Hi Bob
In a previous tread, someone said that cooling PRS-10 will only make the
unit consume more energy, since it has to maintain the lamp hot.
Is this wrong?
Before I read the previous tread, I was thinking of electronic
thermostat/fan but...
Em 20/10/2020 10:34, Bob kb8tq escreveu:
Hi
Not very well organized message here …. I’m still working on my first cup of
coffee …:)
First up: What do I do?
My “lab” is in the basement of our house. There is HVAC running pretty much
year round. The temperature “wanders” between about 20 and 25 C over a
year. It cycles about 2 or 3 C at roughly a 1 to 2 hour rate. I have three 5065
Rb’s running full time on a rack. I have a couple of GPSDO’s running on another
rack. Various OCXO’s get tested on a bench. All of it is “out in the open”.
I have looked into various approaches to temperature control ( thermo electric
heaters ….). I have a big pile of water pumps, TEC’s, piping, and radiators
siting in a dark corner of the room. The more I looked into the power required
to do that, the less I liked the idea ….
I do have a few cardboard boxes and a small pile of “beach towels” that I use.
If the measurement is particularly sensitive, I put a towel over the test set.
If
the device is large, I put a box over it. That generally works quite well to
reduce
fast changing temperature from drafts in the room.
======
Are you trying to lock the PRS-10 to the UCCM? If so, don’t bother, the net
result is a PRS-10 that is not very stable. Just use the UCCM’s output.
Practically speaking, the PRS-10 is a 0-50C device. It’s no different than other
Rb’s in that respect. It also is no different than the rest in making those
limits
really hard to pull out of the data sheet. They talk about 65C baseplate, but
that’s not ambient.
The UCCM has an OCXO in it that is (probably) rated too 70C. It depends a
bit on what version / vendor you happen to have. In still air the OCXO is ok
to 50, in moving air, it should be all right at 70. No idea how the rest of the
parts
on the board will do above 50C / moving air. ( = I’d bet there will be problems
above 50C).
The PRS-10 needs a heatsink on it or it will quickly die. It will live a longer
life
with a better / bigger heatsink. With a small one, figure 2 years. With a fan or
something large, figure > 5 years of life. That’s all in a indoor situation
with a
HVAC system keeping the ambient around 20 to 25C. The small heatsink
gets around 50, the bigger one + fan may get down to 40C.
The real issue with both the PRS-10 and the UCCM is to reduce the temperature
swings. The UCCM will track out long term changes. You just need to protect it
from quick changes. Depending on which PRS-10 you have the temperature
performance
may be better or worse (per the specifications on the part). A wooden box with
heaters / fans can do this, there are other ways.
A lot depends on just how crazy it gets where you are in Brazil does it get
above
40C? ( I looked it up, the record is 44.7C …. yikes … ). Ideally you want to
keep
the heatsink around 40C. That’s not going to happen if it’s 40 out. Running the
part at 55 C all the time is not a good idea.
More or less, you normally need 10C of “headroom” on any sort of heated device
(like a heated wooden box). You also have a practical problem coming up with
a heatsink that gets to < 10C relative to ambient. The net is that your heatsink
is at “max ambient” + 20C. For a somewhat more practical heatsink, the numbers
are worse …. sorry about that.
What’s a “big” heatsink? Think of something the size of the Rb with a high speed
fan attached. As noted in various posts the magnetic field from the fan will not
make the Rb happy, some sort of ducting to keep it a bit further away might be
a good idea. That sort of thing probably *does* get you within 10C of ambient .
The bottom line is that this sort of gear normally gets used in a location that
has
heating / cooling. Temperature may swing from 10 to 30, but that’s about it.
They
generally have pretty big fans moving air around. A cell phone “base station” is
one example of this sort of setup.
Probably enough for now…..
Bob
On Oct 20, 2020, at 7:36 AM, Luiz Alberto Saba <[email protected]> wrote:
Hi Bob
I will try with the toroid setup (as soon as I receive the transformer) and see
what happens.
I'm really lost in respect of thermal insulation (or cooling).
I'll use a PRS-10 and a Samsung UCCM gpsdo.
A single mass of copper (or alluminium) wil be effective for PRS-10?
I saw in an old trend here people saying that forced cooling is not good for
the PRS-10.
Closed box or ventilation holes?
It will operate at ambient temperature, in Brazil...
If you or someone has some hints about what to do in this respect, I'll be very
grateful.
Thanks
Luiz Alberto Saba
Em 19/10/2020 17:35, Bob kb8tq escreveu:
Hi
One weird issue with big caps in a linear supply:
As you increase the size of the caps on the bridge rectifier, the ripple
voltage goes down. The ripple *current* goes up. In some cases (say ripple
voltage at 1%) it can go up by 10X to 100X of the minimum value.
Why does this matter?
Magnetically coupled signals and current bounce in the ground are both very
real effects. They can be tough to get rid of ….
There is no one solution to any of this …. :(
Bob
On Oct 19, 2020, at 12:41 PM, Luiz Alberto Saba <[email protected]> wrote:
Hi Bob
The phase noise is not particularly important in my case.
It will be used mainly to calibrate ocxo/rb.
I'm pendig to a good toroid/bridge/huge caps and something like LT-1083.
Em 19/10/2020 12:38, Bob kb8tq escreveu:
Hi
What will the output of the box be used for? If low phase noise is an objective,
then one needs to be pretty careful about power supply noise. If low phase noise
is not a “need” in this case, then (good) switchers can be used for everything.
What’s a “good” switcher? You want one that is reliable ( = does not catch fire
on a regular basis). You also want to get the noise down into the 10 mv p-p
range.
That generally means filtering the output a bit. Past that the usual
requirements
of line / load regulation and efficiency come into play.
Bob
On Oct 19, 2020, at 10:24 AM, Luiz Alberto Saba <[email protected]> wrote:
Hi guys
Planning to build my "reference box" with a rubidium & a gpsdo.
SMPS will do a good job or the transformer+bridge+capacitor+regulator is best
suited for this application?
In the "test phase" I've used a switched ps for the rubidium and a conventional
for the gpsdo.
Thanks
Luiz Alberto Saba
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