effect of using two LNA in series?

[devil6600]

hi

can anyone tell me effect of using two LNA is series? for example if a single LNA provides gain of 10dB then using two in series will increase gain to 20dB or in between 10-20dB? for some reason i think this is not true but i don't know why?

thanks

 

[WimRFP]

You should get about the sum of the two.

There can be some deviation as the gain is mostly measured as an insertion gain (so driven from 50 Ohms and with 50 Ohms load). As the input and output impedance of the LNA may deviate from 50 Ohms, the first LNA may see something else (then 50 Ohms load). This may affect gain (and circuit stability).

If you change the cable length between the two LNAs (steps of about 1/8 lambda) and this changes the gain, you can be sure you have some impedance mismatch. Note that the gain may increase with certain cable length.

 

[devil6600]

You should get about the sum of the two.

There can be some deviation as the gain is mostly measured as an insertion gain (so driven from 50 Ohms and with 50 Ohms load). As the input and output impedance of the LNA may deviate from 50 Ohms, the first LNA may see something else (then 50 Ohms load). This may affect gain (and circuit stability).

If you change the cable length between the two LNAs (steps of about 1/8 lambda) and this changes the gain, you can be sure you have some impedance mismatch. Note that the gain may increase with certain cable length.

thanks for replying.
so if i use additional LNA with proper cable length then can GPS sensitivity be increased with it? actually i have a gps module with inbuilt LNA so adding another LNA to it with proper cable length can result in increase GPS sensitivity?

 

[vfone]

If you ad an LNA with gain about 15dB and NF below 1dB, followed by a good coax cable, definitely you will get better sensitivity.

Use the freeware program AppCAD to see the effect on sensitivity when add an LNA and cable to your receiver.
After installation go to System-Signals/ NoiseCalc

AppCAD

 

[WimRFP]

Hello,

For a practical application, forget the cable length in between the two LNAs. This was just to explain that due to non-50 Ohms impedances you can have somewhat more or less gain then the sum of the two.

To have benefit of an additional LNA, you should know the noise figures of the LNA, the receiver and the cable loss. The LNA with integral antenna I saw had about 20..30 dB gain and it was never necessary to add additional gain (unless you have very long bad cable inbetween). If your GPS receiver has 3 dB noise figure, and your external LNA has (for example) F=1.5 dB with 10 dB gain, the overall noise figure would be 1.8 dB. Further increase of gain will not bring you much more.

If you have the means you can try it. If it doens't improve, the reason isn't poor sensitivity of the GPS receiver. If you have passive antenna, place the first LNA as close as possible to the antenna to avoid noise increase due to cable loss.

 

[biff44]

LNA is an amplifier optimized for very low noise figure. Typically it is only used right at the antenna connection for a system. Being optimized for low noise figure, it often does not have a very good output power (as evidenced by a low output compression point or intercept point).

If you have a VERY LOW LEVEL SIGNAL, you can put two LNAs together just fine. If the signal is stronger, such as maybe -20 dBm, an amplifier labeled "LNA" would probably make a poor choice as a 2nd amplifier in a chain. You would want an amplifier with a higher output compression point, and would not care much about its noise figure.

 

[buckaroo]

I don't think so, actually, the IP3 of the LNA is limited, as a result, the 2nd LNA will be saturated by large OOB signal;

 

[devil6600]

thanks for all the reply...
i managed to increase gps sensitivity without any LNA. i simply soldered a naked wire around the contact from the gps antenna and wrapping that wire around the gps board. earlier the same board was able to track 4-5 satellites and now it is able to track more than 8 satellite. now my question is what is the effect of adding a wire? the gps itself have an inbuilt LNA so the added wire acts as a loop antenna?
i would really appreciate any information on this...

 

[WimRFP]

The very likely thing is that you have a bad antenna, or the antenna orientation is wrong (for example upside down, less likely interference from own board. You may post data about the antenna (photo of complete assembly?). Sure that the antenna is for the intended frequency band?

 

[tsvetelin_velkov]

Most likely your inbuilt / embedded antenna is not really working, maybe it has a bad soldering, or is broken ... Adding wire as you explain is like adding antenna, but this is probably not very effective as the satellites use circular polarisation, so simple wire wrapped around the board is not the best solution ...
Try to completely remove the embedded antenna and replace with similar one, you can also try to replace it with external one and to see what is the effect. But I really recommend you to use a real antenna and not wires.

Keep in mind that the embedded LNA is build with ESD sensitive components, you can easy destroy it with ESD discharge. Usually the input is well protected, but I would not rely on that, because the high frequency amplifiers are very sensitive.

If you try to connect another LNA in front of your embedded you will most probably saturate the second amplifier and it will just not work. In some cases the signal might be too much for the input of the second LNA and can even destroy it.