1. LO Phase Noise effects on MDS
Gary Lauterbach
AD6FP

2. How could LO phase noise effect MDS?
MDS is determined by Signal/Noise ratio
Only three scenarios are possible
Noise floor increase:
In the presence of strong interfering signals
With only a single weak signal
Signal level decrease
Spreading of signal energy beyond discernable bandwidth
Both: signal decrease and noise increase

3. History of amateur LO phase noise MDS effects
MUD 1996,1997 W8MQW, WA6KBL
Noise floor increases
Theory and negative confirmation with measurements
MUD 2008 W1GHZ
Observations on field MDS tests, no theory of signal or noise effects
MUD 2008 K0CQ
Noise floor increase
Theory with no experimental data
MUD 2004,2009 WA1ZMS
Signal decrease due to spreading
2004: Low BW sub-mmw needs low phase noise
2009: Experimental “confirmation” of W1GHZ observations

4. What is LO phase Noise? Two views: Spectral and Temporal
LO energy spread over spectrum surrounding the LO center frequency
Time jitter of LO waveform zero crossing
Both views are valid and measurements can be translated between them
Can be both deterministic and random
Modulates the LO: AM and FM
FM phase noise creates no additional LO energy

5. Reproducing WA1ZMS results
Brian well documented his experimental setup
But no phase noise plots were included 
Random noise FM modulating a laboratory signal generator
Brian used:
Homemade noise source
HP 8640a signal generator
I wound up using:
HP 3561a random noise source
HP 8662a signal generator
8640a is not sufficiently stable for very close-in phase noise measurements

6. Brians measured spectrum
FM noise modulation producing 2db MDS degradation

7. My measured spectrum of WA1ZMS -2db MDS experiment
144 MHz FM modulated with random noise

8. Phase Noise Measurement
Pair of HP 8662a signal generators
First 8662a is EFC locked through a 0.1 Hz BW PLL to the DUT for PN measurements
Switchable 40 db gain baseband LNA
Second 8662a is a noise modulated source
HP 70210a Spectrum analyzer
HP 3561a random noise source
KE5FX USB baseband digitizer
KE5FX TimeLab measurement software

9. WA1ZMS -2db MDS phase noise plot
Note the 20db/decade phase noise increase
Unlike any real world LO
F^-2 over entire PN range of interest
Integrated power over Hz is a significant % of LO total power

10. Here’s the visible MDS effect
SDR-IQ 10 MHz IF of 154 MHz signal and 144 MHz LO:
Left: clean LO
Right: -52dbc 1 KHz WA1ZMS noisy LO
Signal spreading causes S/N decrease of >1.5 db in 27 Hz BW
Noise floor didn’t go up

11. Why 1 KHz offset PN is a bad metric
1 KHz offset PN is not a predictor of sub 100 Hz PN
Sub 100 Hz offset PN effects “purity” of narrow band weak signals
CW: 30 Hz “ear” BW, <100Hz PN matters
JT: 2.5, 5, 10 Hz BW, <20Hz PN matters

12. What about the K0CQ, WA6KBL Noise floor increase theory?
HP 70210a capture of 1 GHz noise source down converted to 100 MHz IF
Top: clean LO
Bottom: WA1ZMS 1 KHz LO
Noise powers match to <0.5 db

13. Why doesn’t the noise floor increase?
Think of the temporal jitter model:
LO Spectral energy is not simultaneous at all frequencies but is a probability density function (PDF) over time
As the LO jitters its energy is spread over a range of frequencies at different times
The input noise to the down converter has a completely level spectral distribution ergo as the LO jitters in frequency it always down converts a constant input energy

14. N5AC synthesizer PN measurements
Measured with several reference sources:
Cheap SMD CTS crystal oscillator
FOX801BE Downeast recommended source
Vectron 718Y precision OCXO
>20 Hz offset PN the reference doesn’t matter, the PLL loop noise dominates
<-75 1 KHz with all sources

15. N5AC synthesizer PN results

16. N5AC with CTS reference

17. N5AC synthesizer PN on HP E5052 with CTS reference

18. MicroLO PN measurement

19. MicroLO, N5AC and WA1ZMS overlayed PN

20. 100 MHz DFS with Vectron 718Y reference

21. AD6IW 1152 MHz PLL

22. Bare Vectron 718Y OCXO

23. Conclusions
Low LO phase noise is most important in the presence of strong signals
PN at offsets from 1KHz to 1 MHz can be important with strong interfering signals
Don’t use 1 KHz offset PN measurements to determine narrow band MDS effects
Measure your LO you may be surprised
Close-in PN of synthesizers can be better than crystals
Even when multiplied to 10 GHz the N5AC synthesizer is clean enough for no CW MDS degradation (as long as there are no strong adjacent channel signals)

24. Acknowledgements Thanks to:
KE5FX for loaning the baseband USB analyzer and hacking Timelab to support it, it’s a fantastic baseband PN measurement tool
WA1ZMS for answering questions regarding his prior measurements
AD6IW for E5052a results