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Microwave%20Synthesisers

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Biggest problem is discrete spurs. These get multiplied by 20 log N ... DDS will operate at higher frequencies ( 1 GHz) with lower spurs ... – PowerPoint PPT presentation

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Title: Microwave%20Synthesisers


1
Microwave Synthesisers
  • Grant Hodgson G8UBN

Crawley Roundtable 2007
2
Microwave Synthesisers
  • Synthesiser - UK
  • Synthesizer - US

3
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.

4
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

5
Microwave SynthesisersWhat is a synthesiser?
  • Synthesis - the process of combining items into a
    complex whole

6
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

7
Microwave SynthesisersThree basic types
  • Direct synthesis
  • Indirect synthesis -
  • Phase locked loop
  • Direct digital synthesiser

8
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

9
Microwave SynthesisersDirect Synthesis
10
Microwave SynthesisersDirect Synthesis
  • Advantage -
  • Best phase noise performance
  • Can be multiplied (almost) without limit
  • Disadvantages -
  • Very inflexible - frequency cannot be changed

11
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

12
Microwave SynthesisersPhase Locked Loop
13
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

14
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

15
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

16
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

17
Microwave SynthesisersDual Phase Locked Loop
18
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

19
Microwave SynthesisersDirect Digital Synthesiser
20
Microwave SynthesisersDirect Digital Synthesiser
Wanted Actual
21
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

22
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

23
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

24
Microwave SynthesisersGPS Disciplined Oscillator
25
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.

26
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.
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