Thank you for your advice. I'll try it later.
The circuit is part of a mixer that detects shifting in that frequency caused by the alteration of the dielectric present in an electric field generated by two planes (as plates of a capacitor, one the emitting antenna and another the receiving one).
At the exit of this arrangement is the monolithic amplifier. If I extract that wire, I get high frequency oscillations at the low frequency filtered output of the mixer, although not continuous. If I put the wire, the circuit works correctly.
Unfortunately I can not leave the wire for constructive reasons.
What else can I do?
Oscillations are coming from Input side.Since the Input Impedance is matched to 50 Ohm why you have used a extra matching circuit ? Is antenna impedance different than 50 Ohm ??
GND Connections must be very tightly connected to GND layer ( underneath of the main layer ) and this connection should be ensured so no parasitic inductance shouldn't be presented.
Layout drawing will be helpful if it's possible to post here..
According to the layer 1 picture, this elementary principle of RF design isn't considered in the layout. MAR-1 should e.g. use separate vias near both ground pins. The rather large ground trace is likely to bring up oscillations. There are possibly ways to stop oscillations by killing the amplifier gain. To make the circuit work with full performance, the layout needs rework.GND Connections must be very tightly connected to GND layer ( underneath of the main layer ) and this connection should be ensured so no parasitic inductance shouldn't be presented.
Thank you for your advice but, indeed, the manufacturer's suggested layout has been followed and below this layer the mass is connected to GND through some via (those small gray circles) that are not shown in the figure by clarity.
It may be me, but I can't relate the statement to the layer 1 artwork shown in post #10. I understand that the gray points are throughplated vias, but apparently not connected to the MAR-1 ground pins.Thank you for your advice but, indeed, the manufacturer's suggested layout has been followed and below this layer the mass is connected to GND through some via (those small gray circles) that are not shown in the figure by clarity.
At 40 MHz the vias in a MAR-1SM design are pretty much useless, if a good planar ground plane is used.
Remove the series choke (L3A+L3B) and use only the series resistor for the bias (change R2.RF to 330 ohm).
The wire antenna is coupling to the circuit to these series chokes, and make the MMIC to oscillate.
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