Why the NF has worsened when mixer was added?

[liberal]

I design a Receiver as below with MiNi's Product .
>>
>> filter---AMP----MCA1-24---MAR-6--filter--amp2--
>>
>> nowthe receiver NF tested is 14dB.
>>
>> But the first filter and AMP 's Gain and NF tested is 14.8dB and 9dB
>>
>> the MAR-6\ filter and amp2's Gain and NF tested is 28dB and 2.8dB
>>
>> but why the NF is worsened to 14dB when added Mixer MCA1-24,the LO I
>> used is better.the MCA1-24's VSWR is not very good,does it matter?can
>> u tell me why the NFworsened?Thank you.

 

[biff44]

Re: why NF worsened??

Because the mixer has around 8 db of loss.

Search on google for a "cascaded noise figure" tool to help you determine the overall gain and noise figure of the receiver chain. Add ALL of the components during the calculation.

A mixer with 8 db of conversion loss would be inputed as -8 dB gain, and 8 dB noise figure.

Also, there is a chance that you are going to have 3 dB more noise figure than you expect due to image noise--it depends on the frequency plan and the filters you use.

 

[vfone]

Re: why NF worsened??

Something is not ok. Even if you have a high insertion loss input filter (let's say 5dB) and a poor 2dB NF LNA, you get better than 14dB system NF.
Download AppCAD software, go to Signal-Systems/NoiseCalc and verify what is going wrong.

http://www.hp.woodshot.com/