[time-nuts] Divider circuit for Rubidium Standard

Bob Camp kb8tq at n1k.org
Fri May 22 23:08:44 EDT 2015


Hi

To answer the next part of the question - simulation:

Noise wise, Spice is fundamentally a linear analysis program. Logic gates
mostly operate in a non-linear fashion (full on / full off). The noise
models that are commonly used (when you can even find them)
apply to fairly limited “active region” conditions. 

In this case, you have noise in the “DC” region and at audio modulating 
your signal. It’s very much a non-linear case. A conventional Spice 
analysis will quickly lead you astray. You *can* make it work after 
the fact by plugging this and that in here and there. To do that, you 
need to measure some parts and then back fit the dummy elements
in the model to your results.  

To predictively accurately model noise in a gate as it switches  you need:

1) A program that handles non-linear models properly
2) The basic device models for the devices in your IC
3) The non-linear noise add-on for your analysis program
4) The generalized non-linear noise models (DC to GHz) to add onto your device data
5) A way to convert the result into something meaningful (ADEV , phase noise)

Last time I looked, 1 + 3 ran above $125K for something that *might* be accurate. The
models for the IC process devices (2 above) were “sort of” included in that price. 
Adding 4 to the mix was a “cost plus” sort of thing. Obviously number 5 is a
“do it your self” task. 

Far cheaper (at least 10X) to just build one and buy all the brand new gear needed to test it. 
We’re not talking about big buckets of noise, this stuff is all mighty far down. 1x10^-14 is a
long long ways. A simple term like “accurate” gets a major workout in this case. If once 
you have your data, you want to back fit a dummy model - go for it. 

Bob

> On May 22, 2015, at 6:29 PM, xaos <xaos at darksmile.net> wrote:
> 
> Bob,
> 
> This are all great questions.
> 
> 1. Let's assume that it varies from a HP Signal generator
> to a home built device. However, If I was to build it I
> would expect to pay more and get better specs.
> I have a few HP 3325B's and a few 8660C.
> I would probably use those as inputs but not always.
> 
> 2. Let's stick with the basics: 5-10Mhz.
> 
> 3. Again, basics: 1-7V peak to Peak.
> 
> 4. Ok, this is the real important question.
> The answer is, an amplifier of some sort.
> And that amp will probably feed something
> like a Test Instrument or some circuit I
> am building. What are my options?
> 
> For now, I'd like to simulate some of my simple
> designs as well as some designs suggested here.
> Then, see where it goes.
> 
> George
> 
> 
> On 05/22/2015 05:31 PM, Bob Camp wrote:
>> Hi
>> 
>> What is your objective? Put another way:
>> 
>> 1)  How clean is your sine wave source? 
>> 2) What frequency (or range) are you trying to convert?
>> 3) What level range are you trying to work with?
>> 4) What is it going into (how clean is the next stage)?
>> 
>> If you have an optical fountain that is good to 1x10^-15 at 1 second, and
>> you are trying to map Pluto with a radar in your back yard, the answer will
>> be a bit different than if you are starting with a surplus OCXO and trying 
>> to drive a 5334 :)
>> 
>> Bob
>> 
>>> On May 22, 2015, at 4:27 PM, xaos <xaos at darksmile.net> wrote:
>>> 
>>> After reading the posts on this subject I have a question.
>>> First, in my experience I used a rather simple circuit made from
>>> diodes used as limiters and a transistor feeding
>>> a logic inverter. No AGC.
>>> 
>>> So here is my question. What is the proper circuit to use?
>>> I'd like to do a PSPICE and check things out followed by
>>> a prototype.
>>> 
>>> I got that a comparator is out, etc.
>>> 
>>> Cheers,
>>> George H. N2FGX
>>> 
>>> On 04/26/2015 06:51 AM, Bryan _ wrote:
>>>> All:
>>>> 
>>>> Picked up a FE 5680B from Ebay awhile back. Appears to work fine, but is limited to a 1pps output. However there is a point on the PCB that's documented that has a 20Mhz output. There is actually a clean 60Mhz output as well.
>>>> 
>>>> http://www.ebay.com/itm/FEI-fe-5680b-rubidium-oscillator-With-1pps-20mhz-output-ONLY-10mhz-NEED-to-MOD-/291419889143?pt=LH_DefaultDomain_0&hash=item43d9fa9df7
>>>> 
>>>> I would like to tap this 20mhz output and feed it to a divider/buffer circuit for a 10Mhz output at 50ohm. Can anyone recommend a good schematic for such a purpose. I was looking at the project from David partridges web site http://www.perdrix.co.uk/FrequencyDivider/index.html
>>>> 
>>>> Cheers and thanks in advance.
>>>> 
>>>> -=Bryan=- 		 	   		  
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