[Solved] PIN Diode T-R Switch and Power Amplifier for Chinese Transceiver Module - Page 1
Forum section: electronics / rf-microwave

author: josefmtd
author_rank: Contributor
timestamp: February 27, 2019, 01:53:57 am
content: Hello.I am a beginner in RF design and I am currently trying to create a power amplifier board to use with a Chinese FM transceiver module (SR_FRS_1WV). I am trying to connect the ANT output of this module through a power amplifier module (RA07M1317M) that can amplify 20mW to 6W. As I have read in the datasheet of RA07M1317M, it can be supplied by 7.2V, which works for me because I'll be using two Li-Ion batteries as the supply.The FM transceiver module I used apparently have a pin that can be connected to Tx-LED, I reckon from Alinco DR135 that the Tx LED signal is also used to switch PIN diodes T-R switch and provide DC bias to the power amplifier. With this, I thought of doing the same thing in my power amplifier board.I had no idea how to create the circuit, I managed to try to make my own circuit, but I failed badly, so I ended up using Google-Fu and ended up with a stackexchange question:https://electronics.stackexchange.com/questions/166579/pin-diode-t-r-switch-for-use-with-rf-power-amplifier. Reading that stackexchange made me think that I needed a SPST switch for the power amplifier and SPDT switch to connect the antenna with the power amplifier output or directly to the FM transceiver module. I also stumbled upon W2AEW's video and managed to get a clearer view on how to use PIN diode as my RF switch.I tried to follow the stackexchange circuit, and added my own modifications in it as attached. The most notable modification is the stackexchange uses BIAS to enable the PIN diode and make the SPDT switch connects the antenna to Rx and when the BIAS is off, the PIN diode will be off and the power amplifier output will be connected to the antenna. This won't work with the Tx-LED (TXEN) signal that I am planning to use as the BIAS signal, so I thought what if I changed the PIN diode switch position and made the power amplifier connected to the antenna when BIAS (TXEN) is active and made the antenna connected directly to the transceiver output when BIAS (TXEN) is negative. The other modification is I added a Pi Attenuator to reduce 500mW to 20mW and added a 9 pole Chebyshev Low Pass Filter before the Antenna since this Chinese RF module generates a lot of harmonics.I was just about to spin my own board, but I noticed something on my second look, it seems like I made a mistake in my circuit. When the PIN diode (D3) is forward biased, it will create a path to ground from RFIN, I overlooked this detail because I was assuming a separate pin for Tx and Rx. How do I fix my circuit to work as I intended, or do I need to just follow the stackexchange and not reverse the SPDT switch, or should I use an off the shelf SPDT switch?I hope I can get some feedback from you all. Thank you

author: radioactive
author_rank: Regular Contributor
timestamp: February 27, 2019, 02:39:21 am
content: I didn't spend much time analyzing your circuit, but looks like you might just need to replace the C13 coupling cap with another 90 degree filter like you (presumably) have formed with C11,C12,L8. This would look like an open when biased for transmit and a low-pass during receive.

author: josefmtd
author_rank: Contributor
timestamp: February 27, 2019, 02:46:53 am
content: Quote from: radioactive on February 27, 2019, 02:39:21 amWhoa, yeah, I could probably just do that, I might have underestimated the quarter-wavelength transformer versatilityThank you for your input