Mixer: Small IF frequency vs LO/RF 2

[questionguy]

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.

 

[biff44]

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.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[biff44]

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).

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[VEBill]

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

 

[questionguy]

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.

 

[VEBill]

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

 

[zappedagain]

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

 

[VEBill]

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

 

[questionguy]

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.

 

[biff44]

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.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[VEBill]

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.'