[time-nuts] Close-in phase noise (long reply)

Luis Cupido cupido at mail.ua.pt
Wed Dec 17 11:50:01 UTC 2008


Brian,

Was there any particular reason to
go from 5MHz to 20MHz in two steps ?
couldn't be just one x4 stage followed by the filter
(preferably xtal) ?

Luis Cupido.
ct1dmk.


wa1zms at att.net wrote:
> John-
> 
> The BPFs in the 5 to 20MHz chain are just 7-pole LC
> filters with the goal of trying to keep any other
> harmonics other than the desired at least -50dBc.
> Xtal filters would be the better choice, no doubt.
> 
> The -50dBc level is clearly not the best
> that one could get, but was enough for an earlier
> 240GHz project. I just used the same OCXOs and early
> stages of multipliers to get the latest system running
> on 630GHz.
> 
> In the 241GHz system, I ended up building a direct
> frequency synthesizer to get 110MHz from a 10MHz
> drive signal.  At the time, the Freq West PLL
> blocks I used wanted a VHF signal to drive
> the sampling detector to phase lock the L-band
> cavity VCO. The original Freq West units used 5th OT
> xtals for the commercial applications.
> 
> By later experimentation, I found that
> the same sampling detector would also work with a
> much lower frequency reference and still lock the
> loop. The risk however is that the PLL might lock
> on the wrong harmonic of the reference (i.e.: value of N)
> or can have higher reference spur levels since the
> PLL was designed assuming a VHF reference and not an
> HF reference frequency. But this is not a commercial
> design project, and I can live with a difficult alignment
> procedure or initial power-up PLL lock troubles.
> 
> But all this aside, my efforts are currently aimed at
> best close-in noise within the first 1KHz of BW around
> the carrier.
> 
> The PLL bricks all seem to have several kHz of loop BW,
> so my close-in noise going from 20MHz to 1320MHz should
> be only slightly worse than 20Log(n), with n=66 in my case.
> But I'm not ruling out the chance of 1/f noise (or similar)
> showing up from the sampling detector or some other yet-to-be
> determined source.
> 
> However my focus is currently on the 5MHz to 20MHz portion
> of the LO chain and to be sure the gain stages are not
> running near compression. I do still agree with your
> earlier comment about getting the most from that portion
> of the chain.
> 
> -Brian, WA1ZMS
> 
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of John Miles
> Sent: Tuesday, December 16, 2008 7:53 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Close-in phase noise question...more info...
> 
> 
>> More info on the LO chain:
>>
>> 1) 5MHz Wenzel OCXO <--Custom Osc for me.
>> 2) MSA-1105 buffer MMIC and lumped LPF
>> 3) 5MHz to 10MHz 1N5711 diode based doubler
>> 4) MSA-1105 buffer and lumped LPF
>> 5) 10MHz to 20MHz 1N5711 diode based doubler
>> 6) 20MHz BPF
> 
> What kind of BPF?  A really narrow crystal filter would be nice here.  (You
> have basically reproduced the 8568A/B's 20 MHz reference section.)
> 
>> 7) 20MHz drives sampling detector inside surplus Frequency
>> West PLL block to lock 1320MHz cavity oscillator.
> 
> Sounds OK as long as the sampler loop's noise floor doesn't limit you.  I
> haven't measured the in-band residual floor of any bricks but I'd be
> surprised if an SRD multiplier wouldn't be quieter.
> 
>> 8) 1320MHz drives Frequency West SRD multiplier to 6.6GHz.
> 
> If I wanted to get to several GHz with what's in my junk box right now, I
> would do what you did to get to 20 MHz, BPF it with a multipole crystal
> filter, and then use a few more multiplier stages to get somewhere between
> 100 MHz and 1 GHz, a la the 8662A reference section, depending on the choice
> of the next stage.
> 
> That VHF drive signal would go into either an HP 33002A or 33004A SRD
> multiplier, or one of the Picosecond NLTL multipliers (e.g.,
> http://www.picosecond.com/product/product.asp?prod_id=109 ) I picked up in
> their fire sale when they shut down their fab.
> 
>> 9) 6.6GHz to 39.6GHz Milliwave diode multiplier/amp/filter
>> 10) 39.6GHz to 79.2GHz in varactor doubler
>> 11) 79.2GHz to 158.4GHz in varactor doubler
>> 12) 158.4GHz into x4 sub-harmonic mixer
> 
> AFAIK the rest of the chain is fine.  I'd focus on getting rid of the brick
> PLL, or at least taking pains to make sure that it's not the problem, before
> worrying about the MMICs in your early stages.
> 
> Remember that there's no point in optimizing the PN of any one stage much
> below the input-referred residual noise of the following stage.  MMICs, in
> saturation or not, are pretty quiet.  Quieter than sampler loops anyway.
> 
> -- john, KE5FX
> 
> 
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