mixer downconversion - query - RF Cafe Forums

 Post subject: mixer downconversion - query

Posted: Sun Oct 09, 2005 7:08 pm 

Hi guys

I'm trying to downconvert the frequency range : 800MHz to 840MHz to
100Hz to 40MHz with LO being set at 799.9999MHz.

The mixer that i'm using is ZEM-4300 minicircuits.

I swept the RF signal and tried to study the IF signal.

The reading tells me that the IF ZERO has shifted to 1.1KHz.
As i reduce RF from 840MHz towards 800MHz with LO maintained constant , IF frequency reduces respectively from 40MHz towards 100Hz.

With RF at 800.001MHz i expect to see a 1KHz signal but i see a signal that is closer to 60Hz(or DC). When i reduce RF beyond this the IF starts to increase. i.e When RF = 800MHz i expect to see a 100Hz signal but instead i see 1.1KHz signal.

The signals were observed using a Agilent Spectrum Analyser and Dynamic Signal Analyser.

This data tells me that the Zero of the IF spectrum has shifted to 1/1.1KHz. Can anyone tell me what could be the issue in this regard.
The power level of the IF signal has been constant throughout the spectrum.

 Post subject:

Posted: Mon Oct 10, 2005 9:17 am 

Do you have the RF and LO sources locked to the same reference? If not, that alone cann create the discrepency.

The 60 Hz you see has got to be from a poor AC line ground or a strong AC field in the vicinity of your DUT.

Hope that helps.

 Post subject:

Posted: Mon Oct 10, 2005 12:35 pm 

The RF source, LO source and the Spectrum Analyser were locked to a common reference signal.

 Post subject:

Posted: Mon Oct 10, 2005 2:39 pm 

Have you tried making the measurement at a different set of frequencies that don't use your currect frequency settings? I'd try moving the LO to to a new frequency that does not use any of your current numbers, then make the measurement. That would tell you if the signal generator is screwing up and outputting a frequency other than that you tell it to. Have you verified the LO and RF sig gens are actually putting out the exact frequencies you are programming it for? That 1 kHZ step sounds like a sig gen problem.

 Post subject:

Posted: Mon Oct 10, 2005 2:57 pm 

hey my initial settings were RF- 600-640MHz and LO at 599.9999MHz. I have tried at various frequency ranges too but seeing the same result.

Did the check the signal generated from the two sources they provide the right frequency content.

I thought the Spectrum Analyser might not be too sensitive if we try to measure signals near DC so went to OSCOPE measurements to see whether i'm seeing the right frequency but the OSCOPE measurements revealed the same.

I'm left clueless of what is happening in this circuitry.