Microwave%20Synthesisers

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Microwave Synthesisers

• Grant Hodgson G8UBN

Crawley Roundtable 2007
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Microwave Synthesisers

• Synthesiser - UK
• Synthesizer - US

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Microwave SynthesisersWhy would we want a
synthesiser?

• Nearly all current microwave activity is based
  around a crystal-controlled transverter a
  synthesised transceiver.
• This can cause problems when the band is
  fragmented (not contained within a 2MHz segment)
  -
• 13cms - at least 8 different sub-bands
• Personal unattended beacons at 10.4GHz
• 5558MHz / 5760MHz etc.

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Microwave SynthesisersWhy would we want a
synthesiser?

• Synthesisers (usually) allow for great frequency
  agility - the ability to change frequency,
  sometimes very quickly
• For example a DDS can be used to directly
  modulate a carrier
• A synthesised transverter or transceiver would
  allow for gt 2MHz coverage

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Microwave SynthesisersWhat is a synthesiser?

• Synthesis - the process of combining items into a
  complex whole

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Microwave SynthesisersWhat is a synthesiser?

• An oscillator is not a synthesiser
• A frequency multiplier is not a synthesiser
• A frequency converter is not a synthesiser
• However, combing the above can form a synthesiser

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Microwave SynthesisersThree basic types

• Direct synthesis
• Indirect synthesis -
• Phase locked loop
• Direct digital synthesiser

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Microwave SynthesisersDirect Synthesis

• Direct synthesis -
• The frequency is generated with circuit blocks
  performing simple mathematical functions -
• Addition, subtraction, multiplication and
  division
• Can be FM/PM modulated - with care

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Microwave SynthesisersDirect Synthesis
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Microwave SynthesisersDirect Synthesis

• Advantage -
• Best phase noise performance
• Can be multiplied (almost) without limit
• Disadvantages -
• Very inflexible - frequency cannot be changed

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Microwave SynthesisersIndirect synthesis - phase
locked loop

• A variable frequency oscillator is locked to a
  stable reference oscillator
• - but not (usually) on the same frequency
• Undoubtedly the most popular type of synthesiser
  - billions in use worldwide

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Microwave SynthesisersPhase Locked Loop
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Microwave SynthesisersPhase Locked Loop

• Advantages -
• Enormously versatile
• wide range of frequencies can be generated
• can generate outputs directly at microwave
  frequencies - no sub-harmonics to be filtered
• Disadvantages
• Phase noise may be an issue

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Microwave SynthesisersPhase Locked Loop

• Integer -N - the output frequency is an exact
  multiple of the reference frequency
• This gives a channel spacing which is the same as
  the comparison frequency
• To change channels, simply change the
  programmable divider (N).
• Example - 500kHz step size, 1152MHz o/p,
• N1152000000/500000 2304

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Microwave SynthesisersPhase Locked Loop

• Fractional -N - the output frequency does not
  need to be an integer multiple of the reference
• Which allows for higher reference frequencies,
  thus improving phase noise.
• Fractional parts can now be very complex -
• up to 21 binary digits (221 2,097,152)
• Example - 20MHz comparison freq., o/p freq
  2320.905MHz, N116.04525

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Microwave SynthesisersDual Phase Locked Loop

• Instead of dividing the output frequency, a mixer
  is used with a second PLL
• This has the advantage of lower phase noise
• But can be considerably more complex
• Multiple loops can be used - for example in
  commercial signal generators

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Microwave SynthesisersDual Phase Locked Loop
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Microwave SynthesisersDirect Digital Synthesiser

• Consists of three basic parts -
• Counter (phase accumulator - up to 48 bits)
• Sine lookup table (up to 14 bits)
• Digital to Analogue Converter

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Microwave SynthesisersDirect Digital Synthesiser
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Microwave SynthesisersDirect Digital Synthesiser
Wanted Actual
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Microwave SynthesisersDirect Digital Synthesiser

• Dominant feature is the very small step size
  (uHz)
• Other advantages - very fast frequency changes
• Can easily be modulated with FM or PM - AM
  available on some newer Ics
• Output frequency up to 40 of clock frequency

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Microwave SynthesisersDirect Digital Synthesiser

• Biggest problem is discrete spurs
• These get multiplied by 20 log N
• Highest output frequency is 400MHz (1GHz clock)
• Therefore DDS has some limitations as to how much
  it can be multiplied.
• Some can get hot (AD9852 - 3W)
• Requires a high frequency clock - either
  externally multiplied or multiplied on chip

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Microwave SynthesisersGPS Disciplined Oscillator

• Similar to PLL, BUT -
• The reference source has short term instability
• but excellent long term stability
• The VCO (usually an OCXO) has very good short
  term stability but drifts slowly over time
• Therefore, a very long time constant is used -
  gt1000 seconds
• The loop is never really locked - hence the
  term disciplined

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Microwave SynthesisersGPS Disciplined Oscillator
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Microwave SynthesisersHybrid PLL/DDS

• Use a DDS as the reference for an integer-N PLL
• With the right components, this has the
  possibility of giving the best of both worlds -
  with exceptional performance, at more cost than
  either a single DDS or PLL.

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Microwave SynthesisersThe future

• PLLs will have lower noise, lower spurs and
  operate at higher frequencies (currently up to
  8GHz).
• DDS will operate at higher frequencies (gt1 GHz)
  with lower spurs
• Amateur Microwave designs will benefit from
  synthesiser technology.