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Re: [Discuss-gnuradio] SDR/duplexer question

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] SDR/duplexer question
Date: Wed, 30 Aug 2017 16:39:21 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

Well, it kind of is a GNU Radio question, so I'll kind of try to answer it like one:

First of all, there's "Full Duplex (TX and RX simultaneously)" and "Full Duplex (at the same frequency)": Literature, and especially marketing, often is confusing, so be sure which one you're talking about.

No, the full-duplex capability of your SDR just means that you can receive and send at the same time. That just means your SDR can send at the same time as it receives.

A cavity duplex does two things:

1. It's basically a splitter, so that both the RX and the TX chains have one signal path connected. You'll need that splitter – your SDR (most likely, which one that ever might be) doesn't provide one, so you simply can't screw a single antenna to both the RX and the TX port at the same time. And the SDR *will* have separate RX and TX ports.

2. It's a pair of filters that make sure that the receive frequency end up in the RX port, and that the transmit frequencies don't overload the RX (which will be very sensitive compared to the TX power).

If you're in a frequency duplex setting, you might get away without the second feature, if your transmit power is not that much larger than your receive power. Sadly, that ratio, at least for symmetric links, will just be a quadratic function of the distance between your transceivers, so WLOG, so this might not be as cool in practice as in a lab.

For 1.: simply use two antennas. Simple antennas are usually cheaper than splitters.

Then, re 2: well, in fact, many SDRs aren't that bad at living with OOB interference, it really depends on your device.

If you want to build what is a "full-duplex same frequency transceivers", then you'll usually need to do your cancellation of the technologically inevitable¹ TX-RX crosstalk (you can't filter RX from TX, they're on the same frequency) yourself, in software (the "S" in SDR).

Best regards,


¹ expensive solutions like circulators aside; but then again, you're considering cavity duplexers...

On 08/30/2017 04:15 PM, Ricardo Nuszkowski wrote:

If my SDR handles full duplex, do I need a cavity duplexer to use TX/RX on a single antenna. Or is the full duplex capabilities of the SDR enough.

I know this is not a GNU question, but I figured maybe someone could help me out.

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