Electronic – How to share a 900 MHz antenna between two radios

The antenna will work identically for transmitting and receiving, so there is no limitation from the basic physics. The problem is purely one of overdriving a sensitive analog input, which is the receiver input. If the transmitter output is limited to the same power supply that the receiver runs on, this might not matter. It may temporarily make the receiver deaf for a short time after the transmission while its AGC recovers.

Look at the datasheet for the receiver carefully and see what the voltage limits are for the RF input pin. This is actually one of those rare cases where you may be able to use the absolute maximum limits instead of the operating limits if due care is used with all parameters. Your firmware knows when it is transmitting and can know to ignore anything from the receiver during that time and a short time afterward. During this time you only care that the receiver not be damaged.

A better all around solution would be to use a transceiver chip. These have the transmitter and receiver built into the same chip, with a single antenna connection. Whatever protection the receiver might need to not be hurt by transmitter RF levels has already been built in.

When a transceiver is transmitting the PLL frequency is set to Fs and when it is receiving the PLL frequency is set to either a frequency greater than Fs by the intermediate frequency or one lower than Fs by the intermediate frequency i.e

When transmitting:

$$ F_{\text{PLL}} = F_{\text{S}} $$

and when receiving:

$$ F_{\text{PLL}} = \begin{cases} F_{S} - F_{IF} & \text{for low side Injection} \\ F_{S} + F_{IF} & \text{for high side Injection}\end{cases} $$

Remember that we are only permitted to transmit over a very low frequency range e.g 2.39GHz to 2.42GHz so we can not use any of the approaches you propose above because Fs has to be constant.

Further notice that for use the only way to avoid retuning the PLL is to set the intermediate frequency to zero, which will essentially now turn our superhet receiver to a one stage TRF receiver (You can read the the disadvantages of TRF receivers in the link provided).

As for the solution you propose for reducing the IF frequency to a very low frequency so that we can reduce the amount of tuning to be done, you have to remember that the lower the value of our IF frequency the poorer the image channel rejection our system will have so there is a limit to how low our IF can be.We can't just set IF to a very low value.