Microwave Synthesisers Grant Hodgson G8UBN Crawley Roundtable 2007

Microwave Synthesisers Synthesiser - UK Synthesizer - US 11/22/2018

Microwave Synthesisers Why 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. 11/22/2018

Microwave Synthesisers Why 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 11/22/2018

Microwave Synthesisers What is a synthesiser? Synthesis - the process of combining items into a complex whole 11/22/2018

Microwave Synthesisers What 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 11/22/2018

Microwave Synthesisers Three basic types Direct synthesis Indirect synthesis :- Phase locked loop Direct digital synthesiser 11/22/2018

Microwave Synthesisers 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 11/22/2018

Microwave Synthesisers Direct Synthesis 11/22/2018

Microwave Synthesisers Direct Synthesis Advantage :- Best phase noise performance Can be multiplied (almost) without limit Disadvantages :- Very inflexible - frequency cannot be changed 11/22/2018

Microwave Synthesisers Indirect 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 11/22/2018

Microwave Synthesisers Phase Locked Loop 11/22/2018

Microwave Synthesisers Phase 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 11/22/2018

Microwave Synthesisers Phase 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 11/22/2018

Microwave Synthesisers Phase 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 11/22/2018

Microwave Synthesisers Dual 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 11/22/2018

Microwave Synthesisers Dual Phase Locked Loop 11/22/2018

Microwave Synthesisers Direct 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 11/22/2018

Microwave Synthesisers Direct Digital Synthesiser 11/22/2018

Microwave Synthesisers Direct Digital Synthesiser Wanted Actual 11/22/2018

Microwave Synthesisers Direct 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 11/22/2018

Microwave Synthesisers Direct 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 11/22/2018

Microwave Synthesisers GPS 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’ 11/22/2018

Microwave Synthesisers GPS Disciplined Oscillator 11/22/2018

Microwave Synthesisers Hybrid 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. 11/22/2018

Microwave Synthesisers The 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. 11/22/2018