Re: What's the use of a 192 kHz sample rate?

On Sun, 4 May 2008 19:54:57 -0700 (PDT), rickman <gnuarm@[EMAIL PROTECTED]
>
wrote:

>On May 4, 3:40 pm, nos...@[EMAIL PROTECTED]
 (Don Pearce) wrote:
>> On Sun, 4 May 2008 12:28:34 -0700 (PDT), rickman <gnu...@[EMAIL PROTECTED]
>
>> wrote:
>>
>>
>>
>> >On May 4, 12:18 pm, DigitalSignal <digitalsignal...@[EMAIL PROTECTED]
> wrote:
>> >> Hi rickman, can you refer the company who can produce or analyze the
>> >> signals with 140 dB SNR? I have never seen one. I guess they are not
>> >> using any digital technology, or they are look at very low frequency
>> >> signals. Right?
>>
>> >> Jameswww.go-ci.com
>>
>> >Thales Communications, makers of military radios.  This figure came up
>> >in the context of digital interference with the RF sections.  I was
>> >told that the digital noise had to be below -140 dB to prevent
>> >desensitization of the receiver.  These units are *very* overbuilt and
>> >far surpass anything commercial I have seen.  One spur anywhere in a
>> >huge range, 30 MHz to 500 MHz, IIRC and they are back to the testing
>> >room to add more copper tape or to put more resistors in clock lines,
>> >etc.
>>
>> A desensitization level is going to be measured in dBm, and -140 is a
>> very ordinary figure. If external noise interference is present at the
>> input of a receiver, it will add to the inherent noise already there,
>> raising it. This is called desensitization. A common interference
>> requirement is that a desensitization of no more than 1dB is
>> permitted.
>>
>> The -140 in this instance is just a receiver input level, and nothing
>> to do with S/N ratios, dynamic ranges or anything like that.
>>
>> I would be very surprised if anything military surpassed the spec of a
>> commercial design. Far too few units are built to justify the number
>> of trips round the design cycle that are needed to remove every spur
>> from and maximise performance. That fact that they have to bodge units
>> with copper tape to make them work rather bears this out.
>>
>> Certainly when I was designing low noise converters for domestic
>> satellite systems I achieved a guaranteed total noise figure 0f 0.25dB
>> at 12GHz, and the entire unit had a works cost price of 11 dollars.
>> Nothing military has ever come close to that kind of performance or
>> price.
>
>I am not an RF guy and the figure I actually remember was 150 dB.  I
>hedged it a bit as 150 sounded rather extreme to me.  I dunno if 150
>is anything outside of ordinary or not.
>

OK 150 - different bandwidth, then. The basic thermal noise at the
front end of a receiver is -174dBm + 10 log(bandwidth) + noise figure.
A noise or interference level that desensitizes that by 1dB will be
about 10dB lower than that level. You need to do the know the
bandwidth and noise figure to know the significance.

What is significant is that this receiver has a desensitization limit
for interference that it throws at itself. I have never, ever come
across that before - it is always an external spec; you design the
internals so it doesn't interfere.
 
>When you say that it costs too much to go around the design cycle many
>times, you clearly don't know much about the military procurement
>process.  On the last radio that they built while I was there, they
>were up to rev 14 of the board that had nothing but the UI controller
>and external interfaces.  You need to remember that often the
>development process is cost plus and the customer is *asking* for
>tough specs.  It is only when they can't be delivered that they back
>off.
>
Iteration 14 and the company was still in business? Anyone can afford
to do it again if they can't do it right. Was this a "cost-plus"
contract? Three or perhaps four was more the number I had in mind.

>What is really funny is that you are getting wrapped around the axle
>about my use of this figure when that was really just an aside to an
>aside of my original point.  Funny how these discussions get so far
>off topic.
>
If you like. It was clearly a number you threw in because it sounded
impressively big, even though you hadn't a clue what it meant.

>Did you read my post which used the original 140 dB figure?

Yes. It was nonsense.

d
-- 
Pearce Consulting
http://www.pearce.uk.com