ADC limitations for bandpass/IF sampling

I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling for
downconversion.  Is there any limiting factor on the ADC that I need to
consider?  e.g., does the ADC need special circuit?  or small
sample-and-hold delay time? 

CJLam

"clam" <jethrolam@gmail.com> wrote in message
news:tK2dnfBZWp3GuQLenZ2dnUVZ_vmdnZ2d@giganews.com...
> I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling for
> downconversion.  Is there any limiting factor on the ADC that I need to
> consider?  e.g., does the ADC need special circuit?  or small
> sample-and-hold delay time?
>
> CJLam

You need to be sure that the analog bandwidth of the ADC is higher than
1MHz. You should find this as a parameter in the data sheet.
Also, I'm assuming your BW of the RF signal is not much more than about
5kHz.

Cheers
Bhaskar

Bhaskar Thiagarajan wrote:
> "clam" <jethrolam@gmail.com> wrote in message
> news:tK2dnfBZWp3GuQLenZ2dnUVZ_vmdnZ2d@giganews.com...
> 
>>I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling for
>>downconversion.  Is there any limiting factor on the ADC that I need to
>>consider?  e.g., does the ADC need special circuit?  or small
>>sample-and-hold delay time?
>>
>>CJLam
> 
> 
> You need to be sure that the analog bandwidth of the ADC is higher than
> 1MHz. You should find this as a parameter in the data sheet.
> Also, I'm assuming your BW of the RF signal is not much more than about
> 5kHz.
> 
> Cheers
> Bhaskar
> 
> 

I assume Bhaskar means at least the sample acquisition time is less than 
.5 microsecond [ That's how I would apply Nyquist to the situation. ]

Given that assumption and assuming the 20kHz sampling rate is not locked 
to the 1 MHz carrier, I can not see what information could be obtained.

Obviously I'm missing something.

Bhaskar Thiagarajan wrote:
> "clam" <jethrolam@gmail.com> wrote in message
> news:tK2dnfBZWp3GuQLenZ2dnUVZ_vmdnZ2d@giganews.com...
> 
>>I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling for
>>downconversion.  Is there any limiting factor on the ADC that I need to
>>consider?  e.g., does the ADC need special circuit?  or small
>>sample-and-hold delay time?
>>
>>CJLam
> 
> 
> You need to be sure that the analog bandwidth of the ADC is higher than
> 1MHz. You should find this as a parameter in the data sheet.
> Also, I'm assuming your BW of the RF signal is not much more than about
> 5kHz.
> 
> Cheers
> Bhaskar
> 
> 

I assume Bhaskar means at least the sample acquisition time is less than 
.5 microsecond [ That's how I would apply Nyquist to the situation. ]

Given that assumption and assuming the 20kHz sampling rate is not locked 
to the 1 MHz carrier, I can not see what information could be obtained.

Obviously I'm missing something.

Bhaskar Thiagarajan wrote:
> "clam" <jethrolam@gmail.com> wrote in message
> news:tK2dnfBZWp3GuQLenZ2dnUVZ_vmdnZ2d@giganews.com...
> 
>>I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling for
>>downconversion.  Is there any limiting factor on the ADC that I need to
>>consider?  e.g., does the ADC need special circuit?  or small
>>sample-and-hold delay time?
>>
>>CJLam
> 
> 
> You need to be sure that the analog bandwidth of the ADC is higher than
> 1MHz. You should find this as a parameter in the data sheet.
> Also, I'm assuming your BW of the RF signal is not much more than about
> 5kHz.
> 
> Cheers
> Bhaskar
> 
> 

I assume Bhaskar means at least the sample acquisition time is less than 
.5 microsecond [ That's how I would apply Nyquist to the situation. ]

Given that assumption and assuming the 20kHz sampling rate is not locked 
to the 1 MHz carrier, I can not see what information could be obtained.

Obviously I'm missing something.

Bhaskar Thiagarajan wrote:
> "clam" <jethrolam@gmail.com> wrote in message
> news:tK2dnfBZWp3GuQLenZ2dnUVZ_vmdnZ2d@giganews.com...
> 
>>I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling for
>>downconversion.  Is there any limiting factor on the ADC that I need to
>>consider?  e.g., does the ADC need special circuit?  or small
>>sample-and-hold delay time?
>>
>>CJLam
> 
> 
> You need to be sure that the analog bandwidth of the ADC is higher than
> 1MHz. You should find this as a parameter in the data sheet.
> Also, I'm assuming your BW of the RF signal is not much more than about
> 5kHz.
> 
> Cheers
> Bhaskar
> 
> 

I assume Bhaskar means at least the sample acquisition time is less than 
.5 microsecond [ That's how I would apply Nyquist to the situation. ]

Given that assumption and assuming the 20kHz sampling rate is not locked 
to the 1 MHz carrier, I can not see what information could be obtained.

Obviously I'm missing something.

Bhaskar Thiagarajan wrote:
> "clam" <jethrolam@gmail.com> wrote in message
> news:tK2dnfBZWp3GuQLenZ2dnUVZ_vmdnZ2d@giganews.com...
> 
>>I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling for
>>downconversion.  Is there any limiting factor on the ADC that I need to
>>consider?  e.g., does the ADC need special circuit?  or small
>>sample-and-hold delay time?
>>
>>CJLam
> 
> 
> You need to be sure that the analog bandwidth of the ADC is higher than
> 1MHz. You should find this as a parameter in the data sheet.
> Also, I'm assuming your BW of the RF signal is not much more than about
> 5kHz.
> 
> Cheers
> Bhaskar
> 
> 

I assume Bhaskar means at least the sample acquisition time is less than 
.5 microsecond [ That's how I would apply Nyquist to the situation. ]

Given that assumption and assuming the 20kHz sampling rate is not locked 
to the 1 MHz carrier, I can not see what information could be obtained.

Obviously I'm missing something.

On Tue, 13 Dec 2005 13:43:23 -0600, "clam" <jethrolam@gmail.com>
wrote:

>I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling for
>downconversion.  Is there any limiting factor on the ADC that I need to
>consider?  e.g., does the ADC need special circuit?  or small
>sample-and-hold delay time? 
>
>CJLam

Yes, the sample-and-hold time is important, but shouldn't be too
difficult for a 1MHz input.   For IF sampling this is one of the more
important parameters.

As Bhaskar mentioned, the input BW of the front end has to support the
desired input spectrum.   Getting a narrow enough IF filter at 1MHz to
support such a low signal bandwidth might be tricky, so I'm assuming
you've got reasonable control of the input spectrum.

Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

Richard Owlett wrote:
> Bhaskar Thiagarajan wrote:

>> You need to be sure that the analog bandwidth of the ADC is higher than
>> 1MHz. You should find this as a parameter in the data sheet.
>> Also, I'm assuming your BW of the RF signal is not much more than about
>> 5kHz.
>>
>> Cheers
>> Bhaskar
>>
>>
> 
> I assume Bhaskar means at least the sample acquisition time is less than 
> .5 microsecond [ That's how I would apply Nyquist to the situation. ]

No. He means that the electronics between the analog input and the 
sampler will pass 1 MHz without significant attenuation or phase shift. 
(It's also important that the aperture uncertainty be as good as would 
be needed to sample at the full (2 MHz) rate.)

> Given that assumption and assuming the 20kHz sampling rate is not locked 
> to the 1 MHz carrier, I can not see what information could be obtained.
> 
> Obviously I'm missing something.

Yes. Read the chapter on sub-band sampling in Rick's book. (You located 
it on the shelf behind you.)

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

"Richard Owlett" <rowlett@atlascomm.net> wrote in message
news:11pubcn12vjih5a@corp.supernews.com...
> Bhaskar Thiagarajan wrote:
> > "clam" <jethrolam@gmail.com> wrote in message
> > news:tK2dnfBZWp3GuQLenZ2dnUVZ_vmdnZ2d@giganews.com...
> >
> >>I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling
for
> >>downconversion.  Is there any limiting factor on the ADC that I need to
> >>consider?  e.g., does the ADC need special circuit?  or small
> >>sample-and-hold delay time?
> >>
> >>CJLam
> >
> >
> > You need to be sure that the analog bandwidth of the ADC is higher than
> > 1MHz. You should find this as a parameter in the data sheet.
> > Also, I'm assuming your BW of the RF signal is not much more than about
> > 5kHz.
> >
> > Cheers
> > Bhaskar
> >
> >
>
> I assume Bhaskar means at least the sample acquisition time is less than
> .5 microsecond [ That's how I would apply Nyquist to the situation. ]

No  - as Jerry has already explained, one of the important things that
limits undersampling is the BW of the front-end of the ADC. Usually the
analog front-end of an ADC will be several tens of times (say 10x) the max
sampling rate it can support.

The sampling rate only needs to be higher than twice the BW of the signal
for satisfying Nyquist (this relates to my 5kHz remark...I used 5k and not
10k to give room for practical considerations like filter roll-off and
such).

Cheers
Bhaskar

>
> Given that assumption and assuming the 20kHz sampling rate is not locked
> to the 1 MHz carrier, I can not see what information could be obtained.
>
> Obviously I'm missing something.
>