[time-nuts] SMPS or conventional?

Tue Oct 20 14:45:14 UTC 2020

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 <las at intercat.com.br> 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 <las at intercat.com.br> 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 <las at intercat.com.br> 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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