的VHF接收机的实验测试检测器电路。

意外发现接收VHF FM检波器的可能性。有一天，我走在莫斯科，诺维高里的Terletskiy公园，我决定听广播 - 我有一个简单的矿石收音机，没有调谐部分（这个电路描述在“收音机”，2001年1号图3 ）。接收机具有长度为约1.4m的拉杆天线。想知道是否可以用这个短天线接收无线电广播？有可能听到，但是微弱地同时操作两个电台。但是令我惊讶的是，接收的音量每隔5 ... 7 m就会周期性地上升和下降，并且每个广播电台的音量都不同！

已知在波长为几百米的LW和MW波段中，这是不可能的。我不得不停止在接收点与最大音量的一个电台，并仔细听着。原来，这是“Radio Nostalgie”，100.5 MHz，从附近城市Balashikha广播。天线之间没有视线。如何使用AM检波器接收FM传输？进一步的计算和实验表明，它是完全可能的，不依赖于接收器。

简单的便携式FM矿石收音机以与电场的指示器完全相同的方式制造，但是代替测量装置，需要连接高阻抗耳机。增加检测器电路和谐振回路之间的耦合的调节以调节接收信号的最大量和质量是有意义的。
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最简单的矿石收音机
适合这些要求的接收机电路图如图1所示。该电路非常接近上述接收机的电路。只有VHF谐振回路已添加到电路中。
图1.

VD1，VD2 - GD507A - 一个旧的USSR锗高频二极管，电容为0.8 pF（反向电压为5V），反向电阻的恢复时间不超过0.1 uS（在直接脉冲= 10 mA，U反脉冲= 20V，Icutoff = 1mA）
该装置包括直接连接到谐振回路L1C1的伸缩式天线WA1。天线也是谐振槽的一个元件，因此为了获得信号的最大功率，必须调整天线的长度和振荡电路的频率。在某些情况下，特别是当天线的长度是波长的大约1/4时，将天线连接到调谐线圈L1的抽头是有用的（通过找到线圈的最大体积信号）。
与检测器的耦合可以通过微调器C2进行调节。实际上，检测器由两个高频锗二极管VD1和VD2构成。该电路与倍压整流器电路完全相同，但是如果只有微调电容器C2的值为高，但是然后谐振电路L1C1的负载将过大，则检测到的电压将加倍，并且其品质因数Q将为低。结果，电路箱L1C1中的信号电压将较低，并且音频音量也将较低。
在我们的情况下，耦合电容器C2的电容足够小，并且不发生电压倍增。为了使检测器电路与谐振电路最佳匹配，耦合电容器的阻抗必须等于检测器的输入电阻与谐振电路L1C1的谐振电阻之间的几何平均值。在这种情况下，检测器获得高频信号的最大功率，这对应于最大音频音量。
电容器C3在检测器的输出处分流较高频率。检测器的负载是直流电阻不小于4K欧姆的耳机。整个单元组装在一个小金属或塑料外壳中。长度不小于1m的伸缩天线安装在壳体的上部，并且用于电话的连接器或插孔附接在壳体的底部。请注意，电话线是偶极天线（对重）的后半部分。
线圈L1是无框的，它包含5匝直径为0.6 ... 1mm的漆包铜线，缠绕在直径为7 ... 8mm的心轴上。您可以通过拉伸或压缩线圈L1的匝数来调整必要的电感。更好地使用具有空气电介质的可变电容器C1，例如，具有两个或三个可移动和一个或两个固定板的1KPVM型。其最大容量小，可在7 ... 15 pF的范围内。如果可变电容器具有更多的板（电容较高），则建议去除任何板，或者将可变电容器与恒定电容器或微调电容器串联，这将降低最大容量。
电容器C2是陶瓷微调电容器，例如容量为2 ... 7pF的KPK或KPK-M。也可以使用其他微调电容器。微调电容器C2可以用类似于C1的可变电容器代替，并且可以用于“即时”调整耦合以优化无线电接收能力。
二极管VD1和VD2可以是GD507B，D18，D20（它是旧的USSR锗高频二极管，这种二极管可以用现代的肖特基二极管代替）。分流电容器C3是陶瓷的，其容量不是关键的，并且可以具有在从100到4700pF的范围内的值。
接收器的调整很简单。通过旋转可变电容器C1上的旋钮调整收音机，调整电容器C2以获得最大音频音量。谐振槽L1C1的调谐将改变，所以所有操作必须重复几次，同时找到无线电接收的最佳位置。它不一定是电场具有最大强度的相同地方。这应该更详细地讨论，并解释为什么这个接收机可以接收FM信号。
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干扰和FM转换为AM


如果我们的接收机（图1）的振荡电路L1C1将被设置为使得FM信号的载波频率落在谐振曲线的斜率上，则FM可以转换为AM。让我们找到振荡电路的Q值。假设谐振电路L1C1的带宽等于频率偏差的两倍，对于上部和下部VHF频带，我们得到Q = F0 /Δ2f= 700。

矿石无线电中的振荡电路的实际Q可能小于700，因为它自己的Q（约150 ... 200）的低Q因子，并且因为谐振回路被天线和输入阻抗。然而，FM变成AM的弱变换是可能的，因此，如果其振荡回路在频率上向上或向下失谐一点，接收机将几乎不工作。

然而，有更有力的因素有助于将FM转换为AM，这是一种干扰。当接收机在无线电台的视线中时，很少会发生，在大多数情况下，视线被建筑物，丘陵，树木和其他反射物体遮蔽。这些物体散射的几个无线电波束到达接收机的天线。即使在天线的视线有一些反射信号（当然，直接信号也来了）。总信号取决于求和分量的幅度和相位。

如果它们的相位相同，即它们的路的差是波长的整数的倍数，则两个信号被求和，并且如果它们的路的差是相同的数，则两个信号在它们相反相位时被减去的波长加上半波长。但波长，以及频率在FM变化！光束的差异及其相对相移将变化。如果路的差大，则即使频率的小变化也导致相位的显着偏移。基本几何计算导致以下关系：Δf/ f0 =λ/4ΔC，或ΔC= f0 /λ/4Δf，其中ΔC-相位移±π/ 2所需的方向的差AM信号的全部和，Δf-频率偏差。完全AM是振幅信号从两个信号的振幅之和到它们的差的总变化。如果我们考虑频率乘以波长f0λ等于光速c：ΔC= c /4Δf，可以进一步简化该公式。

现在很容易计算出获得双光束FM信号的完全AM，光束方式之间的足够的差异大约是一公里。如果路的差较小，则AM的深度按比例减小。嗯，但如果方法的区别更多？然后，在调制音频信号的一个周期期间，干扰信号的总幅度将通过高频和低频通过几次，并且当将FM转换为AM时失真将非常强，直到接收到声音时完全不清楚FM通过AM检波器。

对FM广播接收的干扰是极其有害的现象。它不仅产生信号的伴随寄生AM，如上所述，但是它产生寄生相位调制，即使我们得到良好的FM接收器也会导致失真。这就是为什么将天线放在正确的位置，在那里只有一个信号占主导地位是如此重要。使用定向天线总是更好，因为它增加了直接信号的幅度，并减少了来自其他方向的反射。

只有在这种情况下，使用非常简单的检测器无线电接收机，干扰才起到有用的作用，并且允许我们收听无线电广播，但是无线电广播可以被弱听到或具有显着的失真，并且无线电广播不能在任何地方被听到，但只在某些地方。这解释了在Terletskiy公园的无线电广播的音量的周期性变化。
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矿石检波器带有频率检波器的无线电接收器
改进接收的根本方式是使用频率检测器而不是振幅检测器。在图2中示出了基于单个高频锗晶体管VT1的具有简单频率检​​测器的便携式检测器无线电接收器的电路。使用锗晶体管是因为它的结工作在约0.15伏的低电压，这允许检测非常弱的信号。硅转换的结在大约0.5V的电压下工作，并且具有硅晶体管的接收器的灵敏度低得多。
图2.

VT1-GT313A - 具有hfe = 10 ... 230（在DC：Uke = 3V，Ie = 15mA），hfe = 3 ... 10（在f = 100mHz处）的旧的USSR锗高频晶体管， Ukb = 5V，Ie = 5mA）
如在先前的设计中，天线连接到输入槽电路L1C1，可变电容器C1用于调谐功能。来自输入振荡电路的信号到达晶体管VT1的基极。另一个振荡电路L2C2与输入振荡电路L1C1电感耦合。储能电路L2C2与可变电容器C2谐振。由于这两个槽之间的电感耦合，谐振槽L2C2中的振荡相对于跨输入tnak电路L1C1的信号相移90°。从线圈L2的抽头，信号到达晶体管VT1的发射极。旁路电容器C3和高阻抗耳机BF1连接到晶体管VT1的集电极。
晶体管在其基极和发射极具有信号的正半波时开始导通，并且发射极上的瞬时电压大于其基极电压。同时，平滑的检测电流通过收集器网络中的耳机。但是当信号的相移在谐振回路中为90°时，信号的正半波仅部分重叠，因此检测到的电流达到由信号电平确定的最大值。
利用频率调制，根据频率偏差，相移也改变，对应于振荡电路L2C2的相位 - 频率响应。当频率在一个方向上偏移时，相移减小，并且基极和发射极处的信号的半波重叠得更多，结果，检测到的电流增加。当频率偏差在相反方向上时，其减小信号的半波的重叠，并且电流减小。因此发生信号的频率检测。
检测器的增益直接取决于谐振槽L2C2的品质因数Q，品质因数Q应该尽可能高（在我们如前所述的700的极限），因此与晶体管的发射极的耦合是弱的。当然，这样的简单检测器不抑制接收信号的AM。事实上，它的检测电流与输入端的信号电平成比例，这是一个明显的缺点。但无论如何，这是非常简单的电路。
就像以前的电路一样，接收机内置在一个小外壳中，在外壳顶部安装了伸缩天线，耳机插座位于外壳底部。可变电容器的旋钮位于前面板上。这些可变电容器不应该组合成一个单元，因为可以通过单独调谐获得更大的音量和更好的接收质量。
线圈L1，L2如果是无框架的，则在直径8mm的心轴上用铜线0.7mm（AWG 21）缠绕。 L1包含5匝，L2 - 5 + 2匝。如果可能，线圈L2缠绕有镀银线以提高品质因数Q，线的直径不是关键的。通过压缩或拉伸线圈L1和L2来调节线圈的电感，以使FM无线电台处于可变电容器调谐范围的中间。线圈L1和L2之间的距离在15 ... 20mm的范围内（线圈的轴线是平行的），通过弯曲它们的端子来调节该距离，焊接到可变电容器。
利用这种接收机可以做很多有趣的实验，探索用检波器接收VHF无线电广播的可能性，探索无线电波在城市地区的传播等。可以做实验来进一步改进接收机。然而，具有膜的高阻抗耳机的声音质量差。因为它开发了更好的接收机，它提供更好的声音质量，并允许您使用不同的外部天线，通过馈线连接到接收器。
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无线电接收机由无线电波的能量供电
试验一个简单的水晶无线电设备，反复不得不确保检测到的信号的功率足够（几十或几百微瓦）在耳机提供非常大的声音。但是接收质量不好，因为没有频率检测器。这个问题在第二接收机（图2）中被部分地解决，但是信号强度也被低效地使用，因为晶体管由正交高频信号供电。因此，决定在接收器中使用两个检测器：包络检测器（为晶体管供电）和频率检测器 - 以改善信号检测。

图3.

C1, C2 - 2.2...15 pF, C3 - 0.15 uF, C4 - 1 uF, C5 - 1 nF, R1 - 130 k
接收机的电路图如图3所示。通过双线线路连接到接收器的外部天线（偶极子），由带状VHF电缆制成，阻抗为240 ... 300欧姆。电缆和天线之间的阻抗匹配是自动进行的，并且通过选择线圈L1的适当抽头来执行输入振荡电路L1C1的阻抗匹配。一般来说，馈线到输入槽电路的不平衡连接减少了天线馈线系统的噪声干扰，但是由于接收机的低灵敏度，这没有关系。存在使用耦合线圈或平衡 - 不平衡变换器的馈电器的平衡连接的公知方法。
作者的折叠偶极子由常规的隔离连接线制成，偶极子放置在阳台上，在具有最大场强的地方。进料器的长度不超过5米。以这样小的长度，馈线中的损耗可以忽略，因此，可以成功地使用平衡线路。
输入振荡电路L1C1被调谐到信号的频率，并且基于高频二极管VD1，通过振幅检测器对L1C1两端的高频电压进行整流。由于FM信号的幅度具有恒定值，因此实际上不需要对整流的DC电压进行平滑。然而，为了在无线电信号的多径传播的情况下去除可能的寄生振幅调制（参见关于干扰的上述故事），平滑电容器C4的电容被选择为足够大。整流的DC电压用于为晶体管VT1供电。为了控制电流消耗并且对于信号电平指示使用模拟电流计PA1。
接收器的正交频率解调器用晶体管VT1和移相器振荡电路L2C2来实现。来自线圈L1的抽头的高频信号通过耦合电容器C3施加到晶体管VT1的基极，并且它的信号从相移线圈L2的线圈L2的抽头施加到晶体管VT1的发射极。移位槽电路L2C2。检测器的工作与之前的设计完全相同。为了增加频率解调器的增益，在晶体管VT1的基极上通过电阻器R1施加偏移电压，并且因为使用耦合电容器C3。注意，电容器C3具有足够的电容（0.15μF） - 该电容被选择为分流低频电流，即，用于使声频的晶体管VT1的基极接地。这增加了晶体管的增益并增加了接收的体积。
晶体管VT1的集电极电路中的输出变压器T1的初级绕组用于将晶体管的高输出阻抗与耳机的低阻抗匹配。此收音机可使用立体声耳机TDS-1（8..16欧姆）或TDS-6（8欧姆）。两个听筒（左通道和右通道）并联连接。旁路电容器C5用于对集电极电路中的高频电流进行滤波。按钮SB1用于在调谐输入振荡电路和搜索信号时缩短晶体管VT1的集电极电路。耳机中的声音同时消失，但是指示器PA1的灵敏度显着增加。
接收器的设计可以非常不同，但无论如何它需要前面板与两个可变电容器C1和C2的旋钮（每个电容器具有单独的旋钮）和按钮SB1。为了减少对调谐的手感，期望制造金属板或覆铜层压板的前面板。它也可以作为接收器的公共线。可变电容器的转子应该与面板具有良好的电接触。天线插座X1和电话插座X2可以放置在接收器的前面板上或侧面或后面。其尺寸取决于可用的组件。所以，让我们说几句话。
电容器C1和C2是KPV型，最大容量为15 ... 25 pF。电容器C3-C5是陶瓷的。
线圈L1和L2是无框的 缠绕在直径为8mm 的心轴上（见图4），

L1包含5，L2包含7匝。绕组的长度为10 ... 15 mm（通过调整长度做一些调整）。使用0.6 ... 0.8 mm（AWG 20..23）的漆包铜线，但最好使用镀银线，特别是线圈L2。抽头由1和1.5匝（L1）和1匝（L2）组成。线圈可以彼此同轴或轴线平行地布置。调整线圈（10 ... 20 mm）之间的距离。即使在线圈之间没有电感耦合的情况下，接收器也将工作 - 通过晶体管的结电容的电容耦合就足够了。音频变压器T1是TAG-3，其具有10:1或20:1的绕组比。晶体管VT1可以由具有不低于400MHz的最大工作频率ft的任何锗晶体管代替。也可以使用p-n-p晶体管，例如GT313A，在这种情况下，指示器PA1和二极管VD1的极性应该反转。二极管可以是任何锗类型，高频。作为指示器PA1，可以使用具有50..150mA电流范围的任何电流表。将振荡回路调谐到无线电台的频率，调节线圈的抽头和线圈之间的距离以获得最佳结果最大音量和最好的接收质量）。在阳台上，具有上述天线的接收器提供了两个站的高质量接收，其中具有来自距离不小于4km的无线电中心的最强信号，并且没有直接的视线（被建筑物遮蔽）。晶体管的集电极电流为30 ... 50mA。当然，VHF晶体无线电设备的可能设计不限于上述。相反，这个电路应该被认为是在这个有趣的领域的第一个实验。当使用放置在屋顶上并且瞄准无线电台的高效天线时，即使在距离电台相当远的距离处，也可以获得足够的信号强度。这为耳机提供了高质量的接收，在某些情况下，您可以获得扬声器接收。可以通过使用更有效的检测电路并使用高质量的谐振回路，特别是螺旋谐振器作为谐振电路来改进这种接收机。
Polyakov，莫斯科

cheungbx

正在实验中, 试了 3DQ 能响, 但是 沒有 文中所說的效果, 現在 只欠三极管.

drzhu

先收藏了，慢慢看，谢谢！

drzhu

请问原文是英文还是俄语？

老宋外贸

先收藏了，慢慢看，谢谢！

cheungbx

drzhu 发表于 2017-1-10 01:07
请问原文是英文还是俄语？

原文是英文

cheungbx

谢谢梁老师加分。我的枝术水平低，不明白怎样能把3DQ用在这个电路，所以直接把 PNP管子换成3DQ是失败了. 请梁老师有空指教一下。

L.D.XIONG

cheungbx 发表于 2017-1-10 08:21
谢谢梁老师加分。我的枝术水平低，不明白怎样能把3DQ用在这个电路，所以直接把 PNP管子换成3DQ是失败了. 请 ...

3DQ要正常工作关键是源极那个线圈，而且双栅并联好处理些。FM线路越简单越好成功，所以源极线圈不调谐。

cheungbx

英文原文: 1/3

FM Crystal Radio Receivers
Radio, 2002, 7
The notion of "crystal radio" is strongly associated with huge antennas and radio broadcasting on long and medium bands, in this article, the author describes the experimentally tested detector circuits of VHF receivers designed to listening to a FM stations.
The very possibility of receiving VHF FM detector was discovered accidentally. One day I was walking in the Terletskiy park in Moscow, Novogireevo, I decided to listen to the broadcast - I had a simple crystal set without resonant tank (this circuit is described in the "Radio", 2001, № 1, Fig. 3). The receiver had a telescopic antenna with length of about 1.4 m. Wonder whether it is possible to receive radio broadcast with this short antenna? It was possible to hear, but weakly, simultaneous operation of two stations. But what is surprised me is the volume of receiving was rise and fall periodically almost to zero after every 5...7 m, and it was different for each radio station!
It is known that in the LW and MW bands, where the wavelengths are hundreds of meters, it is impossible. I had to stop at the point of receive with maximum volume of one of the stations and listen attentively. It turned out - this is "Radio Nostalgie", 100.5 MHz, broadcasting from the near city Balashikha. There were no line of sight between antennas. How does the FM transmission could be received by using the AM detector? Further calculations and experiments shows that it is quite possible and is not depends on the receiver.
A simple portable FM crystal receiver is made exactly the same way as an indicator of the electric field, but instead of measuring device it is necessary to connect a high-impedance headphones. It makes sense to add an adjustment of coupling between the detector circuit and the resonant tank to adjust the maximum volume and quality of the receiving signal.


The simplest Crystal radio

The circuit diagram of the receiver suitable for these requirements is shown in Fig. 1. This circuit is very close to the circuit of the receiver mentioned above. Only the VHF resonant tank has been added to the circuit.
Fig. 1.
VD1, VD2 - GD507A - an old USSR Germanium high-frequency diodes with the capacitance of 0.8 pF (at the reverse voltage of 5V), the recovery time of reverse resistance is no more than 0.1 uS (at the Idirect pulse=10 mA, Ureverse pulse=20 V, Icutoff=1 mA)
The device contains a telescopic antenna WA1, directly connected to the resonant tank L1C1. The antenna is also an element of the resonant tank, so to get the maximum power of the signal it must be adjust both the length of the antenna and the frequency of the tank circuit. In some cases, especially when the length of the antenna is about 1/4 of the wavelength, it is useful to connect the antenna to a tap of the tuning coil L1 (find the suitable tap of the coil by finding the maximum volume of the signal).
The coupling with the detector can be adjust by trimmer C2. Actually the detector is made of two high-frequency germanium diodes VD1 and VD2. The circuit is completely identical to the voltage doubling rectifier circuit, but the detected voltage would be doubled if only the trimmer capacitor C2 value is high, but then the load of the resonant circuit L1C1 would be excessive, and its quality factor Q will be low. As a result, the signal voltage in the circuit tank L1C1 will be lower and the audio volume will be lower too.
In our case, the capacitance of the coupling capacitor C2 is small enough and voltage doubling does not occur. For optimal matching the detector circuit with the tank circuit the impedance of the coupling capacitor must be equal to the geometric mean between the input resistance of the detector and the resonant resistance of the tank circuit L1C1. Under this condition, the detector is getting the maximum power of the high-frequency signal, and this is corresponding to the maximum audio volume.
The capacitor C3 is shunting the higher frequencies at the output of the detector. The load of the detector is headphones with the dc resistance of not less than 4K ohms. The whole unit is assembled in a small metal or plastic housing. The telescopic antenna with the length not less then 1m is attached to the upper part of the housing, and the connector or the jack for the phones is attached th the bottom of the housing. Note that the phone cord is the second half of the dipole antenna (a counterweight).
The coil L1 is frameless, it contains 5 turns of enameled copper wire with diameter of 0.6...1 mm wound on a mandrel with diameter of 7...8 mm. You can adjust the necessary inductance by stretching or compressing the turns of the coil L1. It's better use the variable capacitor C1 with an air dielectric, for example, type 1KPVM with two or three movable and one or two fixed plates. Its maximum capacity is small and can be in range of 7...15 pF. If the variable capacitor has more plates (the capacitance is higher), it is advisable to remove any of the plates, or connect the variable capacitor in series with a constant capacitor or a trimmer, it will reduce the maximum capacity.
The capacitor C2 is ceramic trimmer capacitor, such as a KPK or KPK-M with the capacity of 2...7 pF. Other trimmers capacitors could be used too. The trimmer capacitor C2 can be replaced with a variable capacitor, similar to C1, and it could be used to adjust the coupling "on the fly" to optimize radio receiving capabilities.
Diodes VD1 and VD2, can be GD507B, D18, D20 (it is old USSR Germanium high-frequency diodes. This diodes can be replaced with modern Schottky diodes). The shunting capacitor C3 is ceramic, its capacity is not critical and can have a value in range from 100 to 4700 pF.
Adjustment of the receiver is simple. Tune the radio by turning the knob on the variable capacitor C1 and adjust the capacitor C2 to get the maximum audio volume. The tune of the resonant tank L1C1 will be changed, so all operations must be repeated a few more times, and at the same time find the best place for the radio receiving. It is doesn't necessarily the same place where the electric field has maximum strength. This should be discussed in more detail and explain why this receiver can receive FM signals.
Interference and conversion of FM into AM
If the tank circuit L1C1 of our receiver (Fig. 1) will be set up so that the carrier frequency of FM signal falls on the slope of the resonance curve, the FM can be converted into AM. Let's find the value of Q of the tank circuit. Assuming that the bandwidth of the tank circuit L1C1 is equal to twice the frequency deviation, we obtain Q = F0 /Δ2f = 700 for both the upper and the lower VHF band.
The actual Q of the tank circuit in a crystal radio probably will be less than 700 because of the low Q-factor of its own Q (About 150...200) and because the resonant tank is shunted by the antenna and by the input impedance of the detector. Nevertheless, a weak transformation of FM into AM is possible, thus, the receiver will barely work if its tank circuit detune a little up or down in frequency.
However, there is much more powerful factor contributing to the transformation of FM into AM, - it is an interference. It's very rarely when the receiver is in the line of sight of radio station, in most cases the line of sight is obscured by buildings, hills, trees and other reflective objects. A few radio beams scattered by these objects comes to the antenna of the receiver. Even in the line of sight to the antenna comes some reflected signals (and of course, direct signal comes too). The total signal depends on both the amplitudes and phases of summing components.
The two signals are summed if they are in phase, i.e., the difference of their ways is multiple of an integer of the wavelength, and the two signals are subtracted if they are in opposite phase, when the difference of their ways is the same number of wavelengths plus half wavelength. But the wavelength, as well as the frequency varies at FM! The difference of the beams and their relative phase shift will vary. If the difference of ways is large, then even a small change in frequency leads to significant shifts in the phases. An elementary geometric calculation leads to the relation: Δf/f0 = λ/4ΔC, or ΔC = f0/λ/4Δf, where ΔC - the difference of the ways of the , it's required for the phase shift ±Π/2, to get the full sum of AM signal, Δf - frequency deviation. The full AM is the total variation of the amplitude signal from the sum of the amplitudes of the two signals to their difference. The formula can be further simplified if we consider that the multiply of frequency by the wave length f0λ is equal to the speed of light c: ΔC = c/4Δf.
Now it is easy to calculate that to get a full AM of the two-beam FM signal, the sufficient difference between the ways of beams is about a kilometer. If the difference of ways is smaller, the depth of AM proportionally decreases. Well, but if the difference of ways is more? Then, during one period of the modulating audio signal the total amplitude of the interfering signal will pass several times through the highs and lows, and distortion will be very strong when converting FM into AM, up to complete indistinct of the sound when you receive the FM by an AM detector.
Interference with FM broadcast reception is an extremely harmful phenomenon. It is not only produces a concomitant parasitic AM of a signal, as it is described above, but it is produces the parasitic phase modulation, what leads to distortion even if we got a good FM receiver. That's why it is so important to place the antenna in the right location, where the only one signal prevails. It is always better to use a directional antenna, because it increases the magnitude of the direct signal and reduces reflections coming from other directions.
Only in this case with a very simple detector radio receiver the interference played a useful role and allowed us to listen to the radio broadcast, but the radio broadcast can be heard weakly or with significant distortions, and the radio broadcast can't be heard everywhere, but only in certain places. This explains the periodic changes in the volume of the radio broadcast in the Terletskiy park.

cheungbx

英文原文: 2/3


Crystal Detector Radio Receiver with a frequency detector
A radical way to improve reception is to use a frequency detector instead of an amplitude detector. In Figure 2 is shown a circuit of a portable detector radio receiver with a simple frequency detector, based on a single high-frequency germanium transistor VT1. The germanium transistors is used because it's junctions works at a low voltage about 0.15 Volts, this allows to detect very weak signals. The junctions of silicon transitions works at a voltage approximately 0.5 V, and the sensitivity of the receiver with a silicon transistor is much lower.
Fig. 2.
VT1 - GT313A - an old USSR Germanium high-frequency transistor with hfe=10...230 (at DC: Uke=3 V, Ie=15 mA), hfe=3..10 (at f=100 mHz, Ukb=5 V, Ie=5 mA)
As in the previous design, the antenna is connected to the input tank circuit L1C1, the variable capacitor C1 is used for the tuning function. The signal from the input tank circuit goes to the base of the transistor VT1. The other tank circuit, L2C2, is inductively coupled with the input tank circuit L1C1. The tank circuit L2C2 is tuneble with the variable capacitor C2. Because of the inductive coupling between this two tanks the oscillation in the resonant tank L2C2 is phase shifted by 90° relative to the signal across the input tnak circuit L1C1. From the tap of the coil L2 the signal goes to the emitter of the transistor VT1. A bypass capacitor C3 and high impedance headphones BF1 is connected to the collector of the transistor VT1.
The transistor begins to turn on when its base and emitter has the positive half-wave of the signal, and the instantaneous voltage on the emitter is greater then its base voltage. At the same time the smoothed detected current passes through the headphone in the collector network. But the positive half-wave of the signal is only partially overlapping when the phase shift of the signal is 90° in the resonant tanks, so the detected current reaches the maximum value determined by the signal level.
With frequency modulation, depending on the frequency deviation, the phase shift is also changing, corresponding to the phase-frequency response of the tank circuit L2C2. When the frequency deviates in one direction then the phase shift decreases and the half-waves of the signal at the base and emitter is overlapped more, as a result, the detected current increases. When the frequency deviation goes in the opposite direction, its decreases the overlap of half-waves of the signal and the current decreases. So the frequency detection of the signal occurs.
The gain of the detector depends directly on the quality factor Q of the resonant tank L2C2, the quality factor Q should be as high as possible (in the limit of 700, as we calculated earlier), therefore the coupling with the emitter of the transistor is weak. Of course, such a simple detector does not suppress the AM of the received signal. In fact, its detected current is proportional to the signal level at the input, this is an obvious disadvantage. But anyway it's the very simple circuit.
Just like the previous circuit, the receiver is built in a small housing, on the top of the housing a telescoping antenna is mounted, and the headphone socket in the bottom the housing. The knobs of the variable capacitors is located on the front panel. These variable capacitors should not be combined into one unit, because a louder volume and a better quality of reception can be obtained with separate tuning.
The coils L1, L2 if frameless, they wound with the copper wire 0.7 mm (AWG 21) in diameter on the mandrel of diameter 8 mm. L1 contains 5 turns, L2 - 5+2 turns. If possible, the coil L2 wound with silver plated wire to improve the quality factor Q, the diameter of the wires is not critical. The inductance of the coils is adjusted by compressing or stretching of the coils L1 and L2 to get the FM radio stations in the middle of the variable capacitors tuning range. The distance between the coils L1 and L2 is in the range of 15...20 mm (the axis of the coils is parallel), the distance is adjusted by bending their terminals, soldered to the variable capacitors.
With this receiver can be done a lot of interesting experiments, exploring the possibility of reception of VHF radio broadcasts with the detector receiver, exploring the propagation of radio waves in urban areas, etc. Can be done experiments to further improve the receiver. However, the sound quality in a high-impedance headphones with membranes is poor. Because of it a better receiver was developed, which provides better sound quality and allows you to use a different external antennas, connected to the receiver by feedline.

cheungbx

英文原文: 3/3

Radio receiver powered by the energy of radio waves
Experimenting with a simple crystal radio set, repeatedly had to make sure that the power of the detected signal is sufficiently enough (tens or hundreds of microwatts) to provide a very loud sound in the headphones. But the quality of reception is not good because there is no frequency detector. This problem is partially solved in the second receiver (Fig. 2), but the signal strength is also used inefficiently because the transistor is powered by quadrature high-frequency signal. Therefore it was decided to use two detectors in the receiver: the envelope detector - to power the transistor, and the frequency detector - to improve signal detection.
The circuit diagram of the receiver is shown in Fig. 3. An external antenna (dipoles) connected to the receiver by a two-wire line, made of ribbon VHF cable with the impedance of 240...300 ohms. The impedance matching between the cable and the antenna is performed automatically, and the impedance matching of the input tank circuit L1C1 is performed by selecting a suitable tap of the coil L1. Generally speaking, unbalanced connection of the feeder to the input tank circuit reduces the noiseproofing of the antenna feeder system, but because the low sensitivity of the receiver, it doesn't matter. There is a well-known methods of balanced connections for a feeder with the use of a coupling coil or a balun.
The author's folded dipole was made of a conventional isolated connecting wire, the dipole was placed on the balcony, in a place with a maximum field strength. The length of the feeder does not exceed 5 m. With such a small length the losses in the feeder is negligible, and therefore, the balanced line can be successfully used.
The input tank circuit L1C1 is tuned to a frequency of a signal, and a high frequency voltage across L1C1 is rectified by an amplitude detector, based on the high-frequency diode VD1. Since the amplitude of FM signal has a constant value, there is practically no requirements for smoothing the rectified DC voltage. However, to remove possible parasitic amplitude modulation in case of multipath propagation of radio signals (see above story about the interference), the capacitance of the smoothing capacitor C4 is selected sufficiently large. A rectified DC voltage is used to power transistor VT1. For the control of the current consumption and for a signal level indication is used an analog current meter PA1.
A quadrature frequency demodulator of the receiver is implemented with the transistor VT1 and phase shifter tank circuit L2C2. The high-frequency signal from the tap of the coil L1 is applied to the base of the transistor VT1 through the coupling capacitor C3, and it's signal is applied to the emitter of the transistor VT1 from the tap of the coil L2 of the phase-shifting tank circuit L2C2. The work of the detector is exactly the same as in the previous design. To increase the gain of the frequency demodulator, on the base of the transistor VT1 is applied an offset voltage through the resistor R1, and because of it the coupling capacitor C3 is used. Note that the capacitor C3 has sufficient capacitance (0.15 uF) - this capacitance is chosen to shunt the low-frequency currents, i.e., for grounding the base of the transistor VT1 for the sound frequencies. This increases the gain of the transistor and increases the volume of reception.
The primary winding of the output transformer T1 in the collector circuit of the transistor VT1 is used to match the high output impedance of the transistor to the low impedance of the headphones. A stereo headphones TDS-1 (8..16 ohms) or TDS-6 (8 ohms) can be used with this radio. Both the earpieces (left and right channels) are connected in parallel. The bypass capacitor C5 is used to filter the high-frequency currents in the collector circuit. The button SB1 is used to short the collector circuit of the transistor VT1 while tuning the input tank circuit and the search for a signal. The sound in the headphones at the same time disappears, but the sensitivity of the indicator PA1 is significantly increased.
The design of the receiver can be very different, but anyway it needs the front panel with the knobs of the two variable capacitors C1 and C2 (each capacitor has individual knob) and the button SB1. To reduce hand effect on the tuning, it is desirable to make the front panel of a metal plate or a copper clad laminates. It can work also as a common wire of the receiver. Rotors of the variable capacitors should have good electrical contact with the panel. The antenna socket X1 and the phone jack X2 can be placed either on the front panel or on the side or back of the receiver. Its dimensions are dependent on the available components. So let's say a few words about them.
The capacitors C1 and C2 is KPV type with a maximum capacity of 15...25 pF. The capacitors C3-C5 are ceramic.
The coils L1 and L2 are frameless (see Figure 4), wound on a mandrel of diameter 8 mm, L1 contain 5, L2 contains 7 turns. The length of the winding is 10...15 mm (do some tuning by adjusting the length). The enameled copper wire of 0.6...0.8 mm (AWG 20..23) is used, but it is better to use a silver-plated wire, especially for the coil L2. The taps are made from 1 and 1.5 turns (L1) and from 1 turn (L2). The coils can be arranged coaxially or axis parallel to each other. The distance between the coils (10...20 mm) is adjusted. The receiver will work even in the absence of inductive coupling between the coils - the capacitive coupling through the junction capacitance of the transistor is enough. The audio transformer T1 is TAG-3, it has a winding ratio of 10:1 or 20:1.
The transistor VT1 can be replaced by any germanium transistor with maximum operating frequency ft not lower than 400 MHz. A p-n-p transistor can be used too, for example, GT313A, in this case the polarity of the indicator PA1 and the diode VD1 should be reversed. The diode can be any germanium type, a high-frequency. As the indicator PA1 any ammeter with a current range of 50..150 mA can be used.
Tune the tank circuits to the frequency of a radio station, adjust the taps of the coils and the distance between the coils to get the best result (maximum volume and best quality of the reception). It is useful to adjust the value of the resistor R1 for maximum volume.
On the balcony the receiver with the antenna described above provided high quality reception of two stations with the strongest signal from the radio center at the distance not less than 4 km and with no direct line of sight (obscured by buildings). Collector current of the transistor was 30...50 mA.
Of course, the possible design of VHF crystal radios is not limited to described above. On the contrary, this circuit should be considered only as the first experiments in this interesting field. When using an efficient antenna, placed on a roof and targeted at a radio station, it is possible to obtain sufficient signal strength, even at a considerable distance from the station. This provides a high-quality reception on a headphones, and in some cases, you can get loudspeaking reception. It is possible to improving this receivers by using a more efficient detection circuit and using a high-quality resonant tanks, in particular, spiral resonators as resonant circuits.
V. Polyakov, Moscow