On 1/20/20 12:50 PM, Mark Haun wrote:
On Mon, 20 Jan 2020 15:25:00 -0500
Bob kb8tq <kb...@n1k.org> wrote:
On Jan 20, 2020, at 2:57 PM, Mark Haun <m...@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.
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