Is Full-Duplex possible on the 6000 series?

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Most of my operating is with amateur satellites and the question came up about using my Flex-6300 as a full-duplex interface to transverters. But let me use an example that simplifies the question.

Suppose I am using a Flex-6300 with SmartSDR defining two panadapters. One of the panadapters has slice A on 28 MHz USB connected to the ANT1 port for receive. This panadapter will be used for receive only.

The second panadapter has slice B with TX enabled on 50 MHz USB connected to the XVTR port for transmit. This panadapter will be used for transmit.

Let's assume "Show TX in waterfall" is enabled. When I transmit on slice B, the panadapter with slice B will show the transmit auto on both the spectrum and the waterfall displays. But while I am transmitting on slice B, the panadapter for slice A continues to show the received spectrum on the spectrum display, but the waterfall is black while transmitting and the audio from slice A is muted.

If the spectrum on slice A is displayed, the signal should be available for demodulation. And the demodulated signal should be available for audio output. But the audio output from slice A is muted. If the audio were available, full duplex operation would be possible.

Why not? Is this possible on the 6500 or 6700?

Ron
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Ron Parsons

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Posted 4 years ago

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Steve - N5AC, VP Engineering

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What is and is not possible varies by specific radio -- sometimes there is a hardware limitation such as how the relays engage for specific antenna selections.  There may or may not be coupling issues that vary by port on the radios.  Finally there are software considerations -- not everyone will want to run full duplex and so we have to decide how to present the option (if possible) and make it easy to use and understand.  To date, we've not looked at full duplex operation in detail.  I can say that we've done some preliminary investigation on the 6700 and have found some situations where it works well, but we still need to work out the details and decide how/if we would present the operating mode.

The mute of a slice was added because it was very distracting to those that didn't want to run full duplex (most folks), but it is a software switch and it can be changed when we have time to sort things out and decide what does and doesn't work.
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Steve - N5AC, VP Engineering

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We will be introducing Full Duplex operation in v1.5 which is due out in September.  I'd like to spend a few minutes talking about the specifics.  Full Duplex will allow you to transmit and receive at the same time, but there are limitations.  The FLEX-6700 has the most capability in this area and you will be able to transmit on one antenna and receive on up to two different antennas at the same time.  When you receive while you are transmitting, there are a number of important considerations to ensure good operation.  Here are the considerations:

Isolation and overload: If your transmit energy enters the receiver at sufficient levels with the preamp off (greater than about 5mW in the FLEX-6300 and greater than about 10mW in the 6500 and 6700) or with the preamp on (generally subtract the gain of the preamp from the provided overload point), the radio receiver will overload.  The FLEX-6000 Signature Series radios have a "soft overload" where they will show signs of an overload before going into a complete overload some 5-10dB later.  The key sign of a soft overload are numerous spurs in the panadapter.  There is 40-100dB of isolation inherent in the radio itself.  This will be added to the isolation between your antennas and the total must be below the radio overload point.  More details will be provided, but here's a math example:

Transmitter: 100W = +50dBm
Overload on FLEX-6700 = +9dBm
Isolation from ANT1 to RXA: 90dB
Antenna isolation (at your shack): 40dB

+50 - 90 - 40 = -80dBm ... well below the +9dBm limit (89dB margin)

Transmitter: 100W = +50dBm
Overload on FLEX-6300 with 20dB preamp on: +7dBm - 20 = -13dBm
Isolation from ANT1 to ANT2: 40dB
Antenna isolation (at your shack): 30dB

+50 - 30 - 30 = -10dBm ... we are 3dB above the overload with the preamp on 

We'll show you how to run these calculations, but you can see there is a wide range of possibilities so you will need to do a little math to determine if everything will work for you.  Bandpass filters may also add another 30dB or so of isolation of you are transmitting on one band and receiving on another if your antenna selections are correct.

The radio will also protect itself if you put too much energy in the receiver port and the radio will disconnect the antenna and alert you.

Receiver must be on different frequency from transmitter: The delay that occurs during filtering is guaranteed to drive someone mad if the transmitter and receiver are on the same frequency.  For this reason, we mute the slice that you are transmitting on so you will not hear your own signal.  This may seem counter intuitive at first, but it should fit most use cases.  For example, if you are chasing DX, you can create two slices, a RX slice on the DX and a TX slice where you will TX up (generally) and when you transmit, you can still hear the DX frequency.  You will not hear under your current transmit signal, unless you are in QSK in which case you will hear in-between your transmissions.

Antennas: In all cases, you will need two antennas for full duplex.  We will not be transmitting and receiving on the same antenna.

This is an exciting and new capability for all FLEX-6000 radios, but it will require a little planning and thought to be sure it will work at your location.  We hope this will bring hours of new operating fun!

=================

EDIT:
9/17/15 There are a few errors in the calculations above and rather than just change the text, I'll explain the issue and provide the correct information.  There are really two paths from the transmitter to the receiver.  The first path goes from the transmitter to the transmit antenna, to the receive antenna and into the receiver.  The second path goes from the transmitter directly to the receiver inside the radio.  It is the latter path that prevents most manufacturers from offering full duplex.

For the scenarios above, you should always look at the isolation in the radio AND the isolation outside and take the lower of the two.  

SCENARIO #1
Antenna Path:
Transmitter: 100W = +50dBm
Overload on FLEX-6700 = +9dBm
Antenna isolation (at your shack): 40dB
Receiver Preselector Filters: 20dB (assuming different bands)

+50 - 40 - 20 = -10dBm ... this is acceptable 

Internal path:
Transmitter: 100W = +50dBm
Overload on FLEX-6700 = +9dBm
Isolation from ANT1 to RXA: 90dB

+50 - 90 = -40dBm ... this is also acceptable

So in scenario #1, the configuration will work with full-duplex

SCENARIO #2
Antenna Path:
Transmitter: 100W = +50dBm
Overload on FLEX-6300 with 20dB preamp on: +7dBm - 20 = -13dBm
Antenna isolation (at your shack): 30dB
Receiver Preselector Filters: 0dB (not available in FLEX-6300)

+50 - 30 - 0 = +20dBm ... we are 33dB above the overload with the preamp on.  More antenna isolation is needed

Internal path:
Transmitter: 100W = +50dBm
Overload on FLEX-6300 with 20dB preamp on: +7dBm - 20 = -13dBm
Isolation from ANT1 to ANT2: 40dB

+50 - 40 = +10dBm ... we are 23dB above the overload with the preamp on 

In this case, full duplex may not be run.

More details will be made available in a white paper on Full Duplex and SO2R.  Hopefully I haven't made too many mistakes in the corrected text!
(Edited)