[time-nuts] NLTL comb generators going away...

[Rick Karlquist]

An SRD imposes a phase noise floor that adds to the source's phase
noise floor and the multiplier's input noise floor.  Whatever this
total noise is will be increased by 20 LOG N.  Unless the source
is very low noise and the multiplier equivalent input noise is
very low, you may not see any effects from the SRD.  At Agilent
we didn't have to worry about SRD noise until exotic sources
like the STW were developed.

In the case you site, the source noise was 6 dB below the
equivalent input noise  of the source, resulting in a 1 dB increase
in total noise.  An SRD may or may not have been worse depending
on what the actual noise floor was.

Rick

Mike Feher wrote:
> Over 20 years ago I was the lead guy in the development of the SCOTT
> Milstar
> satellite terminal at Magnavox. Our uplink frequency was from 43.5 to 45.5
> GHz. The PN specifications were essentially almost impossible to meet. The
> terminal had it's own specification and used a Rubidium followed by a 100
> Hz
> crystal filter. Narrow filtering was used after all multipliers and a lot
> of
> heterodyning was used in place of multipliers to come up with the 11 GHz
> required for the PA. The PA was subcontracted out to two different
> manufacturers due to the risks involved and they were each given 1 dB
> above
> what would have been generated within the PA's X4 multiplier in PN
> increase.
> So, instead of 12 dB they were allowed 13 dB of degradation. The
> multiplier
> used was a dual stage varactor type multiplier, where the first stage
> would
> get it up to 22 GHz and the second to the final frequency of the nominal
> 44
> GHz. None of the amplifiers exhibited more than 12 dB of PN increase. In
> fact, they all measured 12 dB. Admittedly, back then, this was not an easy
> measurement to make. This was with 2 different manufacturers and I went to
> both on numerous occasions, one on the east coast and one on the west. So,
> this is why I wonder why an SRD would be that much worse. What is
> happening
> that I am not aware of? Where is this extra residual noise coming from?
> Thanks - Mike
>
>
>
> Mike B. Feher, N4FS
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Rick Karlquist
> Sent: Friday, December 28, 2007 9:00 PM
> To: Discussion of precise time and frequency measurement
> Cc: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] NLTL comb generators going away...
>
> You need a really clean driver to be good enough to need a NLTL
> instead of an SRD.  Whatever the intrinsic noise of the NLTL is,
> it is lower than any practical source so you can consider it zero.
>
> Rick Karlquist N6RK
>
>
> John Miles wrote:
>> It's hard to read the tea leaves on that.  Dieter Scherer has one note
>> (Generation of Low PN Microwave Signals) that shows the 33004A
>> multiplier's
>> output noise at -140 dBc/Hz to -148 dBc/Hz from 1 to 10 kHz, at 5.5 GHz
>> (N=11).  If that is the case with the 11729's multiplier, then the NLTL
>> part
>> is not really necessary even with an upgraded 640 MHz reference chain.
>> But
>> then, something else must be responsible for the 11729's residual PN
>> spec,
>> and I don't see what that would be.
>>
>> Of course, I'd assume that PSPL's measurements were taken with the
>> crappiest
>> SRD part they could find. :)
>>
>> It appears that the 11729's 640-MHz PA can drive the 7123 adequately,
>> although the 12.16 GHz comb line is somewhat close to the spec limit of
>> -20
>> dBm.  The 7113-110 is a better replacement for the 33004A in the 11729C,
>> since it would actually be operating in spec.  They have several of
>> those
>> in
>> stock (I just ordered 2 of the 8 they had left).
>>
>> -- john, KE5FX
>>
>>> -----Original Message-----
>>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
>>> Behalf Of Richard (Rick) Karlquist
>>> Sent: Friday, December 28, 2007 10:08 AM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] NLTL comb generators going away...
>>>
>>>
>>> The SRD (step recovery diode) definitely degrades the phase
>>> noise beyond 20 LOG N, according to Agilent engineers I have
>>> talked to.  They mentioned a number like -150 dBc/Hz or something.
>>> Having said that, you also have to be very careful to design
>>> the driver (especially with NLTL) or it will become the limiting
>>> factor.
>>>
>>> Rick Karlquist N6RK
>>>
>>
>>
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