IQ demod / IF or Direct conversion
 

I've simulated a circuit and a little confused... If I have an BPSK
RF signal at 1KHz, and I mix it with an LO of .9KHz, then I should get
100Hz still in BPSK format? PSpice says otherwise. If I filter the IF
for lowpass at 100Hz, it doesnt look like anything I could possible
work with and doesnt sync to anything. If I pass everything below
200Hz, it looks more like a sign wave, that's syncronous with the
modulation.

On the other end of the spectrum (no pun intended), if I went with
direct down converstion, what I get at the output of the mixer is a
sign that it twice the RF freq that' shifts 'dc offset'. Should I be
filtering for the DC offset?
Does anyone have a link to actual waveforms that I should see in a
typical IQ demodulation scheme? Or at least some good docs for
understanding the waveforms and what I should be looking for?

Mikal wrote:

I've simulated a circuit and a little confused... If I have an BPSK
RF signal at 1KHz, and I mix it with an LO of .9KHz, then I should get
100Hz still in BPSK format? PSpice says otherwise. If I filter the IF
for lowpass at 100Hz, it doesnt look like anything I could possible
work with and doesnt sync to anything. If I pass everything below
200Hz, it looks more like a sign wave, that's syncronous with the
modulation.

On the other end of the spectrum (no pun intended), if I went with
direct down converstion, what I get at the output of the mixer is a
sign that it twice the RF freq that' shifts 'dc offset'. Should I be
filtering for the DC offset?
Does anyone have a link to actual waveforms that I should see in a
typical IQ demodulation scheme? Or at least some good docs for
understanding the waveforms and what I should be looking for?

Don't multiple post. If you must make the message visible on multiple
newsgroups then cross-post it.

And think twice before you cross-post.

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

"Mikal" wrote in message
ups.com...
I've simulated a circuit and a little confused... If I have an BPSK
RF signal at 1KHz, and I mix it with an LO of .9KHz, then I should get
100Hz still in BPSK format? PSpice says otherwise. If I filter the IF
for lowpass at 100Hz, it doesnt look like anything I could possible
work with and doesnt sync to anything. If I pass everything below
200Hz, it looks more like a sign wave, that's syncronous with the
modulation.

When mixed down to 100 hz, will there be enough bandwidth at that frequency
for the BPSK to work ? I have no idea but it does take a certain ammount of
bandwidth for a given ammount of data. Also are the components of the
circuit of the correct value to pass frequencies below 100 hz ?

Sorry.
How do I 'cross post'? My reasoning for doing what I did, what that
not everyone goes to every newsgroup, and the topic fit in two
newsgroups. So... how do I cross post.
..and thanks for the information. I'm actually using PSpice, so I
can make a 'perfect' filter just by editing a 'block'. Let me push
some more sims based on your feedback.
(Yea, 1khz RF? actually, I used that freq just for sim :).

Mikal wrote:
Sorry.
How do I 'cross post'? My reasoning for doing what I did, what that
not everyone goes to every newsgroup, and the topic fit in two
newsgroups. So... how do I cross post.

It depends on your newsreader. Generally you just put multiple
newsgroups in the "to" section. Your newsreader will automatically post
to all the groups, and the item will carry that information.

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

I could be mistaken, but I think "cross posting" would be to enter a
separate post to EACH newsgroup thread, separate from any other newsgroup
thread...

This avoids the following:
Including multiple newsgroups in the posting of a single post--causes any
reply to that post to be sent to ALL the newsgroups that single post has
been posted to...

Warmest regards,
John

"Mikal" wrote in message
oups.com...
Sorry.
How do I 'cross post'? My reasoning for doing what I did, what that
not everyone goes to every newsgroup, and the topic fit in two
newsgroups. So... how do I cross post.
..and thanks for the information. I'm actually using PSpice, so I
can make a 'perfect' filter just by editing a 'block'. Let me push
some more sims based on your feedback.
(Yea, 1khz RF? actually, I used that freq just for sim :).

"John Smith" wrote in message
...

This avoids the following:
Including multiple newsgroups in the posting of a single post--causes any
reply to that post to be sent to ALL the newsgroups that single post has
been posted to...

Which is basically what you want ... all the newsgroups being involved in
the conversation. When you cross post, most newsreaders can recognize that
and mark the posting already read in the other newsgroup. While not
everyone may be subscribed to all the newsgroups, a lot of people will if
they are related subjects. This avoids them reading the same post multiple
times. It also avoids duplicate responses. If you multiply post and get a
valid response in a newsgroup I don't read, I may well repeat essentially
the same response.

If it is appropriate to cross-post (and it is often overused), then it seems
far better than multiply posting essentially the same message to different
groups.

...

John Smith wrote:

I could be mistaken, but I think "cross posting" would be to enter a
separate post to EACH newsgroup thread, separate from any other newsgroup
thread...

This avoids the following:
Including multiple newsgroups in the posting of a single post--causes any
reply to that post to be sent to ALL the newsgroups that single post has
been posted to...

Warmest regards,
John


Take the trouble to do a web search on the phrase "cross post" and
you'll get not only a definition but an explaination of why it's better
than multiple posting.

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

I have worked out my own methods--but I am sure others may use that... good
post...

Warmest regards,
John

"Tim Wescott" wrote in message
...
John Smith wrote:

I could be mistaken, but I think "cross posting" would be to enter a
separate post to EACH newsgroup thread, separate from any other newsgroup
thread...

This avoids the following:
Including multiple newsgroups in the posting of a single post--causes any
reply to that post to be sent to ALL the newsgroups that single post has
been posted to...

Warmest regards,
John


Take the trouble to do a web search on the phrase "cross post" and you'll
get not only a definition but an explaination of why it's better than
multiple posting.

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

Mikal wrote:

Sorry.
How do I 'cross post'? My reasoning for doing what I did, what that
not everyone goes to every newsgroup, and the topic fit in two
newsgroups. So... how do I cross post.
..and thanks for the information. I'm actually using PSpice, so I
can make a 'perfect' filter just by editing a 'block'. Let me push
some more sims based on your feedback.
(Yea, 1khz RF? actually, I used that freq just for sim :).


I don't think Google lets you cross post. You need a real news
server and software to do it.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida