Impact of intermodulation interference when use a low-noisee-amplifier in TPMS

[Liam.Zhai]

Due to the bad sensitivity of PCB-antenna-receiver (433.92MHz),I choosed a low-noise-amplifier to improve it.

However, the amplifier bring a new issue( Intermodulation interference)that the receiver need to  set closely to a TBox  which is applying 868MHz signal.The sensitivity of receiver(433.92MHz) get worse if i turn on the 868MHz signal.

For all i known, intermodulation signals(433) are generated by combining 433 and 868 , but its signal power is much smaller than that of 433 main frequency signal.
So why the intermodulation signal affect the sensitivity of 433 ?

[MarkT]

Is any frequency-selectivity before the LNA, ie is the antenna tuned with good Q ?  It might just be the LNA is overloading.

[Liam.Zhai]

Hi,thank you for your suggestions!
I try to answer your question ,if there is any item not clear ,please let me know!

Is any frequency-selectivity before the LNA, ie is the antenna tuned with good Q ?

no,there is no frequency-selectivity or filter before the input of LNA . We usually consider the impedance and SWR of the antenna, matching the impedance to 50 ohms with a SWR less than 1.1 .

 It might just be the LNA is overloading.

How can I determine if the LNA is overloaded ?

[Uky]

You will need two signal sources. One for the signal of interest that can have the output adjusted for the requirement of the product (but see below).
The other for the interferer. If your system is using 50 Ohms, use a resistive power splitter. (that will add another 6dB of loss)

Then add at least 10 dB attenuation in each
signal generator branch in order to acheive good isolation between the two sources.

Then modulate the 433 generator witha a signal that the receiver/decoder in the other end will decode and
that will produce some output that you can verify for proper signal transfer.

Then increase the 868 signal level until the 433 receiver starts missing decodes.
You will most likely need to sweep the 868 signal over its frequency band in order to measure and check the relationship "Signal/Interferer".

Then you can decide if your design is good enough.

 

[Liam.Zhai]

Yeah...As you suggested , i used two signal generator and a spectrum to try to find the difference when the noise signal is on (please see the attachment ).

From the spectrum which is connected to the output of  LNA , I can clearly see another signal( Marker 2 ) combined  by 835 MHz and 433 MHz signal , when I scan the frequency setup on the up generator and keep the 433 generator , the frequency of the another signal will move to 433 MHz when I set the upper generator to 867.3 MHz.

I assume that signal is a intermodulation interference ,and it truly reduce my receiver sensitivity by 5-8 dB .

But , the power of the intermodulation is 20dB lower than my 433 MHz(when I turn on the upper generator power to the biggest) .
So , based on my rudimentary understanding of signals , even there is a intermodulation exist ,its power is not enough to affect the quality of the real signal .

So I was wondering if you can explain how the  intermodulation  affect it or teach me how avoid this ?
Thank you very much !!

[Uky]

Not enough information.

You will need to provide much more details about your application
in order to evaluate the problem.