Zener Diode As Regulator

              

Zener Diode

          We know that "reverse biased" diode blocks current in the reverse direction, but will suffer from premature breakdown or damage if the reverse voltage applied across it is too high. However, the Zener Diode or "Breakdown Diode" as they are sometimes called, are basically the same as the standard PN junction diode but are specially designed to have a low pre-determined Reverse Breakdown Voltage that takes advantage of this high reverse voltage. The zener diode is the simplest types of voltage regulator and the point at which a zener diode breaks down or conducts is called the "Zener Breakdown Voltage" ( Vz ).

Doping concentration of    P+N DIODE > ZENER DIODE > TUNNEL DIODE> PN DIODE

               The Zener diode when biased in the forward direction it behaves just like a normal signal diode passing the rated current, but as soon as a reverse voltage applied across the zener diode exceeds the rated voltage of the device, the diodes breakdown voltage V_B is reached at which point a process called Avalanche Breakdown occurs in the semiconductor depletion layer and a current starts to flow through the diode to limit this increase in voltage.

The current now flowing through the zener diode increases dramatically to the maximum circuit value (which is usually limited by a series resistor) and once achieved this reverse saturation current remains fairly constant over a wide range of applied voltages. This breakdown voltage point, Vz is called the "zener voltage" for zener diodes and can range from less than one volt to hundreds of volts.

Zener Diode I-V Characteristics:

The Zener Diode is used in its "reverse bias" or reverse breakdown mode, i.e. the diodes anode connects to the negative supply. From the I-V characteristics curve above, we can see that the zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the zener diodes current remains between the breakdown current I_Z(min) and the maximum current rating I_Z(max).

The Zener Diode As Regulator:

Zener Diode Acts as a Voltage Regulator only in the Reverse Breakdown Region.A voltage regulator should maintain constant voltage across terminals of load irrespective of fluctuation in Load or Supply.To act as a voltage regulator the zener diode must satisfy two conditions:

   (i) Current through zener diode should be grater than or equal to  I_Z(min),knee current.

   (ii)Voltage across terminals of zener diode should be Vz, Breakdown voltage.

               The resistor, R_S is connected in series with the zener diode to limit the current flow through the diode with the voltage source, V_S being connected across the combination. The stabilised output voltage V_out is taken from across the zener diode. The zener diode is connected with its cathode terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor R_S is selected so to limit the maximum current flowing in the circuit.

                  With no load connected to the circuit, the load current will be zero, ( I_L = 0 ), and all the circuit current passes through the zener diode which in turn dissipates its maximum power. Also a small value of the series resistor R_S will result in a greater diode current when the load resistance R_L is connected and large as this will increase the power dissipation requirement of the diode so care must be taken when selecting the appropriate value of series resistance so that the zeners maximum power rating is not exceeded under this no-load or high-impedance condition.

                The load is connected in parallel with the zener diode, so the voltage across R_L is always the same as the zener voltage, ( V_R = V_Z ). There is a minimum zener current for which the stabilization of the voltage is effective and the zener current must stay above this value operating under load within its breakdown region at all times. The upper limit of current is of course dependant upon the power rating of the device. The supply voltage V_S must be greater than V_Z.

          One small problem with zener diode stabiliser circuits is that the diode can sometimes generate electrical noise on top of the DC supply as it tries to stabilise the voltage. Normally this is not a problem for most applications but the addition of a large value decoupling capacitor across the zeners output may be required to give additional smoothing.

            Then to summarise a little. A zener diode is always operated in its reverse biased condition. A voltage regulator circuit can be designed using a zener diode to maintain a constant DC output voltage across the load in spite of variations in the input voltage or changes in the load current. The zener voltage regulator consists of a current limiting resistor R_S connected in series with the input voltage V_S with the zener diode connected in parallel with the load R_L in this reverse biased condition. The stabilized output voltage is always selected to be the same as the breakdown voltage V_Z of the diode.

Zener Diode Voltages

As well as producing a single stabilised voltage output, zener diodes can also be connected together in series along with normal silicon signal diodes to produce a variety of different reference voltage output values as shown below.

Zener Diodes Connected in Series:

        The values of the individual Zener diodes can be chosen to suit the application while the silicon diode will always drop about 0.6 to 0.7V in the forward bias condition. The supply voltage, Vin must of course be higher than the largest output reference voltage and in our example above this is 19v.

       
Zener Diode Clipping Circuits:

            Thus far we have looked at how a zener diode can be used to regulate a constant DC source but what if the input signal was not steady state DC but an alternating AC waveform how would the zener diode react to a constantly changing signal.

               Diode clipping and clamping circuits are circuits that are used to shape or modify an input AC waveform (or any sinusoid) producing a differently shape output waveform depending on the circuit arrangement. Diode clipper circuits are also called limiters because they limit or clip-off the positive (or negative) part of an input AC signal. As zener clipper circuits limit or cut-off part of the waveform across them, they are mainly used for circuit protection or in waveform shaping circuits.

For example, if we wanted to clip an output waveform at +7.5V, we would use a 7.5V zener diode. If the output waveform tries to exceed the 7.5V limit, the zener diode will "clip-off" the excess voltage from the input producing a waveform with a flat top still keeping the output constant at +7.5V. Note that in the forward bias condition a zener diode is still a diode and when the AC waveform output goes negative below -0.7V, the zener diode turns "ON" like any normal silicon diode would and clips the output at -0.7V as shown below.

Square Wave Signal:

               The back to back connected zener diodes can be used as an AC regulator producing what is jokingly called a "poor man's square wave generator". Using this arrangement we can clip the waveform between a positive value of +8.2V and a negative value of -8.2V for a 7.5V zener diode. If we wanted to clip an output waveform between different minimum and maximum values for example, +8V and -6V, use would simply use two differently rated zener diodes.

                 Note that the output will actually clip the AC waveform between +8.7V and -6.7V due to the addition of the forward biasing diode voltage, which adds another 0.7V voltage drop to it. This type of clipper configuration is fairly common for protecting an electronic circuit from over voltage. The two zeners are generally placed across the power supply input terminals and during normal operation, one of the zener diodes is "OFF" and the diodes have little or no affect. However, if the input voltage waveform exceeds its limit, then the zeners turn "ON" and clip the input to protect the circuit.

Superheterodyne Receiver

                     In electronics, a superheterodyne receiver (often shortened to superhet) uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency (IF), which can be more conveniently processed than the original radio carrier frequency. Virtually all modern radio receivers use the superheterodyne principle.

Block diagram:

               The diagram contains a RF amplifier, a variable frequency local oscillator(LO), a frequency mixer, a band pass filter and intermediate frequency (IF) amplifier, and a demodulator plus additional circuitry to amplify or process the original audio signal (or other transmitted information).For AM The Intermediate frequency(IF) is 455Hz.

principle of operation:

         The principle of operation of the superheterodyne receiver depends on the use of heterodyning or frequency mixing. The signal from the antennal i.e RF Signal (fs) is filtered sufficiently at least to reject the image frequency (see below) and possibly amplified by RF Amplifier. A local oscillator(LO) in the receiver produces a sine wave( i.e fl ) which mixes with that signal, shifting it to a specific intermediate frequency (IF= fl – fs OR  f_IF = f_LO - f_RF), usually a lower frequency(i.e Mixer performs Down Conversion Here).The IF signal is itself filtered and amplified and possibly processed in additional ways. The demodulator uses the IF signal rather than the original radio frequency to recreate a copy of the original information (such as audio).An AF amplifier used to amplify audio signal.

Image Frequency & its suppression:

                   One major disadvantage to the superheterodyne receiver is the problem of image frequency. In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Image frequencies can be eliminated by sufficient attenuation on the incoming signal by the RF amplifier filter of the superheterodyne receiver.

                                  

                                         f_img(Image Frequency) ₌ fs +2* f_IF 

               For example, an AM broadcast station at 580 kHz is tuned on a receiver with a 455 kHz IF. The local oscillator is tuned to 580 + 455 = 1035 kHz. But a signal at 580 + 455 + 455 = 1490 kHz is also 455 kHz away from the local oscillator; so both the desired signal and the image, when mixed with the local oscillator, will also appear at the intermediate frequency. This image frequency is within the AM broadcast band.

          Practical receivers have a tuning stage before the converter, to greatly reduce the amplitude of image frequency signals; additionally, broadcasting stations in the same area have their frequencies assigned to avoid such images.

Image Rejection Ratio:

                     To determine the suppression factor of tuned ckt image rejection ratio is used. The image rejection ratio, or image frequency rejection ratio, is the ratio of the intermediate-frequency (IF) signal level produced by the desired input frequency to that produced by the image frequency. The image rejection ratio is usually expressed in db.

            Mathematically,

where Q=Quality factor, f_img=Image frequency, fs=RF Signal frequency

                                                                  

Note that IMRR is not a measurement of the performance of the IF stages or IF filtering (selectivity); the signal yields a perfectly valid IF frequency. Rather, it is the measure of the bandpass characteristics of the stages preceding the IF amplifier, which will consist of RF bandpass filters and usually an RF amplifier stage or two.

NOTE: The image frequency should be suppressed before the mixer stage.Practically IRR Should be as high as possible ,so the tuned circuits are connected in cascade.if X is the IRR Of tuned ckt 1 &Y is the IRR of tuned ckt 2 then IRR of the cascaded stage is  X*Y.To improve the IRR either Q Factor or IF should be increased but to increase IRR we practically prefer to increase IF Because increasing Q Factor causes decrease in Bandwidth.

GSM Network Architecture

            A GSM network is made up of multiple components and interfaces that facilitate sending and receiving of signalling and traffic messages. It is a collection of transceivers, controllers, switches, routers, and registers.

            A Public Land Mobile Network (PLMN) is a network that is owned and operated by one GSM service provider or administration, which includes all of the components and equipment as described below. For example, all of the equipment and network resources that is owned and operated by Cingular is considered a PLMN.

Mobile Station (MS)

The Mobile Station (MS) is made up of two components:

Mobile Equipment (ME) This refers to the physical phone itself. The phone must be able to operate on a GSM network. Older phones operated on a single band only. Newer phones are dual-band, triple-band, and even quad-band capable. A quad-band phone has the technical capability to operate on any GSM network worldwide. 

                  Each phone is uniquely identified by the International Mobile Equipment Identity (IMEI) number. This number is burned into the phone by the manufacturer. The IMEI can usually be found by removing the battery of the phone and reading the panel in the battery well.

                  It is possible to change the IMEI on a phone to reflect a different IMEI. This is known as IMEI spoofing or IMEI cloning. This is usually done on stolen phones. The average user does not have the technical ability to change a phone's IMEI.

Subscriber Identity Module (SIM) - The SIM is a small smart card that is inserted into the phone and carries information specific to the subscriber, such as IMSI, TMSI, Ki (used for encryption), Service Provider Name (SPN), and Local Area Identity(LAI). The SIM can also store phone numbers (MSISDN) dialed and received, the Kc (used for encryption), phone books, and data for other applications. A SIM card can be removed from one phone, inserted into another GSM capable phone and the subscriber will get the same service as always.

            Eadch SIM card is protected by a 4-digit Personal Identification Number (PIN). In order to unlock a card, the user must enter the PIN. If a PIN is entered incorrectly three times in a row, the card blocks itself and can not be used. It can only be unblocked with an 8-digit Personal Unblocking Key (PUK), which is also stored on the SIM card.

Base Transceiver Station (BTS) - The BTS is the Mobile Station's access point to the network. It is responsible for carrying out radio communications between the network and the MS. It handles speech encoding, encryption, multiplexing (TDMA), and modulation/demodulation of the radio signals. It is also capable of frequency hopping. A BTS will have between 1 and 16 Transceivers (TRX), depending on the geography and user demand of an area. Each TRX represents one ARFCN.

            One BTS usually covers a single 120 degree sector of an area. Usually a tower with 3 BTSs will accomodate all 360 degrees around the tower. However, depending on geography and user demand of an area, a cell may be divided up into one or two sectors, or a cell may be serviced by several BTSs with redundant sector coverage.

           A BTS is assigned a Cell Identity. The cell identity is 16-bit number (double octet) that identifies that cell in a particular Location Area. The cell identity is part of the Cell Global Identification (CGI), which is discussed in the section about the Visitor Location Register (VLR).

                             

120 ° Sector

The interface between the MS and the BTS is known as the Um Interface or the Air Interface.

Um Interface

Base Station Controller (BSC) - The BSC controls multiple BTSs. It handles allocation of radio channels, frequency administration, power and signal measurements from the MS, and handovers from one BTS to another (if both BTSs are controlled by the same BSC). A BSC also functions as a "funneler". It reduces the number of connections to the Mobile Switching Center (MSC) and allows for higher capacity connections to the MSC.

          A BSC my be collocated with a BTS or it may be geographically separate. It may even be collocated with the Mobile Switching Center (MSC).

                                                    Base Station Controller

The interface between the BTS and the BSC is known as the Abis Interface

Abis Interface

The Base Transceiver Station (BTS) and the Base Station Controller (BSC) together make up the Base Station System (BSS).

Base Station System

Mobile Switching Center (MSC) - The MSC is the heart of the GSM netowrk. It handles call routing, call setup, and basic switching functions. An MSC handles multiple BSCs and also interfaces with other MSC's and registers. It also handles iner-BSC handoffs as well as coordinates with other MSC's for inter-MSC handoffs.

Mobile Switching Center

The interface between the BSC and the MSC is known as the A Interface

Gateway Mobile Switching Center (GMSC)

There is another important type of MSC, called a Gateway Mobile Switching Center (GMSC). The GMSC functions as a gateway between two networks. If a mobile subscriber wants to place a call to a regular landline, then the call would have to go through a GMSC in order to switch to the Public Switched Telephone Network (PSTN).

Gateway Mobile Switching Center

For example, if a subscriber on the Cingular network wants to call a subscriber on a T-Mobile network, the call would have to go through a GMSC.

Connections Between Two Networks

The interface between two Mobile Switching Centers (MSC) is called the E Interface

Home Location Register (HLR) - The HLR is a large database that permanently stores data about subscribers. The HLR maintains subscriber-specific information such as the MSISDN, IMSI, current location of the MS, roaming restrictions, and subscriber supplemental feautures. There is logically only one HLR in any given network, but generally speaking each network has multiple physical HLRs spread out across its network.

Visitor Location Register (VLR) - The VLR is a database that contains a subset of the information located on the HLR. It contains similar information as the HLR, but only for subscribers currently in its Location Area. There is a VLR for every Location Area. The VLR reduces the overall number of queries to the HLR and thus reduces network traffic. VLRs are often identified by the Location Area Code (LAC) for the area they service.

Visitor Location Register 

Location Area Code (LAC)

A LAC is a fixed-length code (two octets) that identifies a location area within the network. Each Location Area is serviced by a VLR, so we can think of a Location Area Code (LAC) being assigned to a VLR.

Location Area Identity (LAI)

An LAI is a globally uniqe number that identifies the country, network provider, and LAC of any given Location Area, which coincides with a VLR. It is composed of the Mobile Country Code (MCC), the Mobile Network Code (MNC), and the Location Area Code (LAC). The MCC and the MNC are the same numbers used when forming the IMSI.

                                                 

Cell Global Identification (CGI):

The CGI is a number that uniquely identifies a specific cell within its location area, network, and country. The CGI is composed of the MCC, MNC, LAI, and Cell Identity (CI)

               The VLR also has one other very important function: the assignment of a Temporary Mobile Subscriber Identity (TMSI). TMSIs are assigned by the VLR to a MS as it comes into its Location Area. TMSIs are unique to a VLR. TMSIs are only allocated when in cipher mode.

The interface between the MSC and the VLR is known as the B Interface and the interface between the VLR and the HLR is known as the D Interface. The interface between two VLRs is called the G Interface.

B & D Interfaces

Equipment Identity Register (EIR) - The EIR is a database that keeps tracks of handsets on the network using the IMEI. There is only one EIR per network. It is composed of three lists. The white list, the gray list, and the black list.

              The black list is a list if IMEIs that are to be denied service by the network for some reason. Reasons include the IMEI being listed as stolen or clonedor if the handset is malfunctioning or doesnt have the technical capabilities to operate on the network.The gray list is a list of IMEIs that are to be monitored for suspicous activity. This could include handsets that are behaving oddly or not performing as the network expects it to.The white list is an unpopulated list. That means if an IMEI is not on the black list or on the gray list, then it is considered good and is "on the white list".

The interface between the MSC and the EIR is called the F Interface.

Equipment Identity Register

Authentication Center (AuC) - The AuC handles the authentication and encryption tasks for the network. The Auc stores the Ki for each IMSI on the network. It also generates cryptovariables such as the RAND, SRES, and Kc. Although it is not required, the Auc is normally physically collocated with the HLR.

Authentication Center

There is one last interface that we haven't discussed. The interface between the HLR and a GMSC is called the C Interface. You will see it in the full network diagram below.This completes the introduction to the network architecture of a GSM network. Below you will find a network diagram with all of the components as well as the names of all of the interfaces.

Full GSM Network