Many thanks to SWLing Post contributor, Tom Ally, who shares the following news via the Cornell Chronicle:
Engineers devise two-way radio on a single chip
Two-way communication requires, of course, both send and receive capabilities. But putting them in the same device requires a filter between the send and receive circuits to provide signal isolation.
Without a significant filter, communication would be impossible.
“Your transmit signal is [10 to the power 14] times stronger than your receive signal,” said Alyosha Molnar, associate professor of electrical and computer engineering (ECE). “That’s 100 trillion times stronger – that’s a really hard problem.”
But researchers in Molnar’s lab have offered up a solution.
Molnar and collaborator Alyssa Apsel, professor of ECE, have devised a method for both transmitting and receiving a radio signal on a single chip, which ultimately could help change the way wireless communication is done.
Their work is described in “A wideband fully integrated software-defined transceiver for FDD and TDD operation,” published online Jan. 27 in the Institute of Electrical and Electronics Engineers’ Journal of Solid-State Circuits. Doctoral student Hazal Yüksel and Dong Yang, Ph.D. ’15, are co-lead authors.
Separating the send and receive bands is difficult enough, but the problem is compounded by the ever-increasing number of bands in the latest devices. From GPS to Bluetooth to Wi-Fi, each band requires a filter to stop the strong transmit signals from drowning out reception.
Molnar and Apsel have come up with an ingenious way to separate the signals. Their idea lies in the transmitter – actually a series of six subtransmitters all hooked into an artificial transmission line. Each sends its signal at regular intervals, and their individually weighted outputs are programmed so that they combine to produce a radio frequency signal in the forward direction, at the antenna port, while canceling out at the receive port.
The programmability of the individual outputs allows this simultaneous summation and cancellation to be tuned across a wide range of frequencies, and to adjust to signal strength at the antenna.
“In one direction, it’s a filter and you basically get this cancellation,” Apsel said. “And in the other direction, it’s an amplifier.”
“You put the antenna at one end and the amplified signal goes out the antenna, and you put the receiver at the other end and that’s where the nulling happens,” Molnar said. “Your receiver sees the antenna through this wire, the transmission line, but it doesn’t see the transmit signal because it’s canceling itself out at that end.” […]
30 dB isolation is not much. But if you have a very large-signal resistant receiver and a very low-noise transmitter…
These days most full-dupley systems use time multiplex. Cell phones are one example. Analog amateur radio repeaters use frequency multiplex, i.e. the repeater and the users transmit on different frequencies.
Nevertheless it is quite interesting to do duplex on one channel: you need only half of the bandwidth.
BTW: Delay lines have found interesting usage over the time: Early wide-band scopes used delay lines to operate several tubes in parallel without the added load capacity. Or in the analog PAL TV system an accoustic delay line made of glass delayed the color signal by one TV line. This way the phase error from one line could be substracted from the phase error of the next line. This converted phase (color) errors to amplitude (saturation) errors that were much less obvious. That is why it was called PAL (Phase Alternate Line) and not NTSC (Never The Same Color). The PAL delay lines were very stable. This was used in the 1980s by Karl Braun, DJ3DT and his developer Jörg Menning, DK1FE, to stabilize the VFO of the Braun SE400 VHF transceiver. Instead of a stable frequency you can also use a stable delay time…
Some neat technology here. A few years ago I’d seen a VID where a student set-up TX / RX from the same place on the same FREQ, but instead of a set of big-ass expensive duplexers, he used a nulling technique. Looks like that’s employed here.