[time-nuts] Odd-order multiplication of CMOS-output OCXO

[jimlux]

On 1/20/20 12:50 PM, Mark Haun wrote:
> On Mon, 20 Jan 2020 15:25:00 -0500
> Bob kb8tq <kb8tq at n1k.org> wrote:
>> On Jan 20, 2020, at 2:57 PM, Mark Haun <mark at hau.nz> wrote:
>>> Agree except you were starting from the VFOV numbers for the 100-MHz
>>> version.  If you use their numbers for the 10-MHz version and add
>>> 20 dB for an ideal 10x multiplication, for comparing with the ABLNO
>>> spec at 100 MHz, you end up with
>>>
>>> offset    VFOV405 @ 10M, ideal 10x multiply     ABLNO @ 100 M
>>> 10        -100                                  -88
>>> 100       -120                                  -118
>>> 1k        -140                                  -141
>>> 10k       -145                                  -160
>>> 100k      -145                                  -161
>>
>> If indeed -145 is “good enough” then you have moved out of the “good
>> phase noise” region into fairly generic sort of specs. A “couple of
>> dollar” oscillator will give you -145 sort of noise floors.
> 
> True enough, but remember that my motivation for using the OCXO in the
> first place was to combine the required phase-noise spec with
> OCXO-class frequency stability (this is for narrowband coherent
> modulation schemes on the shortwave bands where short-term stability of
> ~ 10^-10 is nice to have).  The alternative is what Attila said,
> VCXO phase locked to an OCXO.  The advantage of doing it this way is
> that I [potentially] reduce complexity, board space, and power.
> 
> Hypothetically, sure, any old 80-MHz OCXO with "generic" phase-noise
> performance would suffice.  But hobbyists can't just pick up the phone
> and order something like that; we're limited to surplus/used stock,
> where 80-ish MHz is unusual.  And of course most surplus/used OCXOs
> would require high voltage (5V or above), high power (half a watt or
> more), or both.
> 
> Sorry, I didn't plan to expound on my design rationale at such length,
> but you seemed curious :)
> 
> 

Learning about design rationale is what this list is all about.

As Bob and others have pointed out over the years, oscillator 
manufacturers will happily give you what ever you specify, for a price. 
But it's not unusual to have a low volume application where you're 
willing to take what you can get, as long as it meets some other 
requirement (cost, delivery time).  The problem is that it's hard to 
convey all the trades in a requirements or spec document. "Sure, I'm 
happy to have high far out noise, but *I* care about noise between 
10-1000 Hz" or "I don't care about absolute frequency stability over 
temperature or long term, because I'm putting it in an oven and I'm 
going to be able to discipline/measure it, but I want really good close 
in noise"

The amateur radio person wanting to multiply their reference up to 10 
GHz for narrow band CW is a fine example - They probably have way to 
measure frequency, so absolute stability isn't all that important. And 
they want low power (because you're sitting on some mountain top with 
batteries).  And, there's only a few hundred people in the entire world 
who would conceivably be interested in it, and maybe 3 who would try, 
and they all want to spend less than $100.

Vectron, Abracon, etc. do not have a business case that contemplates 
this idiosyncratic market (nor should they).

But it is useful to know what the "care-abouts" are, because often, 
there is lore (that cannot be used as a spec) about stuff that might work.

For my space OCXO need, I started by asking manufacturers if they could 
make me a small OCXO with the heater disconnected - so I didn't have to 
spend heater power or have a 15V power supply. I wound up with a OCXO 
that *did* have a heater, but the heater power is negligible.  And, as a 
side effect, it actually gives me visibility into the internal state of 
the device, because by knowing just bus current vs time, I can tell 
approximately what temperature things were at when it was turned on.