LO Phase Noise effects on MDS Gary Lauterbach AD6FP

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

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

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

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

Brians measured spectrum FM noise modulation producing 2db MDS degradation

My measured spectrum of WA1ZMS -2db MDS experiment 8662a @ 144 MHz FM modulated with random noise

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

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 1-100 Hz is a significant % of LO total power

Here’s the visible MDS effect SDR-IQ 10 MHz IF of 154 MHz signal and 144 MHz LO: Left: clean LO Right: -52dbc PN @ 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

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

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 -52dbc @ 1 KHz LO Noise powers match to <0.5 db

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

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 dbc @ 1 KHz with all sources

N5AC synthesizer PN results

N5AC with CTS reference

N5AC synthesizer PN on HP E5052 with CTS reference

MicroLO PN measurement

MicroLO, N5AC and WA1ZMS overlayed PN

100 MHz DFS with Vectron 718Y reference

AD6IW 1152 MHz PLL

Bare Vectron 718Y OCXO

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)

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