Microwave Synthesisers

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.