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Fusion Talk-through

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FTT Diagram

General

The following refers to the Yaesu FTM-400XDE and will apply to some other Yaesu Fusion dual band mobile rigs.

The specification says that it works in cross-band talk-through mode and that is useful for establishing a RAYNET radio network.  What may not be obvious is that talk-through will be in FM mode only.  You can set VFO A to digital mode and VFO B to FM mode, but when you enable talk-through, both bands are in FM.

To extend a radio link from a disaster area to a Fusion digital gateway node, it may seem reasonable to have a 12.5 kHz bandwidth digital radio in the disaster area linked to a digital gateway node via a 25 kHz bandwidth FM talk-through station.  It won't work, as the mode linking to the digital gateway is not digital.

The solution is below.  It allows a Fusion gateway on a 2m frequency to be onward linked on a 70cm FM frequency. 

Requirements

A Yaesu FTM-400XDE dual band mobile radio or similar model in digital mode and in range of a Fusion gateway.  It has a 10 way mini-DIN packet data socket.
A 70cm FM mobile rig with a standard 6 way mini-DIN packet data socket.
A dual band 2m / 70cm aerial.
A VHF / UHF duplexer filter to allow a 2m radio and a 70cm radio to use the same aerial at the same time.
A very simple interface cable described below.  Click icon "FTT Diagram" at the top left to see the circuit diagram.

Block Diagram

Packet Data Ports

The interface cable plugs into these ports, but has nothing to do with packet data.  It is just a means of connecting to the Squelch Condition, PTT, transmit audio and receive audio pins.

Live Mikes

Check if the microphones pick up room noise.  Unplug them if they do.

Simple

Isolation using opto-couplers and audio transformers was considered and abandoned.  The interface circuit is simple and works.  The two transistors were chosen on the basis of which were closest to hand.  They are Motorola 0829-1 or 2N909.  Almost any small signal NPN transistor should work.

Squelch Condition Pin

This has almost zero voltage with the squelch closed.  With the squelch open, it is about +4.7V.  If short circuited to earth / frame / ground, then the pin will supply a maximum current of 1 mA.

PTT Pin

Earthing this pin makes the radio switch to transmit.  If short circuited to earth / frame / ground, then the pin will supply a maximum current of about 0.7 mA.

Configuring the Yaesu FTM-400XDE

The factory default is that the packet data port will only work with VFO B, i.e. the lower band in the display.  To make it work with either VFO:-

Setup / Data / 2 Data Band Select / Data / Change SUB BAND to MAIN BAND.

If configured for factory defaults except for the above, talk-through will work fine in one direction, but the other direction will stutter.  As the FTM-400 switches to transmit, it then switches to receive etc.  To fix:-

Setup / Data /  4 Data Squelch / TX / Change ON to OFF.

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Page updated on 09 January 2017

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