i put 100nF capacitors between each amplifier but the problem it the same . and for the moment they aren't in a box , just on the circuit boardThere is not enough isolation between input and output. The circuit is oscillating.
Could be many reasons, box resonance, insufficient power supply isolation for two.
This device is a wideband amp you have to take care over the full operating BW and beyond.
Try some absorber in the box.
I wanted to amplify small signals of the order of -80dBm in order to attack a detector which starts to work from -40 dBm.What is your operating frequency ? If 15GHz as is said, 100nF is not a capacitor at that frequency, it's an inductor.
Therefore the amplifier chain is oscillating.
Cascading 2 amplifiers at that frequency needs very carefully done analysis and implementation.You can not connect them simply as opamps.
How you have implemented the board, show us here including the details.I wanted to amplify small signals of the order of -80dBm in order to attack a detector which starts to work from -40 dBm.
my system is simple, an antenna, amps and a detector.
I would like to separate the stages of amps but in this case, what can I put between?
I work at 10Ghz.
I put everything on a circuit board because I don't have space constraints.
since the amp is internally matched should I still put elements between them?
thanks
here's my schematic and my board .How you have implemented the board, show us here including the details.
-Screening
-Grounding
-Coupling and Decoupling
-Transmission
-Connectors
-Soldering
-Components used..
All above are very critical at 10 GHz.Oscillation may occur very quickly if the precautions are not taken into account.
Hi Bigboss ,-C10-C12 Capacitors are 100nF that is not appropriate values for 10GHz.You should use few pF range for instance 5.6pF ( more or less)
-Transmission Lines are not adequate for 10GHz. Instead, Microstrip Transmission Lines have to be used instead of Coplanar Waveguide.CPW can be used around up to 3GHz.
-C1,C2,C14,C15,C16,C17 and others should be selected High Quality Microwave Capacitors form a well known Manufacturer such as ATC, Johansson or Kyocera..
Grounding of those capacitors has to be reinforced with more vias around GND pads.Also they have to be placed closer to the IC.
-Pay attention to discontinuity of Coupling Capacitor connection to Transmission Lines.
Send me Gerber file of your layout, let me simulate with s-parameters of HMC772.
hi Vfone,The low value capacitors C1,C2,C14,C15,C16,C17 should be placed closer to the ICs, as ADI recommend on their datasheet and the evaluation board.
You placed them more than twice from the recommended position.
hello,Give me the details of substrate, VIAs hole diameter.
I looked at the output with a spectrum analyzer as you advised me and indeed I have peaks on the whole band which explains the phenomenon of oscillations.Have you looked at the output on spectrum analyser?
My initial comment about oscillation was based on an output and the noise figure being significantly higher than epected.
Checking the figures again -25dBm is exactly what you should get assuming 45dB gain 1.5dB noise figure and 15GHz bandwidth
-174+10*Log10(15e9)+45+1.8 = -25.4dBm.
IIRC it was late at night when I replied.
If it is not oscillating then you are lucky. When desiging high gain amplifiers at tha frequency I tried to keep to not much over 30dB before breaking the circuit and addig some screening between stages to keep things stable
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