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Tracking 3

Tracking is a process where the local oscillator frequency follows the signal frequency to maintain a constant intermediate frequency. Tracking errors occur if this frequency difference is not precise. Two point tracking uses a padder capacitor and trimmer capacitor to adjust the local oscillator frequency above and below the desired value. Three point tracking uses both capacitors adjusted at three frequencies to minimize tracking error across the entire band. This results in radio stations appearing at their correct positions on the receiver dial.

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Disha Goel
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174 views12 pages

Tracking 3

Tracking is a process where the local oscillator frequency follows the signal frequency to maintain a constant intermediate frequency. Tracking errors occur if this frequency difference is not precise. Two point tracking uses a padder capacitor and trimmer capacitor to adjust the local oscillator frequency above and below the desired value. Three point tracking uses both capacitors adjusted at three frequencies to minimize tracking error across the entire band. This results in radio stations appearing at their correct positions on the receiver dial.

Uploaded by

Disha Goel
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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TRACKING

TRACKING

A process in which the local oscillator frequency


follows or ‘tracks’ the signal frequency to have a
correct frequency difference
IF  f o  f s
Tracking errors
• In AM receivers all the tuned circuits are ganged so that
only one tuning control can be used to vary the capacitors
simultaneously.
• The local oscillator frequency (fo) must be precisely
adjusted to a value which is above the signal frequency (fs)
by IF i.e.
f o  f s  IF

OR IF  f o  f s

• If this tuning is not done precisely then the frequency


difference i.e. (fo – fs) is not constant. This type of error is
known as “tracking errors”.
Superhet tracking

It is the ability of LO to oscillate either above or below the


selected RF carrier by an amount equal to the IF through the
entire frequency band.
Superhet tracking
Solution to the problem of tracking

• Tracking can be two point tracking or three point tracking

TRACKING

TWO POINT TRACKING THREE POINT TRACKING


1. Padder tracking
2. Trimmer tracking
Two Point Tracking

Padder Tracking
Trimmer Tracking
Padder Tracking
Cp

Ls Cosc

• By adding padding capacitor Cp in series with the


local oscillator section, its effective capacitance
decreases, hence local oscillator frequency (fLO )
increases.

• fLO is more than required value, thus tracking error is


positive.
Trimmer Tracking
tracking error

+
Ls CT
min max

0 freq
-

Trimmer capacitor CT is used in parallel with the local


oscillator section, thus effective capacitance increases,
fLO decreases.
Two point tracking can be obtained by selecting Lo &
CT. fLO is lower than desired, hence tracking error is
negative.
Three point Tracking
Cp tracking error

+
Ls CTR 600

0 950 1500
Freq. kHz
-

• By using both trimmer and padder capacitors and adjusting


oscillator inductance correctly at a point between minimum and
maximum RF frequencies, a ‘three point tracking’ curve can be
obtained.
• The three frequencies of correct tracking are normally 600 kHz,
1500 kHz and the geometric mean of the two, i.e. 950 kHz.
• It is possible to keep the tracking error below 3KHz
Three point Tracking

It is possible to keep the tracking error below 3kHz by using


Cp and CT.

The process of adjusting Lo, CT, Cp at three points on the dial is


known as ‘Receiver Alignment’.

Thus radio stations will appear at their correct position on the


dial of the receiver.

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