help with multiplexers
 

Hi,

array processing / analog to digital cards

I would like to process signal from an antenna array with 8 elements.
Initially I was thinking of using an A/D card to digitize, with 8
input ports for 8 antenna array. Each antenna would have its cable
coming into each port (assuming no RF analog) on to the card

But recently a friend told me that a "multiplexer" can be used to
multiplex the 8 antenna signals into one and piped to a single port of
A/D card. That way a single port (cheaper) card could be used and less
hassle with cabling.

Assuming the bandwidth to be received/processed is 30 Mhz and the A/D
is 70Mhz, can someone pls guide me:

1. Can such processing be done using multiplexers?
What are the drawbacks in terms of losing signal info, etc.

2. What are the important specs/considerations when dealing with
multiplexers in such a setup?

3. Can you recommend a brand?

4. If multiplexers are not useable, can you suggest a cheaper
alternative to buying a 8 port A/D card with 8 a/d chips, etc.

Thanks

Russ

help with multiplexers
 

wrote:
Hi,

array processing / analog to digital cards

I would like to process signal from an antenna array with 8 elements.
Initially I was thinking of using an A/D card to digitize, with 8
input ports for 8 antenna array. Each antenna would have its cable
coming into each port (assuming no RF analog) on to the card

But recently a friend told me that a "multiplexer" can be used to
multiplex the 8 antenna signals into one and piped to a single port of
A/D card. That way a single port (cheaper) card could be used and less
hassle with cabling.

Assuming the bandwidth to be received/processed is 30 Mhz and the A/D
is 70Mhz, can someone pls guide me:

1. Can such processing be done using multiplexers?
What are the drawbacks in terms of losing signal info, etc.

2. What are the important specs/considerations when dealing with
multiplexers in such a setup?

3. Can you recommend a brand?

4. If multiplexers are not useable, can you suggest a cheaper
alternative to buying a 8 port A/D card with 8 a/d chips, etc.

Thanks

Russ

The multiplexer works like a switch: it connects channel 1 to the ADC,
then channel 2, then channel 3, etc. Because you have eight antennas to
sample, it reduces the ADCs effective sampling rate by a factor of eight.

The multiplexer itself won't have perfect isolation from one channel to
the next, and the ADC (and the multiplexer's back end) won't settle
infinitely fast, so if you're switching things quickly you will see some
settling-time induced cross-talk between antennas. These two effects
may be severe unless you spend as much on your multiplexer as you would
on seven ADC channels.

Electronic analog switches like the ones used in multiplexers will
attenuate your signal. A discrete multiplexer using PIN diodes will
minimize this (and be expensive), a multiplexer using monolithic silicon
switches will be worse.

Other than that, and all the stuff I haven't thought of yet, you should
be A-OK.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

help with multiplexers
 

wrote in message
ups.com...
Hi,

array processing / analog to digital cards

I would like to process signal from an antenna array with 8 elements.
Initially I was thinking of using an A/D card to digitize, with 8
input ports for 8 antenna array. Each antenna would have its cable
coming into each port (assuming no RF analog) on to the card

But recently a friend told me that a "multiplexer" can be used to
multiplex the 8 antenna signals into one and piped to a single port of
A/D card. That way a single port (cheaper) card could be used and less
hassle with cabling.

Assuming the bandwidth to be received/processed is 30 Mhz and the A/D
is 70Mhz, can someone pls guide me:

1. Can such processing be done using multiplexers?
What are the drawbacks in terms of losing signal info, etc.

2. What are the important specs/considerations when dealing with
multiplexers in such a setup?

3. Can you recommend a brand?

4. If multiplexers are not useable, can you suggest a cheaper
alternative to buying a 8 port A/D card with 8 a/d chips, etc.

Thanks

Russ


You seem to think that the multiplexer approach will give you something for
nothing - it won't. If you replace eight 70MHz A/Ds with one A/D and an
8-input multiplexer you'll have to sample eight times faster - i.e. at a
rate of at least 480MHz. What is more, you will need a multiplexer that can
switch at 480MHz - i.e. spend only 2ns on each channel. In that time all
switching transients - and the signal itself - must settle and THEN, the A/D
perform the conversion.

Not mission impossible, but if you need to ask these questions, you might
find this project quite difficult.

John A