| Author |
Message |
clam
Guest
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 Posted:
Wed Dec 14, 2005 1:16 am Post subject:
ADC limitations for bandpass/IF sampling |
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I want to sample a 1MHz RF signal at 20kHz (i.e., IF/Bandpass sampling fo
downconversion. Is there any limiting factor on the ADC that I need t
consider? e.g., does the ADC need special circuit? or smal
sample-and-hold delay time?
CJLam |
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Bhaskar Thiagarajan
Guest
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| Posted:
Wed Dec 14, 2005 1:16 am Post subject:
Re: I'm confused ;) was[Re: ADC limitations for bandpass/IF |
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"Richard Owlett" <rowlett@atlascomm.net> wrote in message
news:11pubcn12vjih5a@corp.supernews.com...
| Quote: | 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
| Quote: |
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.
|
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Eric Jacobsen
Guest
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| Posted:
Wed Dec 14, 2005 1:16 am Post subject:
Re: ADC limitations for bandpass/IF sampling |
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On Tue, 13 Dec 2005 13:43:23 -0600, "clam" <jethrolam@gmail.com>
wrote:
| Quote: | 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 |
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Richard Owlett
Guest
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| Posted:
Wed Dec 14, 2005 1:16 am Post subject:
I'm confused ;) was[Re: ADC limitations for bandpass/IF sam |
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Bhaskar Thiagarajan wrote:
| Quote: | "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. |
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Bhaskar Thiagarajan
Guest
|
| Posted:
Wed Dec 14, 2005 1:16 am Post subject:
Re: ADC limitations for bandpass/IF sampling |
|
|
"clam" <jethrolam@gmail.com> wrote in message
news:tK2dnfBZWp3GuQLenZ2dnUVZ_vmdnZ2d@giganews.com...
| Quote: | 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 |
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Jerry Avins
Guest
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| Posted:
Wed Dec 14, 2005 1:16 am Post subject:
Re: I'm confused ;) was[Re: ADC limitations for bandpass/IF |
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Richard Owlett wrote:
| Quote: | 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.)
| Quote: | 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.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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Steve Underwood
Guest
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| Posted:
Wed Dec 14, 2005 7:44 am Post subject:
Re: I'm confused ;) was[Re: ADC limitations for bandpass/IF |
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Bhaskar Thiagarajan wrote:
| Quote: | "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.
It is important to take care in how you define "ADC" in this context. To |
so many people these days, it implies a chip, and that is how they think
of an ADC.
The key thing is the bandwidth up to the sampler, and the aperature of
the sampler itself. These days the sampler is typically within a
monolithic IC, and you cannot affect its performance. However, if you
are using an ADC chip with a low performance sampler you could put a
high performance external one in front of it, and get the overall result
you need. In fact, you can use a super duper external sampler, and
multiple slower ADCs. You pass successive narrow samples to the ADCs in
turn, and combine their outputs into a final high speed sample stream.
This form of polyphase sampling is much loved by those of a masochistic
tendency, for the pain and misery it can cause. :-) It is a nightmare to
get multiple ADCs like this to track over time and temperatures.
| Quote: | 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.
Everyone misses something, even if its only their lost youth. :-) |
Regards,
Steve |
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Joerg
Guest
|
| Posted:
Wed Dec 14, 2005 7:53 am Post subject:
Re: I'm confused ;) was[Re: ADC limitations for bandpass/IF |
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|
Hello Bhaskar,
| Quote: | 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.
|
If the ADC input circuitry doesn't offer such bandwidth you can sample
and hold. It's really no big deal if done right. I often sample via a
diode bridge driven from a toroid so that I don't have to worry about
the charge injection of CMOS switches.
Regards, Joerg
http://www.analogconsultants.com |
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Jerry Avins
Guest
|
| Posted:
Wed Dec 14, 2005 8:09 am Post subject:
Re: I'm confused ;) was[Re: ADC limitations for bandpass/IF |
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Steve Underwood wrote:
...
| Quote: | Everyone misses something, even if its only their lost youth. :-)
|
Ooh! I have to remember that one!
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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Joerg
Guest
|
| Posted:
Thu Dec 15, 2005 1:16 am Post subject:
Re: I'm confused ;) was[Re: ADC limitations for bandpass/IF |
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Hello Bhaskar,
| Quote: | Neat! I know of a few who've done this although I'm not masochistic enough
(sounds like you are :-))
|
I am not a masochist, at least I hope not. We just didn't have any other
choice back then to get to 12 bits. But I have to confess that I
really like those challenges. And when some folks say that it cannot be
done it's even more fun to get it done :-)
Regards, Joerg
http://www.analogconsultants.com |
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Steve Underwood
Guest
|
| Posted:
Thu Dec 15, 2005 1:16 am Post subject:
Re: Steve Underwood and Newsgroup wierdness - was[Re: I'm co |
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Richard Owlett wrote:
| Quote: | Jerry Avins wrote:
Steve Underwood wrote:
...
Everyone misses something, even if its only their lost youth. :-)
Ooh! I have to remember that one!
Jerry
I was wonder to what Jerry was responding so I did a Google of comp.dsp
for Author=Underwood and found the relevant message.
My ISP provides a Supernews account, that message does not appear.
*HOWEVER* his 12/10 message "Re: Happy Hmas!" appears on Supernews, but
not on Google search ;}
Aside to Steve re his post
Never fought battles you implied but reminded me of two aspects of "good
old days"
-- chopper stabilized op amps
-- multi ampere ground loops in audio racks of a radio station
|
Steve Underwood has no weirdness, but newsgroups generally do. :-)
Google groups usually has everything. However, other news servers
typically seem to miss stuff. Sometimes a little, and sometimes a lot. I
often go to Google groups to figure out what a thread was really about.
Steve |
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Bhaskar Thiagarajan
Guest
|
| Posted:
Thu Dec 15, 2005 1:16 am Post subject:
Re: I'm confused ;) was[Re: ADC limitations for bandpass/IF |
|
|
"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:qJ_nf.33643$q%.17135@newssvr12.news.prodigy.com...
| Quote: | Hello Steve,
This form of polyphase sampling is much loved by those of a masochistic
tendency, for the pain and misery it can cause. :-) It is a nightmare to
get multiple ADCs like this to track over time and temperatures.
Nah, there are situations where you simply have to do it or else the
project would die. Done it a few times.
The trick is not to rely on any manual alignment whatsoever but to run a
little uC alongside and then provide auto-adjust circuitry for all three
variables for all ADCs minus one. Gain, offset and clock phase. It's not
really rocket science. On one project they parked a big TI DSP just for
that purpose. In the end its 'employment rate' hovered around a percent
or two, almost idle. The cost for the analog parts and the DACs to set
the trim levels was about $15 or so, peanuts compared to the cost of one
of the ADCs. This was not a one-off project but it remained in
production for about seven years until a faster and cheaper ADC type
came out where we could get it done without multi-phase.
|
Neat! I know of a few who've done this although I'm not masochistic enough
(sounds like you are :-))
Sometimes there are no other options and these kinds of approaches are
warranged - but in the case of the OP, I'd bet there are several viable
ones.
Cheers
Bhaskar
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Chris Bore
Guest
|
| Posted:
Thu Dec 15, 2005 1:16 am Post subject:
Re: ADC limitations for bandpass/IF sampling |
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|
The ADC need only be suitable for the 20 kHz sampling.
But the analog hold circuit must be capable of the 1 MHz.
Chris
=====================
Chris Bore
www.bores.com
clam wrote:
| Quote: | 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 |
|
|
| Back to top |
|
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Joerg
Guest
|
| Posted:
Thu Dec 15, 2005 1:17 am Post subject:
Re: I'm confused ;) was[Re: ADC limitations for bandpass/IF |
|
|
Hello Steve,
| Quote: | This form of polyphase sampling is much loved by those of a masochistic
tendency, for the pain and misery it can cause. :-) It is a nightmare to
get multiple ADCs like this to track over time and temperatures.
|
Nah, there are situations where you simply have to do it or else the
project would die. Done it a few times.
The trick is not to rely on any manual alignment whatsoever but to run a
little uC alongside and then provide auto-adjust circuitry for all three
variables for all ADCs minus one. Gain, offset and clock phase. It's not
really rocket science. On one project they parked a big TI DSP just for
that purpose. In the end its 'employment rate' hovered around a percent
or two, almost idle. The cost for the analog parts and the DACs to set
the trim levels was about $15 or so, peanuts compared to the cost of one
of the ADCs. This was not a one-off project but it remained in
production for about seven years until a faster and cheaper ADC type
came out where we could get it done without multi-phase.
Regards, Joerg
http://www.analogconsultants.com |
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| Back to top |
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Richard Owlett
Guest
|
| Posted:
Thu Dec 15, 2005 1:17 am Post subject:
Steve Underwood and Newsgroup wierdness - was[Re: I'm confus |
|
|
Jerry Avins wrote:
| Quote: | Steve Underwood wrote:
...
Everyone misses something, even if its only their lost youth. :-)
Ooh! I have to remember that one!
Jerry
|
I was wonder to what Jerry was responding so I did a Google of comp.dsp
for Author=Underwood and found the relevant message.
My ISP provides a Supernews account, that message does not appear.
*HOWEVER* his 12/10 message "Re: Happy Hmas!" appears on Supernews, but
not on Google search ;}
Aside to Steve re his post
Never fought battles you implied but reminded me of two aspects of "good
old days"
-- chopper stabilized op amps
-- multi ampere ground loops in audio racks of a radio station |
|
| Back to top |
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