Induction Course (Radio)

Limiters

7

LIMITERS

IntroductionLimiting amplifiers are used in transmitter. During a transmission, there may be peaks occasionally which may overload the modulator valves and trip the transmitter. The occasional peaks may be for a short duration and it can not be controlled manually. Modulation of the transmitter also has to be kept very high of the order of 70% or more on average. To take care of all these things, limiting amplifiers are used in transmitter.

AM LIMITER (ME 277 DX)

In AIR Limiting Amplifiers of Meltron make ME 277 DX Limiter are being used in large numbers. Many of the stations may also be using Limiting Amplifiers of BEL make. The principle of working and the modules used in both the Limiting Amplifiers are same. In the Meltron Limiter, integrated circuits (mostly operational amplifiers) are used. Whereas the same function is realised in BEL Limiter using transistors.

Over driving the audio stage of transmitter is protected by the Limiting Amplifier by employing a combination of pre-delay and feed back control. The audio signal is delayed by approximately 300 micro second during which time necessary control signal is calculated and set. The limiter (both Meltron and BEL) uses the combination of compressor and expander circuits, which increases the audio level (density) as is necessary for extending broadcast in fringe area.

Broadcast Specifications of ME 277 DX Limiter

Inputs :Input Impedance 600 Ohms

Nominal Input level- 20 dBm to + 15 dBm

Maximum Input level+ 24 dBm (12.29 V approx.)

Outputs :Balanced floating impedancemaximum 40 ohms.

Nominal output level- 20 dBm to + 15 dBm

Frequency Response30 Hz to 15 kHz + 0.3 dB

Static Settings :

Limiter thresholdAt internal nominal level

Compressor Ratio2 to 6 adjustable

Compressor Gain0 to 18 dB adjustable

Expander (switchable)- 45 to -65 dB adjustable

Expander ratio2 fixed

Dynamic Settings :

Attack time : Less than 100 micro secs. Imperceptible due to 300 micro secs delay of the signal.

Release time : Automatic, programme dependent, switchable to manual and can be adjusted from 0.5 to 7 seconds.

Signal to Noise Ratio :

Gain = 0Expander ON-76 dB

Gain = 18Expander ON-36 dB

Gain = 18Expander OFF-59 dB

Harmonic Distortion :Less than or equal to 0.2%.

Brief working of the Limiting amplifier

Meltron Limiter ME 277 DX is a control amplifier. It consists of a limiter, a compressor and expander. The peaks of the audio signal at the input of the transmitter is prevented for exceeding a predetermined amplitude and Low level signals are amplified.

The audio signal is controlled using a multiplier signal is weighted by a factor 'g' such that the output signal corresponds to the desired function.

Fig. 1

When g = 1, the output = input level

g greater than 1, input is amplified at the output

g less than 1, input is reduced at the output

The factor g depends upon the program signal and is determined in the amplification computer. In the DX Limiter the Gain Control factor 'g' is obtained with a combination of signal delay of about 0.3 milli second and feed back control. As shown in fig. 2, the input signal is delayed before it reaches the multiplier. The amplification factor g is determined during the delay period.

Fig. 2 Block Diagram

The amplification factor g itself is ascertained using a feed back regulation system which consists of a GAIN COMPUTER and Auxiliary multiplier. The auxiliary multiplier and the multiplier employ identical circuitry, so that g serves directly as a factor for both.

The delay time 0.3 milli sec is brief enough not to disturb comparative listening between the input signal and output signal, but sufficiently long to allow the control process to have been completed when the input signal emerges from the delay network, independent of over driving at the input and practically imperceptible to ear.

Static Characteristics

As indicated previously the DX-Limiter includes a limiter, compressor and expander.

Limiter

The limiter prevents all signals from exceeding the pre determined threshold, while signals at power levels are not affected. The limiter responds to peak amplitudes. Short signal peaks that are not indicated by conventional level meters like VU meter because of the time constants employed will nevertheless lead to signal limiting. The limiter will thus protect the transmitting tubes against arcing.

Compressor

The compressor is to increase the loudness of the modulation signal. This enable broadcasting coverage for instance to be extended without increasing transmitter power.

The compressor function is by employing a non constant amplification characteristics at medium level. The closer the level approaches the limiting threshold, the more amplification is reduced.

Expander

An expander is linked directly to the compressor. Low level signals down to level immediately above the noise level are expanded. As a result the signal level may be boosted without raising the noise level. Undesirable increase in noise are there by prevented during pauses in the modulation.

Performance of the ME limiting amplifier can be explained first with reference to its static operating curves.

The amplification factor 1 (0 dB) increases at an angle of 45o up to the limiter threshold and the output level is equal to the input level. In parallel to this line is the line depicting the maximum amplification 18 dB to which the compressor can be adjusted. This is termed as compressor gain. In addition to the gain, the compressor is further characterised by the ratio. Any setting between 2 and 6 (2:1 and 6:1) may be made. If the compressor ratio is 6:1 a 6 dB increase in the input level will produce only 1 dB increase in the output levels. The circuit of the compressor designed such that the operating curve is rounded at the top, advancing smoothly to the limiter region without an abrupt change.

The second region of non-constant amplification is created by the expander, which is effective in the lower range of signal. The change in amplification is designed as an expansion ratio : the expander in the ME 277 DX Limiter exhibits a fixed ratio of 1:2 i.e. if the input level increases by 1 dB, the output level is increased by 2 dB. Signals are expanded in an adjustable range such that the threshold of the expander is generally set above the noise level so that only the signal information is amplified. Increases in the noise during pauses in the modulation are thus effectively prevented.

Fig. 3 Ref.Drg.No:-STI(T)446,(DC198)

Dynamic Characteristics

Among the dynamic characteristics, the release performance of a particular interest. All transient events are completed within the delay time and not perceived an unpleasant.

The ME 277 DX Limiter is characterised by a three segment release curve. It begins with a holding phase (1) of 40 mili sec to prevent the limiter from reacting a new to every half-wave of a low-frequency signal, a process which would inevitably cause higher distortion, immediately thereafter, a rapid release phase (2) follows, which is hardly perceived by the ear and prevents excessive level and loudness losses. The original amplification is restored, however, only after a very long, practically inaudible phase (3).

The additional feature of the DX-Limiter is that the release performance is programme dependent, i.e. controlled from the signal characteristics. Very brief over driving in an otherwise low signal level will be followed by rapid release, where as an extended high-level signal with occasionally over-driven peaks will cause a release phase of very long duration. In this manner, for instance, a short signal impulse will not reduce the level for an in appropriate interval, while dreaded "pumping" effects will be prevented to a great extent for signals frequently exceeding the threshold.

Normal Setting of the ME 277 DX Limiter as recommended by DG:AIR :

Expander thresholds- 50 dB

Compressor ratio2:1

Compressor Gain18 dB

Nominal input level- 6 dBm

Nominal output level+ 6 dBm

Threshold of limiting- 6 dBm

The +6 dBm output of limiting amplifier will be followed by a 6 dB multiple pad for feeding 0 dBm input to the transmitter for100% modulation.

For limiting amplifier alignment, 1 kHz is to be fed from the audio console of control room through the normal programme chain. The level of the audio console at control room should be adjusted such that the VU meter indicate + 3VU. Now the limiting amplifier input will be adjusted at threshold of limiting and the output adjusted to 95% modulation. During normal progamme, the VU meter will peak to zero and as such the peak programme level the input of limiter will always be 3 dB less than the threshold of limiting amplifier.

The above alignment once made, may not be disturbed thereafter.

Caution

If the limiter is kept off, the input will be automatically available at the output terminal. Hence during normal transmission, care should be taken that the limiter is switched ON.

Reference

1) Instruction manual, Meltron ME 277 DX-Limiter

2) DG:AIR, New Delhi. P&D Unit Circular No. 26(1)/84-DTD dated 17/20.2.89.

MELTRON PROTOCOL FOR FINAL MEASUREMENT LIMITER ME 277 DX

MELTRON

Maharashtra Electronics Corporation Ltd.,

Audio Visual Division,

MUMBAI - 93.

Standard Test Conditions

Supply voltage:AC 230 V rms. 50 Hz

Temperature:15o C to 35o C.

Relative Humidity:45 - 75%

1.0General:Visual Examination

2.0Mechanical:Operation of controls and switches

3.0Measuring Parameter:

0 dBu :0.775 V

Source Impedance:600 ohms balanced

Load Impedance:600 ohms balanced

3.1 Limiting Threshold (adjustable):- 20 dBu to + 15 dBu

3.2Max. Input level:+ 24 dBu

3.3Output level Range:- 20 dBu + 15 dBu

3.4 Expander threshold:- 45 to - 65 dB

Adjustability (Below Lim. Threshold)

3.5Compressor Ratio adjustability:(2:1 to 6:1)

3.6Compressor Gain adjustability:(0 to 18 dB)

3.7 Gain/Ratio I & II selectable

4.0Performance Test:

4.1Nominal Input level:- 6 dBu

(Threshold of limiting, LED just OFF, Meter Indication 0)

4.2Nominal Output Level:-56 dBu

4.3Expander Threshold:- 56 dBu

4.4Compressor Gain:18 dB

4.5Compressor Ratio:2:1

5.0Frequency Response

COMP OFF, EXP OFF, REF, FREQ, 1 kHz

Frequency in Hz 30 63125 300 1K 2K 4K 8K 10 K 12K 15K Limits

Input -6 dBu -0.26 -0.06 0 0.02 0 -0.02 -0.08 -0.11 -0.10 -0.09 -0.13 + 0.3

Output +6 dBu

Input -11 dBu -0.25 -0.06 0 0 0 -0.02 -0.09 -0.11 -0.11 -0.04 -0.15 + 0.3

Output +6 dBu

6.0 Signal to Noise Ratio

Unweighted in dBComp. Gain 0

Exp. ONComp. Gain 18

Exp. ONComp. Gain 18

Exp. OFF

Limits767659

Input - 6 dBu

Output + 6dBu77.7677.6260.89

7.0 Distortion

Thd IN %30 Hz1 kHz15 kHzOutput LevelLimits

At Nom. Level (- 6dBu)

Lim. Threshold (-6dBu)0.07%0.06%0.06%+ 6dBu0.3%

At 15 dB above nom. Level (+9 dBu)

Lim. Threshold (+9 dBu)0.13%0.07%0.09%+ 6 dBu1%

8.0 Limiter Specifications

8.1AttacKTime(0 (s)

8.2Release Time(3 sec.) typical (manual)

8.3Limiter Headroom15 dB min.

8.4 Limiter Characteristics

Input level in dBu-6-4-20+2+4+8+10Limits (-6 to +9)

Meter Indication0-2-4-6-8-10-14-16+ 1.5

Output level in dBu6.016.076.116.136.146.146.136.13+ 0.2

9.0Compressor Specification:Nom. Input Level : -6 dBu

Nom. Output level : +6 dBu

9.1Attack time:2.5 ms

9.2Release time:0.5 to 7 sec adjustable

9.3 Compressor Characteristics

INPUT LEVEL IN dBu

Test Conditions-56-46-36-26-16-6

Meter Indication0+10+15+10+50

Typical values in dBu-44-24-9-416

Comp. Gain 18 dB

Comp. Ratio 2:1

Exp : 50-43.91-24.11-9.30-3.481.236.02

10.0Endurance Test:(24 Hrs)

FM LIMITER (EMT 266 TX)

Limiters are commonly employed at the programme input of FM Transmitters to protect against over-deviation. Accurate control of the programme level by a suitable method is absolutely essential for maintaining the peak deviation below the permitted maximum value. Usually the level control proceeds the pre-emphasis. If we consider a reference deviation of + 40 kHz corresponding to 0 dB at 500 Hz., we have a margin of only 5.5 dB. But the pre-emphasis circuit that follows, boosts the signal up to 13.5 dB at 15 kHz thereby exceeding the permissible margin by about 8 dB. If the time constant of level meter is taken into account, the increase would be still higher. Without limitation or peak clipping this would lead to intolerable peak deviations. In case the programme level control is affected after the pre-emphasis, over modulation can be avoided, but the average deviation would be reduced unnecessarily.

It has thus been realised that the conventional limiter together with the constant pre-emphasis of 50 microseconds cannot provide any protection. In conventional limiting amplifiers gain variations affect all sound signals equally i.e. they are not frequency dependent. Tests have shown that the shortcomings of conventional limiters can be avoided by employment of frequency selective form of limiter.

As long as the resulting output signal is much below the nominal level, standard 50 micro second pre-emphasis is employed. When high frequency components of the programme signals exceed the prescribed limits, pre-emphasis is reduced momentarily to avoid overloading. Principle of operation of variable or adaptive pre-emphasis followed in the New Transient Limiter is illustrated in fig. 4. Here pre-emphasis time constant is reduced by shifting the start of the boost sufficiently towards higher frequencies until the established threshold can be maintained. The variable time constant is implemented by a quite simple circuit. The signal is reproduced linearly and over a separate path, with a 6 dB per octave increasing gain characteristics, and the results are added. The level of the portion with increasing gain is controlled by a multiplier and thereby determines the pre-emphasis corner frequency when both portions are summed together.

In AIR VU meters are normally used for aligning and monitoring of levels of programme chain. The signal levels read on the VU meter are not necessarily the true signal levels of the bus they are monitoring. Since the VU meter is an average reading meter, the peaks touching 0VU on VU meter may be actually 6 to 8 dB higher. This is very important factor, which is normally over looked while deciding the threshold point. Till peak reading meters are introduced in the department, sufficient headroom is necessary to accommodate the margin of peak readings. Thus, in order to arrive at proper line up settings, implications arising from various options require due consideration.

Fig. 4

LINE UP PROCEDURE FOR FM TRANSMITTER WITH

EMT 266 TX TRANSIENT LIMITER

Salient Features

The Salient features of these limiters are :

i) The programme signal is internally pre-delayed thereby allowing control circuitry to reduce the total gain of the unit before over driving occurs at the output. This ensures proper limiting without the production of any Transients.

ii) A switchable adaptive pre-emphasis is provided in the limiter itsel,f which can be used for while keeping off the transmitter pre-emphasis. Threshold of pre-emphasis can be set to any value between 0.5 to 5.5 dB above limiter threshold for mid frequency signals.

iii) A switchable fixed de-emphasis circuit is provided. This is to be switched on for measurement purposes or when the pre-emphasis of the transmitter cannot be switched off.

iv) A switchable compressor ON/OFF mode is provided. In the off mode no compressor is used and the limiter limits the signals sharply when the levels exceed the threshold. In the ON mode of the compressor, the sharp limiting at the threshold is made rounded and smooth.

v) Limiter threshold can be set at nominal level which is adjustable between -20 to + 15 dBu (0 dBu = 0.775 V).

vi) A clipper threshold is also available to set the maximum deviation limit.

vii) Release time of 3 sec. Or Auto dependent on programme levels is selectable.

viii) The limiter gets by passed through the main relay contacts just by switching off the main ON/OFF switch,

ix) Output of the limiter is adjustable between -20 dBu to + 15 dBu.

Controls Available/Factory Settings

The following controls are available for adjustment in these limiters. The values/positions of control set in the factory are also given.

1. Input Adj. (R 108 on Input Board) :

The input level can be adjusted by this potentio meter from -20 to + 15 dBu continuously in two plug in ranges -20 to 0 and 0 to +15 dB. Nominal input level has been set to + 6dBu in the range 0 to +15 dBu.

2. Limiter Threshold : (R 510 - on Gain Computer Board).

This is set at nominal input level (+6 dBu in this case).

3. Threshold of adaptive pre-emphasis : (R 439 - on Pre-emphasis Board) :

This is adjustable between 0.5 and 5.5 dB above limiting threshold, factory setting = 2.0 dB.

4. Clipper Threshold : (R-432 on Pre-emphasis Board) :

Adjustable between 1 and 6.5 dB above limiting threshold. Factory setting = 2dB.

5. Output Level : (R-219 - on Output Board) :

Adjustable between -20 to + 15 dBu continuously in two plug in ranges -20 to 0 and 0 to +15 dBu. Adjusted for + 4 dBu (at 500 Hz).

6. Release time : (J 501 - on Gain Computer Board) :

Selected between 3 sec.auto selected for auto mode (Programme dependent).

7. Compressor ON/OFF : (J 502 - on Gain Computer Board)

Selected for 'OFF' mode.

8. Pre-emphasis : (J 403 - on Pre-emphasis Board) :

Selectable 75 to 50 micro second, 50 micro sec. is selected.

9. Adaptive Pre-emphasis ON/OFF : (S 200 on output Board) :

Selected for 'ON' position.

10. De-emphasis ON/OFF : (J 404 on Pre-emphasis Board)

Kept 'OFF'.

Instruments Required

1. Low distortion audio oscillator (alternatively output from tone generator in switching console can also be used).

2. Level meter (alternatively a VU meter available on console/programme input rack can be used).

Line up Procedure for Audio Chain and FM Transmitter :

Switch on the transmitter to the rated power.

Switch off pre-emphasis in transmitter.

Ensure the limiter setting as follows :

PEON

Ad PE50 Micro sec.

Com.OFF

DEOFF

Release TimeAUTO

Note : Unit is calibrated at factory for a nominal input level of + 6 dBu and output level + 4 dBu and need not be disturbed until stated otherwise.

(a) For Co-sited FM Station :

Feed 500 Hz sinusoidal tone via switching console and adjust the input to the limiter as 0 VU with VU meter or level meter. Green LED on limiter (6 dB below threshold) should have tendency to illuminate or will just illuminate. Adjust input level control (INADJ) on the limiter input card if required.

Check the output of the limiter at 'LIM OUT' jack on the console with level meter or VU meter. It should read - 2 dBu. Adjust output control on the output card of limiter if required.

Adjust the AF level adjuster (BCD Switches) on the front panel of the transmitter to get the deviation of + 30 kHz on exciter meter/FKDL demodulator.

Adjust the output level of the switching console again to give 6 dB above 0 VU at 500 Hz. The input of the limiter will be 6 dBu and red LED should have tendency of illumination or will just illuminate.

Check the deviation on Exciter meter/FKDL demodulator. It should be + 60 kHz.

Limiter will now limit all input signal peaks above + 6dBu. The transmitter deviation will be limited to + 60 kHz at 500 Hz and + 75 kHz at high frequencies.

Feed 7 kHz at + 6 dBu and check that deviation does not exceeds. Under these settings green LED will be glowing very frequently, Red LED will be glowing occasionally and yellow LED will be glowing rarely.

Note :If the deviation are not coming as stated above, limiter should be removed from the circuit and calibration may be done according to the technical manual.

(b) For Non-Cosited FM Station :

Feed 500 Hz sinusoidal tone from switching console and adjust the output level as 0 VU at line out jack or STL in jack -in-control room.

Adjust attenuator 'At 2' of programme input rack at transmitter to get -20 dBm at the input of Equalised line Amplifier ("Equalised line Amp. IN")

Adjust gain of Equalised Line Amplifier to get 0 dBm at the input of Limiter (Jack Lim IN").

Align limiter and transmitter as per steps described above from a (i) to a (vi) of co sited FM stations.

Note : In case of 7 kHz at step a (vi) adjust HF gain of line equaliser to give + 6dBu at input of limiter.

Caution Notes :

Pre-emphasis in Transmitter is to be kept OFF.

De-emphasis in Limiter is to be kept OFF.

Once aligned, controls should not be disturbed.

Line up to be checked before every transmission by feeding 500 Hz at 0 VU from control room. FKDL demodulator of FM transmitter should read + 30 kHz deviation.

Whenever Limiter is by passed or removed the pre-emphasis in the transmitter must be switched ON.

STI(T) Publication188004/IC(Radio)/2006STI(T) Publication187004/IC(Radio)/2006

_1052122140.doc

X

g

AUXILARY MULTIPLIER

IN

OUT

MULTIPLIER

X

GAIN

COMPUTER

DELAY

_1052924263.doc

Multiplier

Adaptive

Pre-emphasis

Output

Time

Delay

Gain

Computer

Input

Aux.

Block Diagram of ME 266 X

Transmitter Limiter

Multiplier

_1052924172.doc

Output level

-10

Input level

0

10

20

Static operating curve of the ME 266 X

Transient diameter

(b) Variable Pre-emphasis

-20

-30

dB

20

dB

10

0

-10

-20

-30

dB

-20

-10

10

0

20

30

20

f

200

P

2K

Hz

20K

_1052120049.doc

X

g

INPUT

OUTPUT

gx

x