In some of the small radios I've worked with such as the Nordic Semiconductor 24LE line, there is a significant (100us+) delay between transmitting data and being able to receive a response. By my understanding, a big factor in this delay revolves around the fact that a transmitter requires a PLL to be tuned to the exact desired transmission frequency, while a receiver requires a PLL tuned to a frequency which differs from the desired one by an Intermediate Frequency. Thus, switching between transmission and reception requires retuning the PLL, and the need to retune the PLL represents a significant portion of the turnaround time.
Would it be practical to construct a radio such that the receiver could be configured to receive a frequency either above or below the PLL frequency, and then have a handshake sequence operate something like:
- Transmitter sets PLL to 2315Hz, sets receiver mode to "100Mhz above"
- Expectant receiver sets PLL to 2415Hz and receiver mode to "100Mhz below"
- Transmitter listens for a little while to let its AGC settle.
- Transmitter sends out packet at 2315Hz, while keeping PLL constant
- Receiver hears packet which was sent 100Mhz below its PLL frequency
- Transmitter switches receiver mode to "100Mhz above" while keeping PLL constant
- Receiver sends reply at 2415Hz, while keeping PLL constant.
- Original transmitter receives reply which was sent 100Mhz above its PLL frequency
When using a 100Mhz Intermediate Frequency (which I understand is normal for 2.4GHz communication) this approach would unfortunately be limited to using the upper and lower 50Hz of the 2.4GHz band, but it would seem that a reduction in transmit/receive turnaround time would be helpful. Alternatively, use of a lower Intermediate Frequency would reduce the required separation between transmit and receive frequencies.
Are such approaches used at all, whether in 2.4Ghz radios or in other applications? Do they have any particular problems?