Power Amplifier loading without RF input

[aliahmedrauf]

Hi. I'm working on my RF transmitter. I'm creating RF signal using VCO which is attached to gain block and then fed to moduler Power Amplifier. When i tested this chain in lab on spectrum analyzer using coupler and 50 ohm load my whole chain works fine. But issue arise when i attached antenna at the output. I usually provide bias to my PA first which takes current about 0.3A with no RF in normal condition. But when i attached antenna at the output and attached my VCO board at the input at the PA. I turned on bias for PA (bias for VCO still off so no RF input fed) the amplifier load itself and draws current up to 4A even there is no RF input. I tried to investigate further i removed my VCO board and attached a 50 ohm termination at the input then this issue disappear. Then i removed termination and attached my VCO again but i have attached 20 dB external attenuation between VCO and PA. This issue doesn't appear but it can significantly reduce my desired power. Any one know why this problem is appearing and how can i solve it??

 

[BigBoss]

So your PA is oscillating with Antenna's Impedance.It's not pure 50 Ohm or not even close to.

 

[aliahmedrauf]

So your PA is oscillating with Antenna's Impedance.It's not pure 50 Ohm or not even close to.

Is it the issue with signal generation part?? Issue arises when antenna is attached and signal generator circuit is attached. If i remove signal generator at the input of PA the PA and terminate it then oscillation doesn't appear.

 

[BigBoss]

Is it the issue with signal generation part?? Issue arises when antenna is attached and signal generator circuit is attached. If i remove signal generator at the input of PA the PA and terminate it then oscillation doesn't appear.

We cannot know this. Instability is function of many figures such as Load and Source Impedances, Operating Point, Temperature etc. So you should consider all of those parameters as keyrole players.

 

[aliahmedrauf]

We cannot know this. Instability is function of many figures such as Load and Source Impedances, Operating Point, Temperature etc. So you should consider all of those parameters as keyrole players.

Thanks for help. I'm able to resolve this issue. Obviously it was becasue of the mismatch and poor grounding. I have change my board cavity from plastic to metal that improves grounding and eliminated the oscillations.