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Detection of Elements in Organic Compounds

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Detection of Elements in Organic Compounds (Class 11 / 12)

Organic compounds contain carbon and hydrogen. In addition to these they may contain nitrogen, halogen, sulphur etc. Nitrogen, halogen and sulphur can be detected by Lassaigne's test. This is the most dependable test for the detection of nitrogen, halogen and sulphur. The test is conducted using sodium fusion extract. Sodium fusion extract is prepared as follows.

Preparation of sodium fusion extract

A small piece of metallic sodium is heated slowly to melt in an ignition tube. About 0.2g of the substance is added to it and then it is heated first gently and then strongly. When the tube is red-hot, it is plunged into about 10mL. of distilled water in a china dish. The ignition tube breaks and excess of sodium reacts with water. The solution is boiled and filtered. The filtrate is known as sodium fusion extract. The following tests are done with the sodium fusion extract.

Detection of nitrogen

A little sodium fusion extract is boiled with about the same volume of freshly prepared ferrous sulphate solution. It is acidified with dil. H2SO4. A prusssian blue colour shows the presence of nitrogen.

Chemistry of the test

During the preparation of sodium fusion extract, nitrogen present in the organic compound is converted to sodium cyanide. This reacts with ferrous sulphate to produce ferric ferrocyamide as follows.

Na + C + N —> NaCN

FeSO4 + 2NaCN —> Fe(CN)2 + Na2SO4

Fe(CN)2 + 4NaCN —> Na4Fe(CN)6

Fe+2 —> Fe3+ + e

4Fe3+ + 3Na4Fe(CN)6 --> Fe4 [Fe(CN)6]3 (Prussian blue) + 12Na+

The solution is acidified to dissolve any green coloured ferrous hydroxide which may be produced by the reaction between ferrous sulphate and NaOH. Presence of green Fe(OH)2 may lead to wrong inferences.

Detection of halogen

A little sodium fusion extract is boiled with dil. HNO3 and silver nitrate solution is added to it. Formation of a curdy white precipitate soluble in NH4OH shows the presence of chlorine. A pale yellow precipitate slightly soluble in NH4OH shows the presence of bromine. A yellow precipitate insoluble in NH4OH shows the presence of iodine.

Chemistry of the test

During the preparation of sodium fusion extract sodium halide is formed, if halogen is present in the organic compound.

Na + X --> NaX ( X = CI, Br or I)

The extract reacts with AgNO3 to produce precipitates of silver halide.

AgNO3 + NaX --> AgX + NaNO3

The extract is boiled with dil. HNO3 to decompose any sulphide or cyanide present in the extract. Otherwise they give precipitates with AgNO3 solution.

Detection of sulphur

Sulphur can be detected by two methods using sodium fusion extract.

a) A little sodium fusion extract is acidified with acetic acid and lead acetate solution is added. A black precipitate shows the presence of sulphur.

Chemistry of the test

If sulphur is present in the organic compound then sodium sulphide is present in the fusion extract.

2Na + S —> Na2S

Sodium sulphide reacts with lead acetate to produce black lead sulphide.

Na2S + (CH3-COO)2 Pb —> PbS (Black) + 2CH3-COONa

b) A little sodium fusion extract is mixed with a few drops of sodium nitroprusside solution. A violet colour shows the presence of sulphur.

Chemistry of the test

Sodium sulphide present in the extract reacts with sodium nitroprusside to produce a violet coloured complex,

Na2S + Na2Fe(CN)5.NO ---> Na4Fe(CN)5.NOS (Violet)

Lassaigne's tests are summarised as given below:






1. To one portion of the extract freshly prepared FeSO4 solution is added, heated to boiling, cooled and acidified with dil. H2SO4

Blue or green colouration or precipitate.

Presence of nitrogen.

2. Another portion of the extract is acidified with diI. HNO3, boiled, cooled and AgNO3 solution is added.

i. White curdy precipitate soluble in NH4OH is formed.

Presence of chlorine.

ii. Pale yellow precipitate slightly soluble in NH4OH is formed.

Presence of bromine.

iii. Yellow precipitate insoluble in NH4OH is formed.

Presence of iodine.

3. Freshly prepared sodium nitroprusside solution is added to another portion of the extract.

Violet colouration

Presence of sulphur.

4. Another portion of the extract is acidified with acetic acid and a few drops of lead acetate solution are added.

Black precipitate is formed.

Presence of sulphur is continued.


Digital Modulation Viva Questions

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Digital Modulation Techniques Viva Questions and Answers

1. What do you understand about Digital Modulation.

The mapping of a series of input binary digits into a set of corresponding high-frequency signal waveforms is referred to as digital modulation.

2. Write down the different types of digital modulation techniques.

I. Based on the method of detection, there are mainly 2 types of digital modulation techniques:

1. Coherent digital modulation

2. Non-coherent digital modulation

II. Based on the mapping techniques, the different techniques are:

Binary Scheme

Quaternary Scheme

M-ary Scheme

Hybrid Scheme



1. M-ary ASK

1. QAM


2. MSK

2. M-ary FSK

2. APK



3. M-ary PSK



III. Based on the performance of the modulation scheme and properties of the modulated signal, the different techniques are:

1. Power efficient scheme / Bandwidth efficient scheme.

2. Continuous Phase (CP) Modulation / In phase – Quadrature Phase (IQ) Modulation.

3. Constant Envelope Modulation / Non-constant Envelope Modulation.

4. Linear Modulation / Non-linear Modulation.

5. Modulation scheme with memory / Modulation scheme without memory.

3. Describe the digital communication systems design objectives.

The objectives of digital communication systems design are:

1. Maximum data rate

2. Minimum possibility of symbol error.

3. Minimum transmitted power.

4. Minimum channel bandwidth.

5. Maximum resistance to interfering signals.

6. Minimum circuit complexity.

4. What do you know about the coherent binary modulation technique?

There are three basic types of binary modulation schemes. They are:

1. Binary Amplitude Shift Keying (BASK)

2. Binary Frequency Shift Keying (BFSK)

3. Binary Phase Shift Keying (BFSK)

Coherent binary modulation techniques are defined as binary modulation systems that use coherent detection at the receiver.

The local carrier generated at the receiver is phase synchronized with the carrier at the transmitter incoherent detection.

5. Define ASK, FSK and PSK

ASK: The modulation process in Amplitude Shift Keying (ASK) includes switching or keying the amplitude of the carrier signal following the incoming data.

FSK: The modulation process in Frequency Shift Keying (FSK) involves switching or keying the carrier signal's frequency following the incoming data.

PSK: The modulation process in Phase Shift Keying (PSK) includes switching or keying the phase of the carrier signal following the incoming data.

6. Mention the merits and demerits of BPSK?


• BPSK consumes less bandwidth than BFSK, and it has the lowest likelihood of a mistake. As a result, when compared to BFSK and BASK systems, it delivers great outcomes.

• It has a significant level of noise immunity.


• Since the information in PSK is maintained in the phase, it cannot be detected non-coherently.

7. What are the advantages and disadvantages of BFSK?

Advantages :

• It is simple to implement.

• It has more noise resistant than ASK.


• When compared to BPSK and BASK, BFSK takes a large amount of bandwidth.

8. What are the advantages and disadvantages of BASK?


• BASK is simple to produce and detect.


• It is highly sensitive to noise.


9. What is meant by the Non-coherent binary modulation technique?

Non-coherent modulation refers to a modulation technique in which the detection process does not need the receiver carrier to be phase synchronized with the transmitter carrier. Non-coherent binary modulation techniques are:

1. Differential Phase Shift Keying (DPSK)

2. Binary Amplitude Shift Keying (BASK)

3. Binary Frequency Shift Keying (BFSK)

10. Define DPSK?

Differential Phase Shift Keying (DPSK) is a "Pseudo PSK" technology that is essentially a non-coherent version of BPSK. By merging two fundamental procedures at the transmitter, it eliminates the need for a coherent reference signal at the receiver. They are namely:

1. Differential encoding of the input binary wave

2. Phase shift keying

11. List down the merits and demerits of DPSK?


• At the receiver end, the DPSK system does not require a carrier. As a result, it has simplified the system.

• The necessary bandwidth is lower than for BPSK.


In comparison to BPSK, it has a larger likelihood of a mistake.

• There's a lot more noise.

• Previous bit detection is utilized in DPSK to identify the following bit. As a result, mistakes in pairings are possible.

12. What do you know about the coherent quadrature modulation technique?

Quadrature modulation is an M-ary modulation technique with m=4. The coherent quadrature modulation approach refers to a quadrature modulation system in which coherent detection is used at the receiver. Two quadrature modulation methods that save bandwidth are available. They are

1. QuadriPhase Shift Keying (QPSK)

2. Minimum Shift Keying (MSK)

13. Define QPSK

In QPSK, as in BPSK, the phase contains the information delivered by the transmitted signal. The bits per symbol in QPSK is k = 2, hence m = 2k = 22 = 4. As a result, two quadrature carriers are modulated by two consecutive bits (dibit) in the data sequence.

14. What do you understand about signal constellation?

The signal space diagram is used to analyze any modulation scheme. A map of probable message points is used in the signal space technique. A signal constellation refers to a collection of probable message points.

15. Mention the merits and demerits of QPSK?


•QPSK provides a high level of noise immunity.

• It makes better use of the transmission channel's available bandwidth and has a reduced mistake probability.


• The creation and detection of QPSK are both difficult.

16. Justify Minimum Shift Keying (MSK)?

MSK (Minimum Shift Keying) is a type of binary CPFSK signal. MSK refers to a Continuous Phase Frequency Shift Keying (CPFSK) signal with a deviation ratio of h = 1/2.

17. Define Deviation Ratio.

The deviation ratio is defined as the difference between the frequencies f1 and f2, normalized about the bit rate 1/Tb. The deviation ratio h = 1/2 for MSK.

18.  Explain Minimum Shift Keying (MSK)

The deviation ratio is h = Tb (f1 – f2)

For MSK, we have h = 1/2. Therefore,

1/2 = Tb (f1 – f2)

=> (f1 – f2) = 1/2𝑇𝑏 = 𝑅𝑏/2

Where Rb => bit rate

Hence, the bit rate is half the frequency deviation (f1 – f2). This is the smallest frequency spacing that permits the two FSK signals representing symbols 1 and 0 to be coherently orthogonal in the sense of not interfering with one another during detection. Minimum Shift Keying is the name given to a CPFSK signal with a deviation ratio of half (MSK).

19. Discuss the merits and demerits of MSK?


• The phase of the MSK signal is constant.

• There is no need for sidelobe suppression filters that generate interchannel interference.

• There is no need for sidelobe suppression filters, which produce interchannel interference. 


• MSK signal production and detection are more difficult.

• The neighboring channel interference in a wireless communication system utilizing MSK is insufficient to meet the practical needs of a multiuser communications environment.

20. Mention the two basic steps in a digital receiver? Explain

1) Demodulation and

2) Detection is the two basic phases of a digital receiver.

1. Demodulator:

The frequency down-conversion block is the demodulator. The signal demodulator's purpose is to transform the received waveform X(t) into an N-dimensional vector X = [X1, X2,..... XN], where N is the sent signal waveforms' dimension.

2. Detector

The detector's task was to find out which of the M possible signal waveforms was sent depending on Vector X. The optimum detector minimizes the probability of error.

21. Discuss coherent and non-coherent detection in a digital receiver?

Coherent Detection

The local carrier generated at the receiver is phase synchronized with the carrier generated at the transmitter in coherent detection. The receiver is aware of the exact moments when the modulation changes state. The synchronous detection scheme is coherent detection.

Non-Coherent detection

The detection process in non-coherent detection does not need the receiver carrier to be phase synchronized with the transmitter carrier. Between the transmitter and receiver, there is no time and phase synchronization. As a result, non-coherent detection is an asynchronous scheme.

22. Illustrate about sampled matched filter

The impulse response of a matched filter is a delayed version of the input signal waveform's mirror image (rotated on the t = 0 axis). As a result, the impulse response h(t) of a filter that is matched to s(t) is

A sampled matched filter uses digital techniques and sampled waveforms to implement a matched filter.

23. What do you know about Correlator.

A correlator's primary function is to integrate the received noisy signal with each of the reference carrier signals. The received signal is divided into N-dimensional vectors (X1, X2, .... XN).

24. Define Time Division Multiplexing (TDM)

TDM involves sampling a collection of signals sequentially in time at a common sampling rate and multiplexing them for transmission over a single channel. TDM is a digital management information system. This allows us to combine many digital signals into a single data stream with a higher bit rate, such as computer outputs, digitized voice signals, digitized facsimile signals, and television signals.

25. List the different types of TDM?

TDM is classified into two types. They are

1. Synchronous TDM

2. Asynchronous TDM

1. Synchronous or Deterministic TDM:

Every individual communication channel has a consistent latency and bandwidth. Time slots are employed in a synchronous periodical way and have a fixed length (capacity). ISDN, PDH, and SDH are all approaches that utilize it.

2. Asynchronous or statistical TDM:

Each individual communication channel has a changeable latency and bandwidth. Time slots come in a variety of lengths and are utilized as needed. It is used in protocols such as X25, Frame Relay, ATM, and IP.

26. What is ASCII Framing?

In the ASCII frame for TDM, the American Standard Code for Information Interchange (ASCII) codes in hexadecimal notation are used. Character-oriented protocols are used to organize ASCII packets.

27. Write short notes on EI framing.

• The EI carrier-based framing is a European hierarchy.

• There are a total of 30 PCM encoded speech data channels.

• The data rate on the transmission line is 2.048 Mbps.

• It operates over coaxial cable and uses the A-law algorithm for companding.

28. Write short notes on TI framing

• A North American hierarchy of TI framing is based on TI carriers.

• There are a total of 24 PCM encoded voice data channels.

• It employs the -law algorithm for companding and operates over shielded twisted pair wires with a data rate of 1.544 Mbps.

Pulse Code Modulation (PCM) Viva Questions

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Pulse Code Modulation Viva Questions and Answers

1. Mention channel noise?

The combined effect of thermal noise, interference from other users, and interference from circuit switching transients are termed channel noise. It can enter the transmission path at any moment. It can be represented as Additive White Gaussian Noise (AWGN) with a zero mean and power spectral density of No/2.

Sampling Theorem Viva Questions And Answers

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Sampling Theorem Viva Questions And Answers 

1. Discuss in detail about Formatting?

In digital communication, the first and most important signal processing step is formatting. Formatting converts source data into bits, ensuring that the data and signal processing processes of a digital communication system are compatible.

i. Data that has already been converted to a digital format would be bypassed by the formatting procedure.

ii. A coder converts textual data into binary digits.

iii. Analog data is structured in three steps: sampling, quantification, and coding.

2. What do you know about the sampling theorem.

The sampling theorem may be categorised into two parts:

(i) Sample values at uniform intervals less than or equal to 1/2fm fully represented by a band-limited signal of finite energy with no frequency components higher than fm Hertz.

(ii) A finite-energy band-limited signal with no frequency components greater than fm Hertz may be fully recovered using just the knowledge of its samples obtained at a rate of 2fm samples per second.

3. Discuss in detail about Nyquist Theorem.

The Nyquist theorem can be stated as follows. "To recover the signal exactly from its samples, the sampling frequency (fs) must be at the rate equal to or greater than twice the highest frequency (maximum frequency) component (fm) present in the signal".

Mathematically, it can be written as fs≥2fm.

4. What does sampling mean? Name the various sampling techniques.

Sampling is the process of converting a continuous-time signal into a discrete-time signal. There are mainly three basic types of sampling techniques.

1. Impulse sampling or Ideal sampling.

2. Natural sampling

3. Sample and hold operation or (flat top sampling).

5. What do you know about impulse sampling?  Mention its disadvantage?

The method is known as the impulse or ideal sampling if the sample function is a train of impulses. As a result of this procedure, the breadth of the samples approaches zero. The power content of the immediately sampled pulse is insignificant as a result of this. As a result, this approach is unsuitable for transmission purposes.

6. Explain in detail natural sampling? Discuss its disadvantages?

Natural sampling is the method used when the sampling function is a pulse train or switching waveform. Each pulse in the sampled data series has a changing top according to signal variation in this method. Noise interferes with the top of pulses during transmission. The structure of the top of the pulse at the receiver is very difficult to determine.

7. Discuss sample and hold operation?

The top of the pulse changes according to the signal fluctuation in natural sampling. As a result, the amplitude detection of the pulse is not precise, and mistakes in the signal are introduced. The solution to this problem is to use flat-top pulses. Flat top pulses are generated using a sample and hold circuit.

The sample and hold operation is defined as the convolution of a sampled pulse train with a rectangular pulse of unity amplitude. The flat-top sampled sequence is the outcome of the convolution procedure.

8. Explain about aliasing effect?

Aliasing is the phenomenon in which a high-frequency component in a signal's frequency spectrum assumes the identity of a lower frequency component in the sampled signal's spectrum. Due to the effect of under-sampling (fs< 2fm), aliasing occurs. 

9. How we can prevent aliasing?

Antialiasing filters may be used in two different methods to eliminate aliasing.

(i) A low pass filter is used to preprocess the analog signal. The filter's bandwidth is less than or equal to half of the sampling frequency (fm<fs/2).

(ii) After sampling, a lowpass filter is used to postfilter the analog signal. The aliased words can be removed after sampling when the signal structure is well known.

10. Explain quantization noise?

Analog baseband signal sample values are rounded to the quantizer's nearest allowable representation levels. Quantization noise is a term used to describe the distortion caused by this approximation of the quantized samples. The quantity of noise generated by the quantization process is inversely related to the number of levels utilized.

11. What is quantization? Mention its types.

The quantization technique reduces continuous amplitude values to a limited (discrete) range of acceptable values. In terms of time and amplitude, this is referred to as "discretization." In general, the quantization process may be categorized as follows.

I. Uniform quantization

1. Midtread type

2. Midrise type

II. Non-Uniform quantization

12. What is Uniform Quantization?

The quantizer is referred to as a Uniform or Linear quantizer when the quantization levels are uniformly distributed over the entire amplitude range of the input signal. Throughout the input range, the step size between quantization levels remains the same.

13. Discuss non-uniform quantization?

The quantizer is considered non-uniform or non-linear if the quantization levels are not uniformly distributed throughout the amplitude range of the input signal. The stepsize between quantization levels fluctuates here according to a system of rules.