1 / 26

Microwave Synthesisers

Microwave Synthesisers. Grant Hodgson G8UBN. Crawley Roundtable 2007. Microwave Synthesisers. Synthesiser - UK Synthesizer - US. Microwave Synthesisers Why would we want a synthesiser?.

thadine
Télécharger la présentation

Microwave Synthesisers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 > 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, • N=1152000000/500000 = 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 (2**21 = 2,097,152) • Example :- 20MHz comparison freq., o/p freq = 2320.905MHz, N=116.04525

  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 - >1000 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 (>1 GHz) with lower spurs • Amateur Microwave designs will benefit from synthesiser technology.

More Related