Receiver System Considerations

The input to a wireless transmitter may be voice, video, data, or other information to be transmitted to one or more distant receivers. So, the basic function of the receiver is to demodulate, or decode, the transmitted baseband data. The performance of the receiver depends on the system design, circuit design, and

working environment. To facilitate the discussion, a basic radio receiver as shown in

Figure 2.2 Block diagram of a basic radio receiver.

The antenna receives electromagnetic waves radiated from many sources over a relatively broad frequency range. A preselector bandpass filter can minimize the intermodulation and spurious responses by filtering out received signals at undesired frequencies. The bandpass filter is followed by a low noise amplifier, which has a low noise figure, high gain, and high intercept point, can amplify the possibly very weak received signal and minimize the noise power that is added to the received signal.

Next, a mixer is used to downconvert the received RF signal to a low frequency signal called intermediate frequency (IF). When IF signal is selected by a IF filter, a high gain IF amplifier raises the power level of the signal so that the baseband information can be recovered easily. This process is called demodulation. A local oscillator provides a LO source which should have low phase noise and sufficient power to pump the mixer. The receiver system considerations are listed below [3]:

1. Sensitivity: Receiver sensitivity quantifies the ability to respond to a weak signal.

The requirement is the specified signal-to-noise ratio (SNR) for an analog receiver and bit error rate (BER) for a digital receiver.

2. Selectivity: Receiver selectivity is the ability to reject unwanted signals on adjacent channel frequencies. This specification, ranging from 70 to 90 dB, is difficult to achieve. Most systems do not allow for simultaneously active adjacent channels in the same cable system or the same geographical area.

3. Spurious response rejection: The ability to reject undesirable channel response is important in reducing interference. This can be accomplished by properly choosing the IF and using various filters. Rejection of 70-100 dB is possible.

4. Intermodulation rejection: The receiver has the tendency to generate its own on-channel interference from one or more RF signals. These interference signals are called intermodulation (IM) products. Greater than 70 dB rejection is normally desirable.

5. Frequency stability: The stability of the LO source is important for low frequency modulated (FM) and phase noise. Stabilized sources using dielectric resonators, phase-locked techniques, or synthesizers are commonly used.

6. Radiation emission: The LO signal could leak through the mixer to the antenna and radiate into free space. This radiation causes interference and needs to be less than a certain level specified by the FCC.

2.2.1 Receiver Noise

In many analog circuits, noise figure is a measure of the degradation in the signal-to-noise ratio between the input and output of the component. The noise figure of a system depends on losses in the circuit, kind of the solid-state device, bias applied, and amplification. The noise figure, F, is defined as

o and noise power. By definition, the input noise power is assumed to be the noise power resulting from a matched resistor at T0=290K; that is, Ni=kT0B.

Consider Fig. 2.3 [4], which shows noise power Ni and signal power Si being into a noisy two-port network. The network is characterized by a gain G, a bandwidth

B, and an equivalent noise temperature Te. If we define Nadded as the noise power added by the network, then the output noise power can be expressed as

(

)

G

N₀ = + .

The noise figure can be written as

( )

Figure 2.3 Determining the noise figure of a noisy network.

For a cascaded circuit with n networks as shown in Fig. 2.4, the overall noise figure can be expressed as

1

Figure 2.4 Cascaded noisy circuit with n networks.

2.2.2 Dynamic Range

Dynamic range (DR) is generally defined as the ratio of the maximum input level that the circuit can tolerate to the minimum input level at which the circuit provides a reasonable signal quality. The typical definition of DR is shown in Fig. 2.5 [4]. The dynamic range (DR) is defined as the range between the 1dB compression point and the minimum detectable signal (MDS). If the input power is above this range, the output starts to saturate. If the input power is below this range, the noise dominates.

P_out( dB)

Figure 2.5 Illustrating the dynamic range of realistic mixers, amplifiers, or receivers.

From the 1dB compression point, gain, bandwidth, and noise figure, the dynamic range (DR) of a receiver can be calculated. Expressing the DR in dBm, we can write as

MDS P

DR= _D − .

But, the definition is quantified in different applications differently. Another definition is called the “spurious-free dynamic range” (SFDR). The upper end of the dynamic range is defined as the maximum input level in a two-tone test for which the third-order IM products do not exceed the noise floor. The SFDR is given by

(

)

SFDR= _OIP3− − 3

2 .

where G is the gain of a receiver, POIP3 is the output power at the third-order, two-tone intercept point in dBm.

2.2.3 Third-order Intermodulation

When two signals with different frequencies are applied to a nonlinear system, the output in general exhibits some components that are not harmonics of the input frequencies. Called intermodulation (IM), this phenomenon arises from mixing of the two signals when their sum is raised to a power greater than unity. We are particularly interesting in the third-order IM products at 2w1-w2 and 2w2-w1, illustrated in Fig. 2.6 [1]. These intermodulation products are a troublesome effect in RF systems because

they are difficult to filter from desired channel and may corrupt the desired signal.

Figure 2.6 Intermodulation in a nonlinear system.

The third intercept point (IP3) is a figure of merit for intermodulation product suppression. A high intercept point indicates a high suppression of undesired intermodulation products. Also, it is an important measure of the system linearity. As shown in Fig. 2.7, the magnitude of the IM products grows at three times the rate at which the main signal increases. The third-order intercept point is defined to be at the intersection of the two lines. The horizontal coordinate of this point is called the input IP3 (IIP3), and the vertical coordinate is called the output IP3 (OIP3).

P_in( dB)

Figure 2.7 Growth of output components in an intermodulation test.

For a cascaded circuit, as shown in Fig. 2.8, the following procedure can be used to calculate the overall system intercept point:

y₂( t)

Figure 2.8 Cascaded n nonlinear stages.

1. Transfer all intercept points to system input, subtracting gains and adding losses

dB for dB.

2. Convert intercept point to power (dBm to mW).

3. Assuming all intercept points are independent and uncorrelated, add powers in parallel: