RF transceiver wakeup methods

[wanchope]

Almost all the receiving portion of the RF transceivers are on during operation to maintain certain sensitivity for active RFID applications. Is there a way to shutdown the chip when there is no RF signal, then wake it up while the signal is present? Would using an RF power detector (eg. LTC5505) help?
Please kindly advise.

Wanchope

 

[wanchope]

Or has anybody come across a low-power RF transceiver with carrier detection function at 433 Mhz? I found XE1209 operated at 30-100khz. But could find none for 433 at the moment. Is it technically very difficult to achieve at this kind of frequency (433mhz)?
Any idea is welcome.

Wanchope

 

[throwaway18]

If you can tolerate more latency you can turn off the receiver most of the time eg only turn the receiver on for 0.5s every 10s.

In some pager systems messages are sent in groups and a pager only turns on it's receiver at the right time to receive messaes for the group it is in.
The golay system samples every 1.38 seconds for 250ms looking for the preamble, which lasts 1.42 seconds.

 

[rfsystem]

If you have a medium data rate e.g. 1Mbit/s you can wait for a 16bit preamble. If it is not there go to sleep for further e.g. 100ms. So the ideal duty cycle could be 16us/100ms=160e-6. That is low enough to power your receiver some month with a small battery. The tricky point is the startup time of all receive circuits. They require often more energy than the active receive mode. Think of the startup time of the high frequency crystal oscillator or the synthesizer. Higher data rate is in favor for low energy because you can receive the same information in less time. The higher IF processing frequency does not dominate between 1 and 10MBit/s if you do not use a high power signal processing architecture A/D+FFT+Viterbi!

 

[BigBoss]

It should be observed RF input signal which arrives from LNA. A tanh circuit ( or log amplifier)will wake up the tranceiver. This method is used to get RSSI signal from input RF carrier depending on its level.

 

[wanchope]

Thanks for all your reply.
This application is for person contact tracing purpose. Therefore the receiver needs to capture 100% of the signal. I am afraid using periodic on/off methods will miss some data. That is also why the polling method is the last resort to be considered. I am planning to use 9600 ASK transmitter. Bigboss, as i am going to use a commercial transceiver, do i still need a LNA? I thought the amplification stage is already handle by the chip already. The perfect situation is the receiver is always off until a 'Carrier Detect' is passed back to the MCU before enabling the receiver. I am just wondering any low current circuit (in terms of uA) can provide the 'Carrier Detect' signal. Or any chip has the feature in?

Thanks.

Wanchope

 

[wanchope]

Can I just add a 433 Mhz bandpass filter after the antenna, then I monitor the signal and compare it with a threshold voltage (somehow)? The output then feed to the uP, which will decide whether or not to turn on the transceiver. What do you think? Any idea is welcomed.

regards
Wanchope

 

[throwaway18]

This will be tricky to do well.

There are several problems you may encounter.

Sending some of the signal from the antenna to extra circuitry will reduce the sensativity of your receiver. It would be best to tap the signal after the first amplifier.

There are other radio users around 433MHz who may be transmitting continuously for long periods. You might be unlucky and find there is an amateur radio repeater nearby somewhere in 430-440MHZ putting out several watts into a high up antenna.

The background noise level may vary a lot between locations. If you have a fixed threshold you either kill your batterys in noisy places or fail to receive weak signals that you could have received. You probably want some sort of automatic adjustment depening on wether you are receiving valid data.

You will want to trigger when a couple of microvolts of signal is received from the antenna. That means you need a lot of gain before the comparitor.
It is difficult to have 60dB voltage gain without a little bit of the output leaking back to the input and the amplifier oscillating. If you can get it to work at all, the amplifier stages will have to be built in metal compartments.

 

[wanchope]

Thanks, throwaway18.

Sending some of the signal from the antenna to extra circuitry will reduce the sensativity of your receiver. It would be best to tap the signal after the first amplifier.

Can I add a RF switch just after the antenna to differentiate the signal going into the receiver and the detector circuit?

There are other radio users around 433MHz who may be transmitting continuously for long periods. You might be unlucky and find there is an amateur radio repeater nearby somewhere in 430-440MHZ putting out several watts into a high up antenna.

This is a big problem. But i think it is still better than the case when the receiver is always on. I think the threshold reference need to set carefully to filter some unwanted signal.

The background noise level may vary a lot between locations. If you have a fixed threshold you either kill your batterys in noisy places or fail to receive weak signals that you could have received. You probably want some sort of automatic adjustment depening on wether you are receiving valid data.

You will want to trigger when a couple of microvolts of signal is received from the antenna. That means you need a lot of gain before the comparitor.
It is difficult to have 60dB voltage gain without a little bit of the output leaking back to the input and the amplifier oscillating. If you can get it to work at all, the amplifier stages will have to be built in metal compartments.

Actually I did not realize i need to amplify the signal before comparator. I am going to use a 10dbm ASK transmitter. What would be the waveform pattern? Initially I was thinking to directly tie the output of the bandpass filter to the edge-trigger interrupt pin of the uP. (just have a try). But most likely a more sophicated circuit needs to fit the bill. Any idea?

Thanks.

Wanchope