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ملتقى الإتصالات

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  1. [1]
    ماجد عباس محمد
    ماجد عباس محمد غير متواجد حالياً

    مشرف متميز

      وسام مشرف متميز


    تاريخ التسجيل: Sep 2006
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    ملتقى الإتصالات

    الكل يتمنى أن يكون هناك منتدى للإتصالات
    هنا سأبدأ لكم هذا الموضوع الجديد لمناقشة مواضيع الإتصالات
    وإن شاء اللة سأكمل هنا مجموعة كيف تصمم الدوائر الإلكترونية - على أى حال الإتصالات جزء من الإلكترونيات وتمنياتى بالتوفيق للجميع

  2. [2]
    eng_hazem123
    eng_hazem123 غير متواجد حالياً
    عضو متميز
    الصورة الرمزية eng_hazem123


    تاريخ التسجيل: Oct 2005
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    شكرا أخى ماجد على الرد ولكن دائرة التقاط الحديث التليفونى الارضى بواسطة الراديو موجودة فى المنتدى بالشرح فقط دون وجود لتصميم هذه الدائرة وهى بعنوان (كيف تصبح جاسوس مكالمات)؛
    ارجو الافادة؛وشكرا على اهتمامك بموضوعى

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  3. [3]
    ماجد عباس محمد
    ماجد عباس محمد غير متواجد حالياً
    مشرف متميز


    تاريخ التسجيل: Sep 2006
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    وسام مشرف متميز

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    اخى
    قرات كيف تصبح جاسوس مكالمات
    ولكن تكلم عن خمس لفات ولم يتحدث عن دائرة توصل بها اللفات المذكورة ولا ما هى علاقة سلك التليفون بهذه اللفات ولا الدائرة
    مادام سلك التليفون تحت تصرفك يمكنك أن تركب ما شئت من أجزة ارسال دقيقة ويكفى أن تشترى من السوق ميكروفون لاسلكى وكان يباع بحوالى 10 جنيهات مصرية وهانك العديد من المواقع تعرض مثائل تعمل بترانزستور واحد فهو يعمل على نفس التردد . ثم فك الميكروفون ووصل بدله أى لاقط مغناطيسى ، فقط الدائرة المشروحة أجدر أن تسمى "كيف تفضح هاتفك على الملأ" حيث كل من يملك جهاز استقبال سيستمع للمكالمات واولهم صاحب الهاتف نفسه
    عندما قمت بالرد لم أقصد الإسلوب البسيط هذا الذى يفعلة المرء لنفسه على نفسه ولكن ظننت أن السوال كيف تتجسس اوساط التجسس الدولية على المكالمات الخاصة بدولة ما رغم أن الخطوط سلكية أرضية
    عموما أرجو أن افيد فى اى مجال وشكرا لردكم وبانتظار مزيد من الإستفسارات

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  4. [4]
    X-man
    X-man غير متواجد حالياً
    جديد


    تاريخ التسجيل: Jun 2006
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    ؟؟؟؟؟

    [CENTER][والله خوش فكره لو نفتح منتدى للاتصالات يضمن كل شي عن الاتصالات فقط
    و تحياتي و شكرا:rolleyes: /CENTER]

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  5. [5]
    هبة محمود
    هبة محمود غير متواجد حالياً
    عضو فعال جداً


    تاريخ التسجيل: Apr 2006
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    أرجو أن يتم فتح منتدى خاص لهندسة الإتصالات
    حتى نتمكن من طرح المواضيع التي نحتاج لها في مجال تخصصاتنا

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  6. [6]
    ماجد عباس محمد
    ماجد عباس محمد غير متواجد حالياً
    مشرف متميز


    تاريخ التسجيل: Sep 2006
    المشاركات: 5,226

    وسام مشرف متميز

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    Exclamation

    الأخت هبة محمود
    عصفور فى اليد كما يقول المثل
    لو رأى المشرفون حركة ونشاط فى هذا الموضوع ، بالتأكيد سيقوموا بعمل هذا المنتدى لكن حتى الآن كل ما يقدم هو تأييد وتشجبع ولم يقدم موضوع واحد جاد
    لنبدأ فى طرح تساؤلات ووضع مواضيع جادة ثم نخاطب المشرفون بعد ذلك
    توقعت عندما أضع هذا الموضوع تلقى اسئلة فى مواضيع شتى ولكن ...:)

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  7. [7]
    هبة محمود
    هبة محمود غير متواجد حالياً
    عضو فعال جداً


    تاريخ التسجيل: Apr 2006
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    instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2004/rd73/476finalpro.htm - 96k -

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  8. [8]
    هبة محمود
    هبة محمود غير متواجد حالياً
    عضو فعال جداً


    تاريخ التسجيل: Apr 2006
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    أنا متأسفة يبدو أن الرابط لا يعمل

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  9. [9]
    هبة محمود
    هبة محمود غير متواجد حالياً
    عضو فعال جداً


    تاريخ التسجيل: Apr 2006
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    Modulation is the process of varying a carrier signal in order to use that signal to convey information. The three key parameters of a sinusoid are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. A device that performs modulation is known as a modulator and a device that performs the inverse operation of demodulation is known as a demodulator. A device that can do both operations is a modem (a contraction of the two terms).
    In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represented on a constellation diagram.
    In analog modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.
    Analog modulation techniques
    o
    • Sigma-delta modulation (∑Δ)
    Digital modulation techniques
    Any form of digital modulation necessarily uses a finite number of distinct signals to represent digital data.
    • In the case of PSK, a finite number of phases are used.
    • In the case of FSK, a finite number of frequencies are used.
    • In the case of ASK, a finite number of amplitudes are used. This is very similar to pulse code modulation
    Each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the symbol that is represented by the particular phase.
    These are the general steps used by the modulator to transmit data:
    1. Accept incoming digital data;
    2. Group the data into symbols;
    3. Use these symbols to set or change the phase, frequency or amplitude of the reference signal appropriately;
    4. Pass the modulated signal on for further processing, such as filtering, before transmission.
    At the receiver, the demodulator
    1. Is passed the de-filtered signal;
    2. Determines its phase, frequency or amplitude;
    3. Maps the phase, frequency or amplitude to its corresponding symbol;
    4. Translates the symbol into its individual bits;
    5. Passes the resultant bit stream on for further processing such as removal of any error-correcting codes.
    As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because the transmitter-receiver pair have prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations.
    The principal classes of modulation are:
    • Phase-shift keying (PSK)
    • Frequency-shift keying (FSK) (see also audio frequency-shift keying (AFSK))
    o Minimum-shift keying (MSK)
    o Gaussian minimum-shift keying (GMSK)
    • Amplitude-shift keying (ASK) and its most common form, on-off keying (OOK)
    • Quadrature amplitude modulation (QAM) a combination of PSK and ASK
    • Continuous phase modulation (CPM)
    • Polar modulation like QAM a combination of PSK and ASK, but using a different circuit architecture
    MSK and GMSK are particular cases of continuous phase modulation (CPM). Indeed, MSK is a particular case of the sub-family of CPM known as continuous phase-frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one symbol-time duration (total response signalling).
    Often incorrectly referred to as a modulation scheme, orthogonal frequency division multiplexing (OFDM) usually takes advantage of one of the digital techniques. It is also known as discrete multitone (DMT). When OFDM is used in conjunction with channel coding techniques, it is described as Coded orthogonal frequency division multiplexing (COFDM). OFDM is strictly a channel access method and not a modulation scheme.
    Pulse modulation
    These are hybrid digital and analogue techniques.
    • Pulse-code modulation (PCM)
    • Pulse-width modulation (PWM)
    • Pulse-amplitude modulation (PAM)
    • Pulse-position modulation (PPM)
    • Pulse-density modulation (PDM)
    Miscellaneous techniques
    • The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation.
    • Adaptive modulation
    • Wavelet modulation
    • Trellis coded modulation (TCM) also known as trellis modulation

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  10. [10]
    هبة محمود
    هبة محمود غير متواجد حالياً
    عضو فعال جداً


    تاريخ التسجيل: Apr 2006
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    Amplitude is a nonnegative scalar measure of a wave's magnitude of oscillation, that is, magnitude of the maximum disturbance in the medium during one wave cycle
    the displacement y is the amplitude of the wave.
    Sometimes this distance is called the "peak amplitude", distinguishing it from another concept of amplitude, used especially in electrical engineering: the root mean square (RMS) amplitude, defined as the square root of the temporal mean of the square of the vertical distance of this graph from the horizontal axis. The use of peak amplitude is unambiguous for symmetric, periodic waves, like a sine wave, a square wave, or a triangular wave. For an unsymmetric wave, for example periodic pulses in one direction, the peak amplitude becomes ambiguous because the value obtained is different depending on whether the maximum positive signal is measured relative to the mean, the maximum negative signal is measured relative to the mean, or the maximum positive signal is measured relative the maximum negative signal and then divided by two.
    For complex waveforms, especially non-repeating signals like noise, the RMS amplitude is usually used because it is unambiguous and because it has physical significance. For example, the average power transmitted by an acoustic or electromagnetic wave or by an electrical signal is proportional to the square of the RMS amplitude (and not, in general, to the square of the peak amplitude).
    There are a few ways to formalize amplitude:
    In the simple wave equation

    A is the amplitude of the wave.
    The units of the amplitude depend on the type of wave.
    For waves on a string, or in medium such as water, the amplitude is a displacement.
    The amplitude of sound waves and audio signals conventionally refers to the amplitude of the air pressure in the wave, but sometimes the amplitude of the displacement (movements of the air or the diaphragm of a speaker) is described. The logarithm of the amplitude squared is usually measured in dB, so a null amplitude corresponds to -∞ dB.
    For electromagnetic radiation, the amplitude corresponds to the electric field of the wave. The square of the amplitude is proportional to the intensity of the wave.
    The amplitude may be constant (in which case the wave is a continuous wave) or may vary with time and/or position. The form of the variation of amplitude is called the envelope of the wave.

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