Suspected to have LO Leakage Problem

Hi all,

I’ve a problem to integrate a mixer, BCO and IF signal of 10MHz as the inputs.

However, the result obtained was as shown in Picture 001 in the attachment.

Is the result correct? Shall the output be a single mixed spectrum (4.803GHz) only? I suspect it is due to LO leakage problem.

What should I do to get the correct output signal?

Picture 005 shows the spectrum of the IF BPSK signal (10MHz)






Any help and advise is appreciated.

Thanks

What is a BCO?

Sorry typo error.

BCO = VCO

It is not that clear what you are trying to do. If I assume you are trying to transmit 4.803 ghz, by upconverting a 10 mhz bspk if signal with a 4.793 ghz lo, then yes, the LO leakage is huge. It is big enough that the receiver will have trouble ingnoring it. Also, you have an equal power other modulated signal (at 4.783 MHz), that will jam other users.

What type of mixer do you have? If you have access, try varying the LO power up/down to get best rejection. Make sure you have a "balanced" mixer that serves to null out the LO leakage. Bump up the IF power ot the mixer to get bigger sidebands out.

The signal is too close to the LO to easily bandpass filter it. But there are other tricks to reduce the LO leakage if you don't mind more complexity.

What type of mixer did you use ???
For these kind of applications, you should use double balanced mixers.

I think you should take 2nd harmonic of IF frequency lower at input of mixer. It is either mixed before the input, or happend in the mixer.
You can decrease 1,2,3dB for the IF power, see what happens on the image spur.

BigBoss said:

What type of mixer did you use ???
For these kind of applications, you should use double balanced mixers.

Click to expand...

I'm using DMY0207M Miteq mixer. It's a double balance mixer.

The datasheet can be found here: **broken link removed**

Thanks!!!

The LO signal seems to be too huge when compared to the IF signal.
BPSK constellation looks correct. RF spectrum is too low in power.
1. reduce the LO power level and try increasing the IF power level.
2. check the mixer module alone by connecting IF and LO signals from two separate signal generators by supplying the power levels as given in the datasheet.
3. Check if all the modules connecting to mixer are 50 ohm?

that miteq mixer should work perfectly, unless you blew it out somehow. Try replacing it with another just like it.

Other than that, you are giving it the wrong power levels.

Put +7 dBm into the LO port and 0 dBm into the IF port. Double check you are using the right ports.

What is the nature of the signal going into the IF port? Is the 10 mhz signal bpsk modulated (i.e. a modulated 10 MHz sine wave), or are you using the mixer as the bpsk modulator itself (IF is a +/- ~ 0.5V square wave)???

Hi,

I've tried replacing the IF by using test equipment (to generate 10MHz BPSK modulated signal). So, is this the correct output that I should get after integrating the IF with LO of 4.793GHz on a double balancedmixer?

did you supply IF as a tone or a modulated signal. if it is a tone-- fig shows mixer is not acting as a double balanced. since all the sidebands are seen.
if you are supplying a modulated signal -- check in VSA the spectrum and the constellation.

I supplied the IF with modulated signal. OK, will do check the output on the VSA.

Thanks!