Could transmit/receive turnaround be improved by having a heterodyne receiver switchable between high-side and low-side injection

Question

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?

Answer 1

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.

Answer 2

This sounds like overkill but couldn't you use two transceivers - one permanently set to receive and one permanently set to transmit then neither have to shift their respective PLLs. There may be issues connecting them to the same antenna but something in the back of my mind tells me that a few of these "types" of transceivers have separate receive and transmit output lines and you use external components to join them to one common antenna. It might be worth a try just to see what response you get.

I'll also add (and I haven't looked at the tech specs for these devices in ages) that there may be a "planned" delay to allow for ringing to die down (following a RF transmission) on the components that connect rx and tx to the antenna and you may in fact gain nothing.

Answer 3

The Nordic Radios are, I suspect zero IF radios, where the signal is down converted to 0Hz in quadrature, so there isn't an IF frequency. This lets you integrate the receiver into a single chip. The retuning is to allow for half duplex operation, so that different units are transmitting and receiving on different frequencies. 100uS retuning speed isn't too bad, you've still got synchronization issues between different radios to allow for. Even if you could cut that in half, how many bits would you be able to transmit in that time?