1. Microwave Synthesisers
Grant Hodgson G8UBN
Crawley Roundtable 2007

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

3. 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.
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4. 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
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5. Microwave Synthesisers What is a synthesiser?
Synthesis - the process of combining items into a complex whole
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6. 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
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7. Microwave Synthesisers Three basic types
Direct synthesis
Indirect synthesis :-
Phase locked loop
Direct digital synthesiser
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8. 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
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9. Microwave Synthesisers Direct Synthesis
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10. Microwave Synthesisers Direct Synthesis
Advantage :-
Best phase noise performance
Can be multiplied (almost) without limit
Disadvantages :-
Very inflexible - frequency cannot be changed
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11. 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
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12. Microwave Synthesisers Phase Locked Loop
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13. 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
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14. 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= / = 2304
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15. 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 = MHz, N=
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16. 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
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17. Microwave Synthesisers Dual Phase Locked Loop
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18. 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
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19. Microwave Synthesisers Direct Digital Synthesiser
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20. Microwave Synthesisers Direct Digital Synthesiser
Wanted
Actual
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21. 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
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22. 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 (AD W)
Requires a high frequency clock - either externally multiplied or ‘multiplied’ on chip
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23. 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’
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24. Microwave Synthesisers GPS Disciplined Oscillator
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25. 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.
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26. 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.
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