This article will go over all you need to know in the A Level Physics which includes advantages of digital signalling, analogue signalling, converting between digital to analogue and analogue to digital, sampling, polarisation, sensing circuits, resolution, bandwidth and a sampling summary including sampling rate and channel capacity (I think are in chronological order). Your more than welcome to skip to the parts that are of most help to you. As well as that, you may also want to look at other articles I have done for Physics AS revision. If you want a quick look at this article, I would suggest you look at Sampling Summary at the bottom although that is not an excuse to just read that: you will be missing out loads of information and detail from the main article.

There are many advantages to digital signalling. However, before we get onto that, we should know what is an analogue and digital signal.

**Digital Signals **

The parts of a digital signal can have one of two values, 0 or 1.

Examples of digital devices include on/off switch, digital ammeter, digital clock, optic fibre and telephone cable. Time is on the x-axis.

A digital code will look like this:

1001101001010011010111100101010100

Because there are only different possibilities.

**Analogue Signals**

The parts of an analogue signal can have any value within a fixed range of values . For my example, it’s range is from 0-3 but this can vary from analogue signal to analogue signal. They basically take any values within it’s range.

Examples of analogue devices include dimmer switch, thermometer, analogue ammeter and speedometer. Time is on the x-axis.**Advantages of Digital Signalling**

**Digital signals have better quality**– this is because noise can be easily removed. More information on noise can be found here.**Digital signals allow more information to be carried.****Many signals can be sent down the same optic fibre**– this is called multiplexing.

**Analogue to Digital Conversion**

**Digital to Analogue Conversion**

**Polarisation**

**What happens when you view polarised light through a rotating polaroid filter and why?**

**Sensing Circuits**

**Here is an example of a sensing circuit:**

or more general:

output range / input range

The sensitivity is also know as the **gradient**.

For our example the sensitivity is 2 / 100 and will be measured in Volts per lux.

Therefore, the sensitivity will be 2×10 to the power of -3 Volts/lux.

**Resolution****Resolution = smallest change that can be detected.**

To calculate the resolution:

**What is the smallest possible output?***In our example, it will be 0.01 Volts as our voltmeter is accurate to two decimal places.***What input change does this correspond to ?**

**Bandwidth**

**Bandwidth of a signal is the range of frequencies in a signal.**

**Capacity of a channel is the maximum amount of information it can sends bits per second.**

**As a rough estimation the bandwidth equal to the maximum number of bits per second.**

**Bandwidth Explained**

**square wave**contains a

**fundamental frequency**and all its multiples in its signal and therefore has an extremely high bandwidth.

**fundamental frequency**, new

**bandwidth**and

**number of bits per second**are all

**roughly equal**. (i.e. same order of magnitude).

**Sampling Summary**

**Analogue Signals**

- The parts of an analogue signal can have any value within a fixed range of values

**Digital Signals**

- The parts of a digital signal can have one of only two values, 0 or 1.
- Streams of 0s and 1s can be used to represent any whole number binary code.

**Analogue to Digital Conversion**

- Sampling
- Binary coding
- Further encoding

To sample a wave the height is measured at regular time intervals and put into binary code.

**Number of Bits**

- Number of possible values = 2 to the power of b where b = number of bits.
- The noise limits the maximum number of bits it is worth sending.

Number of possible values = total signal variation / noise variation.

**Sampling Rate**

- The sampling frequency must be
**at least 2 x the highest frequency component present**or else details of the signal may be lost. - The highest frequency present can be identified in a frequency spectrum.

**Channel Capacity**

- This is the rate at which a channel can transmit information, measured in bits per second.
- The amount of information in a signal per second = sampling frequency x bits per sample.

**Bandwidth**

- The range of frequencies used to send information on a particular channel.
- Bandwidth B needed = b bits per second / 2.

**Digital Signals have the advantage that:**

- They can be regenerated easily, reducing the effects of noise.
- They can be processed and encoded.
- They can represent different kinds of information in the same way.

**Further Encoding**

wait… what are the pros and cons of increasing bandwith

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