While testing a direction coupler (i am testing 30 dB DC ), I found that when I am giving signal at port 1 and output at port 2 (through port), it provides insertion loss and at coupled port its providing copuled loss of 30dB which is normal expected operation. But the main problem i am facing is when i am giving signal through coupled port and measure at output the expected signal is 30db less but i am getting this expected result only when i keep the port 1 open. When i terminate port one, then the signal is -55db. Even when i give input to port 2 and measure the coupled port, only when i keep port on open i am getting expected of -30 dB result else the output is -55dB. Why does this happen. isn't a coupler a reciprocal device?
Add a comment
|
2 Answers
\$\begingroup\$
\$\endgroup\$
The coupling of -30 dB is between port 1 and the coupled port (P4 in the figure below). The "coupling" between ports P1 and P3 is called actually called isolation. Ideally, the isolation is infite = so the coupling between P1 and P3 is -infinity dBm.
But the main problem i am facing is when i am giving signal through coupled port and measure at output the expected signal is 30db less
Thats because you are actually measuring the isolation.
but i am getting this expected result only when i keep the port 1 open.
Thats because the signal is going from the coupled port (P4) to the input (P1) and is reflected to P2, because P1 is open and thus the power is reflected.
When i terminate port one, then the signal is -55db.
Your are measuring between port 2 and coupled port. You are now measuring the isolation, which is -infinite dB for an ideal DC.
You should always termimate all ports in RF devices with the characteristic impedance, which is almost always 50 ohms.
The dashed line representes the leaking signal. Ideally theres none = infinite isolation. You're measusing an isolation of 55 dBs.
\$\begingroup\$
\$\endgroup\$
2
Why? what does the datasheet say about the freq. you tested? Yes it is reciprocal.
When ∑ port is terminated isolation between ports 1 and 2 are ~30 dB
Loss between ∑ port is 3.5dB to both ports 1 and 2 unless one port is open then it reduces to 0.5dB on the other port in both directions.
answered Sep 17, 2018 at 7:10
-
\$\begingroup\$ Its a 1GHz coupler. when i terminate the sum port, why does it affect so much? shouldn't it be showing -30dB? \$\endgroup\$– poojaCommented Sep 17, 2018 at 10:31
-
2\$\begingroup\$ Try sweeping it. Your asumptions on something may be invalid, cable etc \$\endgroup\$ Commented Sep 17, 2018 at 13:32