Prev: TMS320LF2407
Next: Wrong question may give WRONG answer -- was[Re: True RMS and averaging.]
From: guru_kaliraj on 18 Nov 2009 06:28
Hi,
In one of my experiment, I need to measure SNR of an ADC which has fixed
sample rate of 2.4MHz & 14 bit. My input sinusoidal frequency is also fixed
(decided by previous stages) and it is 600KHz. I understand that I'm just
taking 4 sample in a cycle (ie. My sample rate is just 4 times my signal
rate).
Will this affect SNR of the ADC? When I calculate SNR for the same --
I got around 36dB only..whereas my theory says it need to be 6N -1 = 83dB.
I'm losing a lot.
I'm quite sure about the algoirthm i'm using while computing SNR (
I'm using FFT method, ignoring harmonic power, summing noise power in the
bandwidth). I verified it with other ADCs with higher sample rate & it got
related to 6N-1. But not in above case.
I'm just wondering whether this sample rate (4 times the signal
freq) will affect SNR or not?.

Regards,
Guru


From: Eric Jacobsen on 18 Nov 2009 11:12
On 11/18/2009 4:28 AM, guru_kaliraj wrote:
> Hi,
> In one of my experiment, I need to measure SNR of an ADC which has fixed
> sample rate of 2.4MHz& 14 bit. My input sinusoidal frequency is also fixed
> (decided by previous stages) and it is 600KHz. I understand that I'm just
> taking 4 sample in a cycle (ie. My sample rate is just 4 times my signal
> rate).
> Will this affect SNR of the ADC? When I calculate SNR for the same --
> I got around 36dB only..whereas my theory says it need to be 6N -1 = 83dB.
> I'm losing a lot.
> I'm quite sure about the algoirthm i'm using while computing SNR (
> I'm using FFT method, ignoring harmonic power, summing noise power in the
> bandwidth). I verified it with other ADCs with higher sample rate& it got
> related to 6N-1. But not in above case.
> I'm just wondering whether this sample rate (4 times the signal
> freq) will affect SNR or not?.
>
> Regards,
> Guru

Just IMHO: If the test signal is synchronous to the sampling, then you
likely do have a problem in that you may be limited in how much
resolution you can discern. If not, and the test signal phase drifts
with respect to the sampling interval, then you just need a long enough
FFT to make use of the quantization steps revealed by the phase drift.

I hope that makes sense. I was trying to think of a theoretical
explanation for this, but I hope the intuitive approach conveys the idea.

--
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com
From: Randy Yates on 18 Nov 2009 13:50
"guru_kaliraj" <guru_kaliraj(a)yahoo.com> writes:

> Hi,
> In one of my experiment, I need to measure SNR of an ADC which has fixed
> sample rate of 2.4MHz & 14 bit. My input sinusoidal frequency is also fixed
> (decided by previous stages) and it is 600KHz. I understand that I'm just
> taking 4 sample in a cycle (ie. My sample rate is just 4 times my signal
> rate).
> Will this affect SNR of the ADC? When I calculate SNR for the same --
> I got around 36dB only..whereas my theory says it need to be 6N -1 = 83dB.
> I'm losing a lot.
> I'm quite sure about the algoirthm i'm using while computing SNR (
> I'm using FFT method, ignoring harmonic power, summing noise power in the
> bandwidth). I verified it with other ADCs with higher sample rate & it got
> related to 6N-1. But not in above case.
> I'm just wondering whether this sample rate (4 times the signal
> freq) will affect SNR or not?.

I don't see why having a sample rate that's a harmonic of the test
signal should do this. What I suspect is happening is that you're
test signal frequency is not falling on a bin and energy is going
from the signal to the nosie due to "splatter."

Can we see your code?
--
Randy Yates % "...the answer lies within your soul
Digital Signal Labs % 'cause no one knows which side
mailto://yates(a)ieee.org % the coin will fall."
http://www.digitalsignallabs.com % 'Big Wheels', *Out of the Blue*, ELO
From: guru_kaliraj on 19 Nov 2009 00:40
Hi Eric,
This explains me clearly why I'm getting less SNR. Yes!. Both sample
rate & signal is not synchronized & hence as you mentioned my signal phase
is playing a role of affecting SNR. Thanks for the reply.
Probably, writing some algorithm so that my sample start & end is
controlled (I'm planning to select samples - to get same value of digital
code at start & end -- and this will make sure that phase drift is
adjusted) will help me to increase the SNR.

Thanks again!

Regards,
Guru

>On 11/18/2009 4:28 AM, guru_kaliraj wrote:
>> Hi,
>> In one of my experiment, I need to measure SNR of an ADC which has
fixed
>> sample rate of 2.4MHz& 14 bit. My input sinusoidal frequency is also
fixed
>> (decided by previous stages) and it is 600KHz. I understand that I'm
just
>> taking 4 sample in a cycle (ie. My sample rate is just 4 times my
signal
>> rate).
>> Will this affect SNR of the ADC? When I calculate SNR for the
same --
>> I got around 36dB only..whereas my theory says it need to be 6N -1 =
83dB.
>> I'm losing a lot.
>> I'm quite sure about the algoirthm i'm using while computing
SNR (
>> I'm using FFT method, ignoring harmonic power, summing noise power in
the
>> bandwidth). I verified it with other ADCs with higher sample rate& it
got
>> related to 6N-1. But not in above case.
>> I'm just wondering whether this sample rate (4 times the
signal
>> freq) will affect SNR or not?.
>>
>> Regards,
>> Guru
>
>Just IMHO: If the test signal is synchronous to the sampling, then you
>likely do have a problem in that you may be limited in how much
>resolution you can discern. If not, and the test signal phase drifts
>with respect to the sampling interval, then you just need a long enough
>FFT to make use of the quantization steps revealed by the phase drift.
>
>I hope that makes sense. I was trying to think of a theoretical
>explanation for this, but I hope the intuitive approach conveys the
idea.
>
>--
>Eric Jacobsen
>Minister of Algorithms
>Abineau Communications
>http://www.abineau.com
>
From: Randy Yates on 19 Nov 2009 07:22
"guru_kaliraj" <guru_kaliraj(a)yahoo.com> writes:

> Hi Eric,
> This explains me clearly why I'm getting less SNR. Yes!. Both sample
> rate & signal is not synchronized & hence as you mentioned my signal phase
> is playing a role of affecting SNR. Thanks for the reply.
> Probably, writing some algorithm so that my sample start & end is
> controlled (I'm planning to select samples - to get same value of digital
> code at start & end -- and this will make sure that phase drift is
> adjusted) will help me to increase the SNR.
>
> Thanks again!

Wow! I'm glad the two of you know what you're talking about. I really
don't have a clue.

In fact, I ran a Matlab simulation of this and found just the opposite,
i.e., that when Fs = 4*f, the measured SNR is way, way bigger than the
actual.

That's because, when the non-quantization noise is much less than the
quantization noise, you're only going to get a repeating sequence of
four values, and that looks like a pure sine wave at Fs/4 without ANY
(wideband) quantization noise. The quantization noise translates into
phase and/or magnitude error of the sine wave.

Additionally, I found, using Octave, that because of the phenomenom of
the previous paragraph, the non-Fs/2 bins of the FFT are identically
zero, and therefore their magnitude is zero, and therefore the log10()
of their magnitude is one of those special double exception values
-Inf, making the plot unusable.

--Randy


>
> Regards,
> Guru
>
>>On 11/18/2009 4:28 AM, guru_kaliraj wrote:
>>> Hi,
>>> In one of my experiment, I need to measure SNR of an ADC which has
> fixed
>>> sample rate of 2.4MHz& 14 bit. My input sinusoidal frequency is also
> fixed
>>> (decided by previous stages) and it is 600KHz. I understand that I'm
> just
>>> taking 4 sample in a cycle (ie. My sample rate is just 4 times my
> signal
>>> rate).
>>> Will this affect SNR of the ADC? When I calculate SNR for the
> same --
>>> I got around 36dB only..whereas my theory says it need to be 6N -1 =
> 83dB.
>>> I'm losing a lot.
>>> I'm quite sure about the algoirthm i'm using while computing
> SNR (
>>> I'm using FFT method, ignoring harmonic power, summing noise power in
> the
>>> bandwidth). I verified it with other ADCs with higher sample rate& it
> got
>>> related to 6N-1. But not in above case.
>>> I'm just wondering whether this sample rate (4 times the
> signal
>>> freq) will affect SNR or not?.
>>>
>>> Regards,
>>> Guru
>>
>>Just IMHO: If the test signal is synchronous to the sampling, then you
>>likely do have a problem in that you may be limited in how much
>>resolution you can discern. If not, and the test signal phase drifts
>>with respect to the sampling interval, then you just need a long enough
>>FFT to make use of the quantization steps revealed by the phase drift.
>>
>>I hope that makes sense. I was trying to think of a theoretical
>>explanation for this, but I hope the intuitive approach conveys the
> idea.
>>
>>--
>>Eric Jacobsen
>>Minister of Algorithms
>>Abineau Communications
>>http://www.abineau.com
>>

--
Randy Yates % "...the answer lies within your soul
Digital Signal Labs % 'cause no one knows which side
mailto://yates(a)ieee.org % the coin will fall."
http://www.digitalsignallabs.com % 'Big Wheels', *Out of the Blue*, ELO
 |  Next  |  Last
Pages: 1 2 3 4 5
Prev: TMS320LF2407
Next: Wrong question may give WRONG answer -- was[Re: True RMS and averaging.]