×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Mixer: Small IF frequency vs LO/RF
2

Mixer: Small IF frequency vs LO/RF

Mixer: Small IF frequency vs LO/RF

(OP)
Hi,

I am designing an FMCW-GPR. My question is about the mixer. We are hoping to keep the IF very low (~10kHz) to make ADC easier, but our LO freq and RF must be ~1.5GHz. The mixer we want to buy does spec the IF as DC-1GHz, but I worry that there will be an issue having the IF so much smaller than the LO/RF. Any advice?

Thanks in advance.

RE: Mixer: Small IF frequency vs LO/RF

As long as it is a DC coupled IF port, you should have no trouble at all.  I would put a lowpass filter on the IF port also.  Otherwise you will have 1.5 Ghz leaking out that port, and possibly screwing up your ADC circuit's operation.  It could be something simple, like an RC filter, as long as the C has a good ground plane attachement right at the mixer case.

www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

RE: Mixer: Small IF frequency vs LO/RF

And, of course, expect to see harmonics coming out the IF port.  If you are looking for a 10 KHz tone, expect the ADC to be seeing 20 KHz, 30 KHz, etc (i.e. make sure your ADC has a high enough clock rate, or you use a GOOD antialiasing filter).

 

www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

RE: Mixer: Small IF frequency vs LO/RF

Using a 10kHz IF, what sort of temporal (distance) resolution are you hoping to achieve?

 

RE: Mixer: Small IF frequency vs LO/RF

(OP)
Thanks all.

VE1BLL: We are shooting for around 1-5cm depth resolution, not really sure what's going to be realistic though. The 10kHz is order of magnitude, because our ADC samples at a max of 100kHz.

 

RE: Mixer: Small IF frequency vs LO/RF

So, ah, what's the Speed of Light where you live?

 

RE: Mixer: Small IF frequency vs LO/RF

Make sure you verify that DC-coupling.  I know of some GHz designers that believe DC is anything below 10 KHz, so they don't see an issue with a series cap on the input!  Or maybe it is the marketing group that decides they can call 0.00001 GHz DC!  I won't name names, unless others are very curious.  

Last time I checked light was still moving 300 microns/picosecond around here.  

John D
 

RE: Mixer: Small IF frequency vs LO/RF

Please excuse my previous post; I was in the wrong domain.

 

RE: Mixer: Small IF frequency vs LO/RF

(OP)
Ok, I'm from Canada, so I thought that might be a joke about the cold making light move slower.

Thanks for the tips everyone.
 

RE: Mixer: Small IF frequency vs LO/RF

FMCW implies he is ramping his frequency.  His return frequency, mixed with the current transmit frequency, can give him a 10 KHz beat note if he is ramping fast enough.  There are, or course, a number of system constraints to deal with, but it is feasible.  I work on systems like that.

www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

RE: Mixer: Small IF frequency vs LO/RF

If my understanding is correct (?), then it's not really a simple beat note (10 kHz or other), unless there's just a single target at a single distance. The baseband signal output from the mixer should be more like a complex spectrum (essentially audio) where the frequency bins are proportional to distance.

But I admit that "FMCW" is exactly what I overlooked with my first two responses. I was thinking in the Time domain, not the Frequency domain. 'My bad.'

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close