Spectrum Analyzer

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Spectrum Analyzer
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Spectrum analyzer

• Type of wave analyzer
• Uses an oscilloscope to display
• Gives a real time picture of spectrum
• Gives graphical display

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Simple spectrum analyzer
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Heterodyne type
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Heterodyne type

• Principle double super heterodyne
• Range in VHF 10 K Hz 300 M Hz
• 1 st Stage If 400 MHz
• i/p image Freq. removed 1st IF LPF used
• Selectivity 1 KHz from 2 nd IF
• 1st LO tuned by varactor diode

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Spectrum analyzer

• Spectrum analyzer should sweep narrow freq
• Span of Freq that is swept dispersion of
  analyzer

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Spectrum analyzer

• Two instabilities
• Drift in 1st LO result- spectrum moves across
  scan. operator can adjust if slow.
• freq in stability. 1st LO - due to noise in
  tuned ckt causes side band. transferred to
  signal. Reciprocal mixing

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Spectrum analyzer

• so 1 st LO is phase locked with a 1 MHz xtal
  oscillator
• 2 nd LO tuning rate narrow determines
  selectivity.
• Display- logarithmic- so special IF amps- called
  log IF amp used

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Spectrum analyzer

• Usefulness of spectrum analyzer dynamic range
• Dynamic range ( Largest- smallest)
• Smallest signal is noise
• Large signal-obtained by inter modulation

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Spectrum analyzer

• MDS -114 10 log (BW/ 1 MHz) NF
• MDS- Min. detectable signal in Db m
• Separation of signals- fn of 2 IF
• Sweep rate- slow for storage displays

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Spectrum analyzer for higher frequencies

• gt1000 MHz
• So oscillator 1000- 2000 Mhz
• YIG oscillator
• Yttrium iron garnet- ferromagnetic molecules
  randomly aligned- used in microwaves

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Spectrum analyzer

• Range extended by harmonic mixing ie, not only RF
  mixed but odd harmonic components

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Spectrum anayzer for high freq
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Fourier transform spectrum analyzer

• Uses Fourier transform
• Uses digital computer
• FFT algorithm
• Also called Fourier analyzer or digital analyzers
• Requires lesser hardware
• Signal can be captured, frozen- can perform
  mathematical operation

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FFT Type

• Not continuous but with finite good
  resolution
• Spectrum found at specific intervals

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FFT Type

• Resolution
• Fr 1/T
• T sampling window time
• F- resolution freq
• No. of data point determine quality of spectrum-
  on Nyquist rate

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FFT Type

• A low freq. Signal for 1s
• FFT spectrum analyzer calculates spectrum as if
  sampled data within a window is repeated
• Bcoz i/p beyond this window - unknown

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FFT Type

• For 1 s data if sampling rate is 1 hz 500
  points will be displayed

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FFT Type

• Dynamic range
• Rd 20 log 2 N
• N- no. of bits in digitalization

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FFT Type

• Since FFT samples a fixed amount of time spectrum
  represented as a window, data periodically
  repeated.

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FFT Type

• Windows open gradually
• Windows. Rectangular, Hamming or Hann window
• selectable windows

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FFT Type

• FFT analyzer restricted to low freq due to
  performance of ADC
• Speed of conversion improved by Flash converter-
  range up to 500 KHz.

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Block diagram
LPF
Attenr
ADC
Computer
DAC
To V
To H plates
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FFT Type
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Frequency measurement
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Freq. Measurement

• principle- gating i/p into a counter of
  predetermined value
• Predetermined value 1s exact
• Gates AND or OR
• Heart is decade counter sync counter or BCD
  look ahead or carry forward type improves speed

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Freq. Measurement

• 7 segment display
• Scan oscillator counter is used
• Display const as long as measure

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Freq .Measurement

• 4 sequence of events can be controlled by time
  base
• Resetting counter
• Opening gate
• Closing count gate
• Storing gate count
• Opening and closing critical, accurate so temp.
  controlled Xtal oscillator used

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• If i/p is weak amplified . Since signal unknown
  amplifier provided

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Time measurement
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Time measurement

• 2 i/p one to close other to open, one internal
  clock
• gives interval b/w signal

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Frequency measurement
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Period measurement
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Period Measurement

• Determine period of pulse b/w rising edge
  falling edge

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Time measurement
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