RADIO FREQUENCY SUBSYSTEM

The RF subsystem usually contains: On receiving side Low noise amplifying equipment and

equipment for routing the received carriers to the demodulating channel.

On transmitting side Equipment for coupling the transmitted

carriers and power amplifier

SYSTEM NOISE TEMPERATURE

Line loss: Consider a lossy line having power loss ‘L’.

Let TL, the effective noise temperature of line. TL = (L-1) TF

Where TF= Thermodynamic temperature (close to 290K)

If ‘G’ is the gain of the lossy line then G=1/L

COMPOSITE NOISE TEMPERATURE

TCOMB = TL + (TR/ GL)

= TL + L TR

=(L -1)TF + L TR

RECEIVING EQUIPMENT

The noise temperature may be determined at two point

At the antenna output, before the feeder losses, temp T1

At the receiver input, after the losses, temp T2

Temperature T1 is the sum of the noise temperature of the antenna TA and the noise temperature of the subsystem (feeder and the receiver in cascade)

T1= TA + TCOMB

TCOMB= TFRX + (TeRX/GFRX)

= (LFRX - 1) TF + (TeRX/GFRX)

T1 = TA +(LFRX - 1) TF + (TeRX/GFRX) At a receiver input, the noise must be attenuated by a factor

LFRX

T2 = (T1/LFRX)

T2 = (TA/LFRX) + TF (1 - 1/LFRX) +TeRX

The noise temperature T2 (noise generated by the antenna and the feeder together with the receiver noise) is called the SYSTEM NOISE TEMPERATURE at the receiver input.

RECEIVER FRONT END BLOCK DIAGRAM

TeRX = TLNA + (TFRX/GLNA) + (TMX/GLNA G1) + (TFRX2/ GLNA GMX G1) +( TIF/ GLNA G1 GMX G2)

substituting G1=1/L1 and G2=1/L2

TeRX = TLNA + (TF(L1-1)/GLNA) + (L1 TMX/GLNA ) + (TF(L2-1) L1/ GLNA GMX) +(L1 L2 TIF/ GLNA GMX )

At a given antenna temperature TA,

The total system temperature T = TA + TeRX

Our main goal is to reduce the total system temperature at receiving side

TeRX = TLNA + (TF(L1-1)/GLNA) + (L1 TMX/GLNA ) + (TF(L2-1) L1/ GLNA GMX) +(L1 L2 TIF/ GLNA GMX )

So at a given antenna temperature, the system noise temperature T is reduced by

1. Minimising the feeder loss (i.e L1 and L2)

2. Increase the gain of amplifier ( eg GLNA)

Contribution of the feeder loss is efficiently reduced by locating the first stage of the receiver as close as possible to the antenna feed

For small stations, frequency conversion and low noise amplification (LNA) can be combined in equipment called LOW NOISE BLOCK (LNB) so that L1=0

LOW NOISE AMPLIFIER

Basic amplifiers: BJTs and FETs BJT cause (shot) noise other than thermal noise and

can provide mediocre performance at high frequency FET noise is mainly due to thermal origin and can be

reduced by selecting

1. the type of semiconductor used

2. the geometric characteristics of the transistor Finally, the appearance of high electron mobility

transistors (HEMTs) has enabled the noise temperature to be further reduced, particularly at high frequencies (20 GHz)

FREQUENCY DOWNCONVERSION

Frequencies are down converted because filtering and signal processing is easy at low frequencies.

Two methods

1. Fullband conversion

2. Carrier by Carrier conversion

TRANSMISSION EQUIPMENT

The available carrier power PT at the antenna

PT = (PHPA)(1/LFTX)(1/LMC)

PHPA: Power of power amplifier

LFTX: Feeder loss between output of the amplifier and the antenna interface

LMC: power loss due to multiple carrier operation

POWER AMPLIFIERS

The power amplifier subsystem uses a tube or transistor power stage which may be associated with a preamplifier and a lineariser.

Tube Amplifier:

1. klystrons

2. Travelling Wave tube Tube amplifiers enable high powers to be produced. The choice between klystron and TWT depends on

required bandwidth (TWT has large bandwidth); for equal powers, the cost advantage is with klystron.

Transistor amplifier Semiconductor amplifiers provide power upto few

100W. Usually use GaAs FET mounted in parallel Used because of their low cost, linearity and wide

bandwidth

POWER AMPLIFIER CHARACTERISTICS

Non Linearity

The maximum output power at saturation in single carrier operation (PO1)sat is the rated power given in the manufacturers data sheet (PHPA).

With solid state amplifiers that cannot be operated at saturation, the maximum output power is specified by 1dB compression point.

When the carriers are modulated, the intermodulation products which fall within the useful bandwidth of the amplifier behave as a noise.

To limit intermodulation noise when several carriers are amplified simultaneously to a value compatible with the overall link budget requirement, amplifiers are operated below saturation region.

The output back-off(OBO), defined as the ratio of the output power delivered on one of the n carriers(PON) to the saturation power, determines the position of the operating point.

OBO=PON/PHPA

The power delivered at the amplifier output for the carrier concerned is

PON = PHPA* OBO The total back-off is sometimes defined as the ratio of

the total power available on all N carriers to the saturation power in single carrier operation.

LINEARISERS

Used in order to limit the effects of amplifier non-linearity.

Combined with the pre-amplifier, or located before it, most linearisers produce amplitude and phase distortion of the signal in order to compensate for the specific characteristics of the power amplifier .

For a given level of intermodulation noise, the lineariser permits a reduction of back-off (in absolute value); that is the amplifier is operated closer to saturation.

The reduction of back-off provides a considerably greater available carrier power for an amplifier of given saturation power and potential cost, power consumption and bulk reduction.

CARRIER PRE-COUPLING

Carrier coupling can be performed at a low power before power amplification.

The power loss LMC caused by multicarrier operation is given as

(LMC)ES = -(OBO)ES

ADVANTAGES The advantages of pre-coupling lie in the simplicity of

coupling and the flexibility to adapt to changes in the number and bandwidth of carriers.

The number of amplifiers is also minimised.

DISADVANTAGES This mode of coupling introduces a source of

intermodulation noise in the earth segment which affects the overall link budget.

The amplifier must have a sufficient bandwidth to amplify the different carriers (this can prohibit the use of a klystron which would otherwise be more economic).

CARRIER POST-COUPLING

Coupling can also be performed after separate amplification of each carrier.

It is then necessary to have as many amplifiers as there are carriers (plus any back-up equipment).

Each amplifier amplifies only one carrier; the amplifiers can therefore operate at saturation.

Since each carrier is amplified separately, the required bandwidth is limited.