SPECTRUM ANALYZERS

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SPECTRUM ANALYZERS

• In what domain does this instrument collect its
  data?
• - frequency domain
• THREE TYPES
• 1. Swept Tuned Frequency
• 2. Fourier (FFT) samples a signal and
  performs DFT.
• 3. Real Time Spectrum analyzer.

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REAL TIME SPECTRUM ANALYZER

• Fixed tuned bandpass filters
• Bar graph or LEDs
• Typically audio band

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SWEPT FREQUENCY SPECTRUM ANALYZER

• R.F. measurements of great bandwidth
• Two methods
• A.)
• Tunable filter for wide range
• Coincides with sweep of CRT
• B.)
• Fixed BPF
• Vary the input frequency.
  

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Types of Measurements

• Modulation
• Noise
• Distortion - Harmonic
• - Intermodulation

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Tuning

• Similar to A.M radio
• Superheterodyne process.
• Heterodyne gt mixing
• Super gt mix to high frequencies.
• Requires a local oscillator.
• Mixer translates to multiple frequencies.
• Difference frquency is targeted by swept tuned
  S.A.

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flo
?
flo-fin
flofin
Mixer
Input Signal
Local Oscillator
L.O. frequency varies due to the ramp of a sweep
generator. Therefore the local oscillator is a
__________.
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Video Filter

• Smoothes the display by averaging the signal.
• Suppresses noise for low level signals.

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I.F. and Local Oscillator

• Select tuning range for the S.A. For example 0
  2.9Ghz.
• The I.F. centre frequency should be greater than
  2.9 Ghz. Select 3.6 Ghz.
• Therefore the L.O. sweep range is _______.
• The difference signal (flo fi) moves from 0 to
  2.9 Ghz.

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fi
flo
flofi
flo-fi
I.F. Filter Response
Image on Spectrum Analyzer
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What generates this signal on an S.A.
Local Oscillator Feedthrough
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Resolution Bandwidth - RBW
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Reducing Resolution Bandwidth
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NOISE AND COMMUNICATIONS

• Where does noise originate in a communication
  system?
• 1. Channel
• DEFINE
• -undesired random variations that interfere
  with the desired signal and inhibit
  communication.
• REMEDIES?
• 1. Reduce Bandwidth
• 2. Equipment
• 2. Increase transmitter power
• 3. Low Noise Amplifiers

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TYPES OF NOISE

• TWO CATEGORIES
• 1. External Noise
• 2. Internal Noise

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EXTERNAL NOISE

• 1. Equipment Noise
• -automobile engines
• - electric motors
• - computers
• - light dimmers
• 2. ATMOSPHERIC NOISE (STATIC)
• - Lightning
• - Noise Blanking
• 3. SPACE NOISE
• - Radiation from the Sun (solar noise)
• - Stellar or Sky Noise (stars)

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INTERNAL NOISE

• What devices or components generate noise?
• -transistors, diodes, tubes, resistors
• - electronic systems and equipment

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TYPES OF INTERNAL NOISE

• Thermal Noise
• - random motion of electrons in a conductor
  due to heat.
• POWER DENSITY of thermal noise is constant
  with frequency. (White Noise)

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THERMAL NOISE CONTIND.

• Half power bandwidth (3 dB)
• Average power
• How can thermal noise power be reduced?
• - decrease temperature
• - reduce bandwidth

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NOISE VOLTAGE

• 
  

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A 300 ohm resistor is connected to the antenna
terminals of a T.V. receiver, at room temperature
(293K), with a bandwidth of 6 Mhz. Determine
noise power and noise voltage applied to the
receiver input.
300 ohm
T.V.
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SHOT NOISE

• Random variations of current flow in active
  devices.
• Examples?
• -Transistors, diodes, tubes, I.C.s
• - due to quantum movement or flow of
  carriers (holes and electrons)(pulses)
• Energy spread over frequency similar to thermal
  noise.

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OTHER INTERNAL NOISE

• Partition Noise
• - where currents separate Ex. BJT
• Excess Noise(Pink Noise)
• -concentration of noise energy at low
  frequencies
• Transit Time Noise
• - at cutoff frequencies, junction delay

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THE SUM OF NOISE FROM DIFFERENT SOURCES

• Take the square root of the sum of the squares
  of the individual voltages.
• Write the equation for total noise voltage.

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SIGNAL TO NOISE RATIO

• Noise power alone is not enough information
• Ratio of Signal to Noise is an important
  communications specification
• S/N is a ratio expressed in dB
• S/N (dB) 10 log PS/PN
• S/N(dB) 20 log VS/VN

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S/N AND NOISE FIGURE

• SN/N
• SINAD (SND)/(ND) Used for F.M. RXs
• Both are power ratios
• Noise Figure NF (S/N)i/(S/N)o
• If expressed in dB NF (S/N)i(dB)-(S/N)o (dB)

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Power is applied to the amplifier without a
signal input. Is noise measured at the output
without a signal?
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Noise Temperature Absolute temperature of a
resistor that when connected to the input of a
noiseless amplifier of the same gain would
produce the same noise at the output of the
amplifier under discussion.
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A receiver produces a (SN)/N (dB) of 14 dB. The
measured output of the receiver with a signal
applied is 5 watts.What noise power does the
receiver produce without a signal applied?
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Ps 100uW Pn 1uW
Ps 1 W Pn 30 mW
Determine the amplifier noise figure as a ratio.
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CASCADED AMPLIFIERS

• Why is noise figure of first stage so important?
• Noise generated in subsequent stages is
  progressively less.

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