Doubts on designedtwo stage low noise amplifier(LNA) measured results

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pusparaga

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Sir,
I have designed the two stage low noise amplifier using Advanced Design System 2009 at 1.7GHz. I have used the followings things to design the two stage LNA.
Transistor used is Avago ATF54143 transistor 3v,60mA
Rogers RT/duroid RO4003C is with following specifications
H(height of the substrate) =0.508mm
T(Thickness of copper cladding)=0.035mm
Er=3.55
TanD=0.0029
First I have designed the single stage LNA and later I have made it two stage by connecting coupling capacitor between two stages. First stage and second stage are the exact copy of each other separated by a coupling capacitor. I have not used Modelithics models of capacitor and resistors for my design of two stage LNA. I have used market available capacitor and resistors equivalent SRF and ESR matching capacitors and resistors to lumped equivalent values,have not used the inductors, instead of inductors, approximate MLIN's I used.
When I generated layout and simulated using ADS2009 , I have got the gain of 25dB, PCB fabricated and placed components measured the two stage LNA, I am totally getting 0dB gain.
What could be the reasons, why I got the measured gain 0dB for two stage LNA. But it was showing in the ADS2009 25dB at 1.7GHz.
What could be wrong,while designing two stage LNA,what could be the reasons behind the getting 0dB gain.
 


You do not indicate how the gain is measured. Cascading two 25 dB amplifiers can easily lead to oscillation, in-band or out-of-band. Take extreme care to use a good ground for both stages, and preferably install each stage in a good metal enclosure joined by connectors.
For DC power input use a regulated power supply, and use capacitor feedthru with more added capacitance inside the enclosure (parallel 1 nF, 0.1 uF and 10 uF).
Testing two cascaded amplifiers with a total gain of >20 dB requires also a good arrangement so the cannot be any coupling from output to input. Good connectors and cables are essential.

Start repeating the gain tests for each stage separately. Check gain flatness across the band and adjust the DC voltage for best results.
When ready, use coaxial attenuators before LNA input and between stages and repeat gain and flatness test. Any rising gain in-band or out-of-band must be corrected. Gradually remove the attenuators while observing the amplifier behavior, and correct all abnormalities.

How the "zero gain" is measured? What type of power meter or detector is used? How well your signal source and detector are matched to 50 Ohms? All such details are important.
 
Dear Sir,
I have used network analyzer for measuring gain(S21). As I told you earlier, I used the two same exact copies of two single stage amplifiers cascaded and they two same exact copies of single stages are separated by a coupling capacitor.
Even I checked gain in both network analyzer and spectrum analyzer,gain was showing "Zero"
jiripolivka said:
preferably install each stage in a good metal enclosure joined by connectors
For DC power input use a regulated power supply, and use capacitor feed thru with more added capacitance inside the enclosure (parallel 1 nF, 0.1 uF and 10 uF).
Click to expand...
I didn't understand above points,what does it mean.Can you tell me in detail
For making ground, I used VIA2 hole ,for making ground from top layer to bottom layer of microstrip. Is VIA2 ground o.k, VIA2 ground with following specification
D1= 0.4000mm
H1= 0.543mm
T1= 0.035mm
R1 =1
W1=0.6500mm
Fabrication generated amplifier layout screen shot is attached
 

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Last edited:


Well, if you used a VNA to measure LNA gain, then please check the input signal level to your LNA. Try not to overload it.

I tried to explain that with 25 dB at 1.7 GHz, there may be a coupling between output and input. Such coupling occurs due to RF connectors and cables, also by a poor DC power filtering. Therefore it is recommended to install each LNA stage in a well made enclosure, connect RF input and output by good connectors (like SMA), and take care that coupling cannot pass through DC lines.
If you try to test your LNAs installed on an open PCB, such coupling can occur easily and it may be difficult to suppress.

Using e.g. 10-20 dB coax attenuators is recommended as I described above. To test a 25 dB LNA, take care that the input level is not larger than -30 dBm. If you test both LNAs, the input level must be < -60 dBm, otherwise the LNA is saturated and some LNAs can be damaged.
 
jiripolivka Sir,
I have used the VNA for measuring gain Whatever I have showed the layout, same thing only , I have taken for PCB. Is that layout O.K., in that layout will coupling not pass the DC lines. My PCB was open, I didn't use any enclosure.
One more doubt I have VIA2 ground ,for making ground, is it VIA2 ground o.k.
As you said in the posting #2 "Take extreme care to use a good ground for both stages".
For making good ground for both stages,what we have to do.
I have showed in the posting # 3 my PCB layout. Is it O.K or any modification , I require to do.
Have I done any mistake in the layout PCB, which is showed in the posting #3. What about coupling losses ,good grounding, DC isolation from first stage to second stage. Please tell me, if there is any mistakes in my PCB layout in posting #3.
 

The layout view you posted looks to me that you may have the transistors connected backward (rotated 180 degrees) so that the Drain and Gate pins are exchanged.



Your layout shows the input signal connected to the Drain and the Gate conncted as the output.
 
RealAEL said:
The layout view you posted looks to me that you may have the transistors connected backward (rotated 180 degrees) so that the Drain and Gate pins are exchanged
Click to expand...
RealSir Sir,
Yes I interchanged the drain and gate pins, now I made it correct.
Any other mistakes, have I done in the layout, what about ground, DC isolation from first stage to second stage.
 

pusparaga said:
RealSir Sir,
Yes I interchanged the drain and gate pins, now I made it correct.
Any other mistakes, have I done in the layout, what about ground, DC isolation from first stage to second stage.
Click to expand...

I am sorry but I cannot see any attachments. Our colleague has noticed that you have installed one ATF transistor in reverse, so my question is why you have written you tested BOTH LNAs and they worked equally? One apparently could not.
Again , open PCB LNAs with 25 dB gain would certainly oscillate when cascaded. Use tight enclosures and good connectors to test. Use good DC blocking as I described.
I cannot see your PCB and how good is your grounding (vias). If you can please attach a picture and possibly the VNA response of each of the stages. ONly if the gain response is flat you may try cascading the LNA stages.
 
jiripolivka said:
Our colleague has noticed that you have installed one ATF transistor in reverse
Click to expand...
Yes I did mistake ,while generating layout. But while placing transistor component to the PCB , I didn't do any exchange, I connected correctly, I am sure.

jiripolivka said:
Again,open PCB LNAs with 25 dB gain would certainly oscillate when cascaded. Use tight enclosures and good connectors to test
Click to expand...
Yes I didn't use enclosure, my PCB was open only. But I used good SMA connectors, they will work upto 18GHz, screen shots of used SMA connectors are shown.

jiripolivka said:
Use good DC blocking as I described
Click to expand...
How to use good DC blocking, For DC blocking I used 100PF capacitors.
jiripolivka said:
I cannot see your PCB and how good is your grounding (vias). If you can please attach a picture and possibly the VNA response of each of the stages. ONly if the gain response is flat you may try cascading the LNA stages.
Click to expand...
Screen shot of PCB is shown, I checked stage by stage and also overall two stage LNA gain, it was showing zero gain in the VNA, I am sure, I don't ahve picture for VNA gain measurement.
My questions are as follows
(1) How to use the good DC blocking, what we can use for good DC blocking other capacitors
(2) Which is the good coupling circuit for cascading two stages, if you have any idea on good coupling circuit , let me know
what circuit we can connect to cascade two stages for getting good response of LNA.
(3) What we can use capacitor other than cascading two stages or isolating DC components from AC components
(4) Which is the good SMA connectors
 

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I cannot see ay attached pictures.
Please check VNA output to your LNA by a power meter.
1. Good DC blocking I have described earlier. Using only 100 pF is not good enough
2. Capacitive coupling is good but use10-dB coax attenuators for first tests
3. You can also use a RF transformer in place of a capacitor
4. SMA connectors are good for 1.7 GHz.

With your VNA you said both stages were measured to have 25 dB gain but you did not show the gain vers. frequency plot.
With a VNA you cannot measure EXACTLY "zero gain" even with one bad stage.
Read VNA manual, measure VNA output power and follow my advices. More talking makes no sense.
 
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