Forums

ADC limitations for bandpass/IF sampling

Started by clam December 13, 2005
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. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
"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. >