LNA input filter problem
I am working on design of two-stage LNA for GPS/GALILEO/GLONASS, specifically for band 1559-1610MHz.
The problem is input filter. The antenna to be used with the LNA is ceramic patch antenna, which is able to catch signal from GSM-1800 band with considerably higher signal level than the one of GPS/Galileo/GLONASS.
What I need to do is to filter off this band, either with some low-pass or band-pass filter. Attenuation of 5-6dB is enough for 1710MHz. As this is input filter, I need the one which would least increase noise figure. I am not able to design anything with microstrip low-pass filter with required attenutation on the very narrow bandwidth 100MHz, which is between 1610MHz and 1710MHz (GSM1800). Though microstrip bandpass filter has required attenuation, its dimensions exceed given maximum dimensions of my PCB 35x35mm FR-4. Standard lumped LC lowpass filter can?t be designed due to low Q inductance on this high frequency. I also can?t find a saw filter for the band of 1559-1610MHz, which wouldn?t influence noise figure.
Can anybody advise me with some solution to reach attenuation at least approximately 5-6dB which wouldn?t increase noise figure?
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Coaxial cable or waveguide? Coax, you might be able to do perhaps 0.3 dB loss. What are you receiving in the pasband, QPSK? BPSK?
vfone:
Thank your for the tip, but I have seen this before and none of these filters meet the requirements.
Noise figure of my LNA can be max 2dB and it will not be satisfactory if it would have first passive unit - filter in the band of 1559-1610MHz with attenuation 3dB.
biff44:
LNA will be directly connected to the pasive ceramic antenna. Coax will be connected at output LNA. I receive BPSK GPS/Glonass signal and BOC Galileo signal.
GPS signals have very low power values and therefore NF of the GPS LNA should ve as low as possible...
You can not use filter after the antenna because Insertion Loss of this filter will be main contributor of Input Referred Noise
You may design a LNA with High Dynamic Range...So GSM signal can not saturate your LNA and then you can put this filter then
That can be a solution...
Almost all of the smartphones on the market (with GPS receiver embeded) use SAW filters in front of the LNA, for multiple reasons.
Some of the filters have maximum 2dB inband insertion loss, with a minimum of 0.45dB, which is reasonable.
Yes.. You're right...I knew the SAW filters have very high attenuations but I think I'm getting older...-))
Anyway...
This filter has minimum loss... Take a look..
TriQuint Semiconductor, Inc. - Product Information 856756
But Bandwidth is narrower than you would..
The reason why cellphones need a filter on GPS input is because the internal 1800 MHz or 1900 MHz cellular transmitter which puts out up to 1 watt. There is also broadband noise caused by the RF PA that the GPS input filter will not remove. There is also WiFi and Bluetooth transmitters in the phone.
If your application does not include transmitter you can get by without an input filter on GPS (with its associated NF degradation). Many high end GPS receivers put a filter after the first LNA to remove outband signals that may create IM further back in the receiver.
There are a few interference sources that are very strong and can cause IM in the GPS receiver, like UHF TV stations and very high power Doppler weather radars. An input filter helps in these cases.
Definitely there are such SAW filters to filter out the 1710MHz interference, I forget the part number at this moment.