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6.3 RECEIVER

CHARACTERISTICS

Sensitivity

-the ability to receive weak signals withan acceptable signal-to-noise ratio (S/N)

- it is expressed in terms of the voltage or

power at the antenna terminals

necessary to achieve a specified signal-

to-noise ratio or some more easily

measured equivalent.

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Selectivity

-the ability to discriminate againstinterfering signals.

Shape factor- the ratio between thebandwidths at 6 dB and 60 dB

down for an ideal IF filter-should be as close as possible to 1.

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Shape factorSF =

where

SF = shape factor

60 = bandwidth at 60 dB downfrom maximum

6 = bandwidth at 6 dB downfrom maximum

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Adjacent channel rejection-another way of specifying selectivity

-commonly used w/ channelized

systems such as CB radio-defined as the number of decibelsby w/c an adjacent channel signal

must be stronger than the desiredsignal for the same receiver output.

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Alternate channel rejection

-also used in systems, such as FMbroadcasting ,where stations in the

same locality are not assigned to

adjacent channelsAlternate channel

-is two channels removed from the

desired one ; also known as the

second adjacent channel

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For example

Figure 6.8 Adjacent and Alternate channels

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Distortion

Harmonic distortion-occurs when the frequenciesgenerated are multiples of those in the

original modulating signalIntermodulation

-when frequency components in the

original signal mix in a nonlinear device,creating sum and differencefrequencies

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Uneven frequency response-any unevenness w/in the IF passbandwill affect the audio frequency

responsePhase distortion

-the signal at the receiver output will

not be in phase w/ the input to thetransitter.

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Dynamic Range-the ratio between two signallevels, expressed in decibels

Blocking-also called desensitization or

desense-reduction in sensitivity to thedesired signal

Audio dynamic range-essentially the usable range ofmodulation depth w/ a givencarrier level

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Spurious Responses

Image Frequencies-is any frequency other than theselected radio frequency carrier

that, if allowed to enter a receiverand mix w/ the local oscillator, willproduce a cross-product frequencythat is equal to the intermediatefrequency

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Assuming high-side injection, =

so = +

For the image, =

therefore =

+

=

= + 2

For low-side injection,= 2

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Image rejection

-the ratio of voltage gain at the input

frequency to w/c the receiver istuned to gain at the image

frequency.

=

= 1 +

where Q= Q of the tuned circuit

= voltage gain at image

frequency

= voltage gain at signalfrequency

=

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-end-

-hayla ^___^