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Listado de Notas de Aplicaci贸n

CONVERSION DBI TO DBD

dBi is the gain measured in relation to an isotropic antenna (an ntenna that radiates the same energy in all directions).

It is usually the gain indicated in the specifications given for our aerials.

dBd is the gain measured with a dipole.

The relationship between dBi (ref. isotropic antenna) and dBd (ref. dipole) is the following:

dBi= dBd + 2.15;  dBd = dBi - 2.3

Example:

How many dBd in a 16 dBi gain antenna?

Answer:
dBd = 16 - 2.15 = 13.85 dBd

and vice-versa:
How many dBi are 22 dBd?

Answer:
dBi = 22 + 2.15 = 24.15 dBi.

ANTENNA PATTERN DIAGRAM

Most of the antenna manufacturers show the pattern diagram of the antennas in their product catalogues.

However some people do have problems in understanding the information that is being shown.

Let麓s see a generic antenna pattern diagram and make some conclusions:

This is called a polar diagram of an antenna radiation pattern.

It is obtained by rotating the antenna from 0 to 360 degrees whilst its output signal level is measured in relation to the signal level of the transmitter.

The black trace shows the behaviour of the antenna at a glance:

The maximum value is obtained when the antenna is pointing directly to the transmitter ( zero degrees).

At both sides of this direction the power received decreases.

The shape of this measurement is the mentioned black lobe.
There might be more than one lobe.

The A/B ratio is an indication of the front-to-back ratio: it measures how the incoming signal is rejected from its rear.

The smaller is A and the greater is B, the better is this parameter.

The blue angle is where the antenna is radiating most of its energy.

It is measured between two points where the radiated power decreases to half its value (-3 dB).

This angle is the parameter known as the antenna beamwidth, which at the same time is a measurement of the directivity of the antenna.

The smaller this angle the greater the directivity.

CASCADING AMPLIFIERS

Why is the number of cascaded amplifiers limited?

This factor is closely related to the minimum input level:

VMin Input Level= C/N Output + Noise = C/N Output + NF +10 log (ktb)

The minimum input level for an amplifier depends on the desired C/N at the output.

Two parameters are essential in computing it:

  • The noise figure of the amplifier: NF
  • and the thermal noise which is a function of both the temperature and the signal bandwidth (1.8 dBμV for B/G standard signals at 20潞c room temperature)

Therefore, let麓s suppose:

  • The desired C/N for the output: 45 dB (very good ratio)
  • Amplifier NF: 9 dB
  • Amplifier gain: 20 dB

At the output the level will be:

1.8 + 20 + 9 + 45 = 75.8 dBuV

Then the minimum input level is:

vI min = 75.8 - gain = 75.8 - 20 = 55.8dBuV.

This is equal to

vI min = SNR output + Noise = SNR output + NF + thermal noise.

Because of this, when several amplifiers are cascaded, the equivalent NF gets higher.

Therefore, the minimum input level gets bigger and bigger until this minimum input level will be equal to the maximum input level accepted by the amplifier to maintain a given intermodulation level.

Once this point has been reached no more amplifiers can be cascaded.

DERATING TABLE

In this table you can find the reduction of the output level according to the amplifier channel loading (number of channels):

CONCEPTO DE RECHAZO

La figura que sigue ilustra este concepto

  • Ambos trazos en rojo y azul muestran las respuesta en frecuencia de dos amplificadores monocanal, separados un canal.
  • Ambos canales interactuan entre ellos debido a la imposibilidad de obtener una respuesta abrupta enun filtro de radiofrecuencia.
  • Parte de la señal de un canal (por ejemplo el canal azul) cae dentro del otro canal. Esta interferencia puede ser visible en la imagen del TV si el nivel de esta señal (azul) es lo suficientemente alto.
  • Sin embargo, si la respuesta del filtro del canal azul es suficientemente buena como para mantener su nivel por debajo de un valor minimo con relacion al nivel de la señal del canal rojo, la interferencia causada no afectará ala imagen en el TV.
  • La figura muestra el "grado de rechazo" del canal azul contra el canal rojo a esa frecuencia en particular.
  • Cuanto mas abrupta es la respuesta, mayor es el grado de rechazo del filtro frente a otras señales interferentes. En otras palabras, la respuesta del filtro es mejor.
  • Siempre existe un compromiso entre esta respuesta y el coste, debido a la complejidad el filtro cuando se requieren mayores rechazos.
  • Se pueden diseñar filtros muy buenos pero con un coste extra que ha de ser evaluado de acuerdo a la aplicación para la cual se diseñan.

       limites del canal
     

Para esta aplicacion concreta, el canal azul rechaza al canal rojo un valor representado por la longitud de este segmento: supongamos que son 28 dB.

El canal rojo esta interferido por el canal azul, pero esta interferencia esta atenuada por lo menos en 28 dB en el limite de su ancho.

 

REJECTION CONCEPT

The figure below illustrates this concept:

  • Both the red and blue lines show the frequency response of two channel amplifiers, spaced one channel.
  • Both channels interact with each other due to the inherent lack of an abrupt response of any filter in radiofrequency.
  • Part of the signal of one channel (e.g. the blue one) bleeds into the another. This interference can be noticed in the TV picture if its level is high enough.
  • However, if the response of the blue filter is as good as to keep its level under a minimum value with regards the red channel signal, the interference it may cause will not be of concern.
  • The figure shows how much the blue filter response "rejects" the red channel at that particular frequency.
  • The more abrupt the response, the higher the rejection of the filter to other interfering signals.

In other words, the filter response is better.

  • There is always a compromise between this response (the complexity of the filter) and the cost.

Very good filters can be designed but at an extra cost that has to be evaluated according to the application that they are intended for.



AMPLIFICATION TYPES

Large TV distribution networks need amplifiers that meet the more stringent requirements of the CATV designers.

One of the most important problems that the installer faces when installing a TV distribution network is the distortion.

These amplifiers must incorporate circuits that are capable to give outstanding low distortion and broad bandwidths.

Single-ended transistor configurations for final amplification stages are not suitable for these applications, because this sort of amplification produces many second harmonics of all the carried channels.

To overcome these distortions, manufacturers of amplifiers use cascode push-pull circuits.

Cascode configurations in combination with the right transistors, working with the optimised emitter current, significantly reduce distortion by eliminating collector non-linearity.

They also have the advantage of low noise transistors combining high gain in the first stage with low emitter resistance and high shunt resistance.

On the other hand a push-pull reduces distortion even further by halving the high frequency emitter current in each half of the circuits and reduces second order beat by cancellation of the second order products of the output signals.

Finally, there is the power doubler configuration.

It is a configuration based on two standard push-pull modules connected in parallel.

Theoretically, this option doubles the output power without any increase of distortion.

However ,in practice, because of the losses introduced by the 75 ohms matching considerations, the output level might not reach the expected theoretical output level.

GHOSTING

ghosting is a common problem in tv reception caused by interference of a reflected signal.

the reflected signal travels a greater/shorter distance before to reach the tv tuner and then arrives a bit later/earlier which produces ghosted pictures either to the left or to the right of the principal picture.

those that appear to the left are generally due to the ingress of the signal into the system after the headend.

mismatches also produce serious ghosting problems, they are present in devices with poor return loss specifications.

related origin can be corroded or faulty connections.

the installer can reduce this effect by re-siting the antenna or by using antennas featuring greater directivity.

in some cases it is necessary the use of stacked antennas to increase the intrinsic directivity of each antenna.

the teletext can also be affected by short delays dropping out it.

the most common origin of the problem is the reflection due to nearby building to the antenna, as indicated in the picture

DERATING TABLE

GHOSTING.- MISMATCHING

ghosting is a common problem in tv reception caused by interference of a reflected signal.

aerials and amplifiers must be impedance matched to the coaxial cable, typically 75 ohms.

whenever a mismatch occurs in the interconnection between coaxial cable and some other element, part of the signal is reflected back.

next is a possible situation where whenever the distance between the antenna and the amplifier is rather big, the use of a good coaxial cable and perfect connections are needed to guarantee an acceptable picture free of ghosting.


the use of our dat45 antenna is the best choice to avoid this
phenomenon, since it features a fully shielded diecast junction box and a f connector.

apart of this feature, there is the option of the mrd (margin raising device) that performs as a built-in preamplifier into the junction box thus ensuring the best matching between antenna and amplification.

this option can be used since the first time the antenna is installed because it only works when it is powered, being null its influence without powering.

in other words, the mrd has no effect on the received signal when it is off but it works as the best preamplifier when it is on:

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