Introduction: Yaesu FT-450D RF Tap Modification for SDR

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About: I am an electronics engineer working in telecommunications in the UK. Generally I work on chip design for digital radio receivers. I like all things arty and consider chip design a bit arty :) Basically I h…

Hello anyone that may be interested,

I think I had better first explain what this instructable is all about. There are thee main components involved in this project as follows:

The Yaesu FT-450D is a modern compact HF/50MHz transceiver capable of covering the 160-6 meter amateur bands with a power output of 100W. Too many features to list, so just Google the radio if you want to know more.

The SDRPlay is a superb wideband Software Defined Radio covering a frequency range of 1KHz to 2GHz and allowing the spectrum to be viewed with a bandwidth of up 10MHz.

SDRPlay : https://www.sdrplay.com/

(I have no connection with the company other than to have purchased their excellent product)

Both these pieces of equipment are superb in their own right. However, the purpose of this instructable is to bring the two pieces of equipment together and to be able to exploit the best of both worlds. By that, I mean being able to use the FT-450D radio as it was intended (as a narrow band radio transceiver) but at the same time being able to use the SDRPlay receiver to visualise the wide band channel.

This inherently poses a problem as both the FT-450D and the SDRPlay need to see an antenna. One approach is to simply use two antennas. A second approach might be to use a single antenna but split the RF path and transmit/receive using in-line switching. A third and preferable approach is to tap off the receive RF path from within the FT-450D using a suitable low noise circuit and present the tapped signal to the SDRPlay. This latter approach results in both the FT-450D and SDRPlay essentially seeing the same antenna. The low noise circuit is only powered while receiving and so during transmit provides substantial isolation protecting the input to the SDRPlay receiver. The low noise circuit has a high impedance input thus presenting a minimum load to the tap point within the FT-450D. This last point is important as suitable tap points within the FT-450D are located either side of passive 50 ohm band pass filters. Any loading or change of impedance introduced by an additional circuit will change the transfer function of the filters and also reduce the power in the wanted signal path.

Most of the off the shelf low noise amplifiers (LNA) available use feedback to generate gain and also have a 50 ohm input impedance - neither of these features are desirable.

A simple high impedance tap circuit has been designed by Dave G4HUP and was available to buy. Very unfortunately, it is my understanding that Dave has passed away. I have taken a part of the design and with modification, produced my own printed circuit board, tested and fitted to my own FT-450D. It is this process which forms the subject matter of this instructable.

Step 1: Creation of LNA Schematic and PCB Layout

Overview

Over the years I have generated a few Printed Circuit Boards (PCB) for products and for home use. In the early days this involved using copper clad board, transfers and special pens to draw the design onto the copper. The board would then be etched in Ferric Chloride to remove exposed copper and leave the wanted tracks. It was also possible to buy light sensitive copper clad board and use a mask to produce a resist before etching. Having a one-off board made commercially was very expensive and required tools that were just not available to hobbyists.

Nowadays, computer tools are free and widely available to design boards in a matter of hours not days. Also fabrication costs have plummeted with many cheap fabricators being available in China and elsewhere outside the UK. However, that said having a single board made is still not that cheap once you include shipping.

Another approach, and the method I have used in this project, is to mill the board using a CNC milling machine. Obviously, you would not buy a CNC machine to make one board but I already had a machine which has been used for lots of other projects involving milling wood, metal and glass.

To mill a PCB using a CNC machine involves using a very fine cutting tool to mill out isolation around the wanted tracks but not to mill away all the copper. This approach is particularly useful when building RF circuits as the remaining copper islands are desirable acting as a ground plane improving stability and performance. I have used a doubled sided copper clad board in this project and have drilled through linking the top and bottom copper surfaces.

PCB design using EasyEDA

I have tried various PCB design packages and had really settled on a package called DipTrace. However, it is ever more popular for design packages to be web-based rather than use a stand alone application. Having not used DipTrace in some time I was a little rusty so looked around on-line and found a web-based design tool called EasyEDA. I found this tool excellent, very intuitive and simple to use. Very easy to generate a schematic in a matter of minutes and then convert to a PCB, the entire process took less than an hour including a few modifications and refinements. The tool designers are obviously hoping you will use the provided fabrication facilities but it is still possible to export a design in industry standard gerber format for use by a subsequent tool chain.

Step 2: Using FlatCAM to Create Geometry and Tooling Paths

After EasyEDA has been used to create the schematic and PCB layout the next step is to create tooling paths and ultimately gcode to control the CNC milling machine. I have tried various pieces of software to achieve this goal and finally settled on FlatCAM. This software is free, stable and quite intuitive to use. Using FlatCAM tooling paths for the board, cutout and drilling can all be created very quickly. There is also a very user friendly geometry editor should anything require a tweak. In the video forming part of this step I show how FlatCAM is used to import gerber files and perform some basic editing. There are many detailed videos available showing how to use the tool end to end. I have only covered the modifications I needed to make specifically for this project.

Step 3: The Milling Process - CNC Machine in Action

Ok, so over the last few steps the following has been achieved:

- The circuit schematic has been captured using EasyEDA.

- From the schematic the PCB layout has been created also using EasyEDA.

- Gerber files have been created for the board and also drill files generated.

- FlatCAM has been used to create/edit path geometry and generate gcode for the board and cutout.

- FlatCAM has been used to import and scale the drill file also resulting in gcode.

So now we have three gcode files for the board, cutout and drilling.

The next stage is to actually start milling some board. The board I have used is double sided fibreglass copper clad board. I could have ordered this on-line but actually found Maplin did quite a nice big sheet for a good price and I had it in my hand within the hour - just wanted to get milling !

My milling machine is a Sable 2015 and I use Mach3 software to control it. To mill the board track relief I used a 0.5mm end mill. For the board cutout and holes I used a 1.5mm end mill. In order to mill right through the board you obviously need something to mill into under the PCB - my mill bed is thick aluminium and you don't want to be milling into that ! I have found for PCB's the best material to use under the PCB is 5mm thick foamboard. You can pick this foamboard up very cheaply on-line or from craft shops. It is easy to cut with a modelling knife and has a very uniform thickness. The copper clad board is mounted on the foamboard using thin double sided tape. The foamboard is also mounted to the CNC bed using the same tape - I have never had a board come free or move during milling.

The 0.5mm end mill is obviously quite fragile and so I keep my feedrate to 60mm/min. I use the same feedrate for the cutout so as not to dislodge the PCB/foamboard sandwich from the securing tape.

Attached is a video showing the milling process action :)

Also attached are three images of the finals boards. One image shows the first attempt at the board and small areas of unwanted copper can be seen most obviously between the transistor pads. The second attempt board these unwanted copper areas have been removed by adding geometry into FlatCAM. The third image show the final board populated with components.

After populating the board was give a very light spray with lacquer to stop the copper tarnishing and discolouring.

Step 4: Frequency Response of Finished Board

The finished populated board was gain characterised using a spectrum analyser. The analyser was set up to sweep the frequency from 10KHz to 30MHz and measure the gain. The gain was also measured with the power off to simulate what happens in the radio when we are transmitting and require good isolation between the FT-450D transceiver and the SDRPlay receiver.

Input level to the LNA was -40dBm

Image 1 - Marker set at 7.1MHz the gain of the LNA is +2.5dB

Image 2 - Power to the LNA off showing >34dB of isolation

Image 3 - Low frequency roll off -3dB down at 1.6MHz

Essentially over the HF amateur bands the LNA is flat 3MHz - 30MHz (was flat up to ~500MHz)

Step 5: Analysing the Yaesu FT-450D for a Suitable RF Tap and Power Point

Before the LNA board can be fitted to the FT-450D a suitable RF tap point and power point must be identified. This was achieved by using the radio service manual and first looking at the receiver block diagram before refining the RF tap point choice using the schematic.

First of all I wanted the SDR to see the antenna connected to the FT-450D before any IF conversion stages so this narrowed down the investigation considerably. Before the first IF mixer there were two obvious points to tap off. Once the Rx signal enters the RF-IF board from the PA board it passes through the following stages :

- Input surge protection

- Switchable (relay) 20dB input attenuation

- A series of eight mutually exclusive switched band pass filters

- Switchable (relay) IPO pre-amplifier

- First stage IF mixer (1st LO driven mixer)

So the two points of interest essentially boiled down to before or after the band pass filtering. I wanted the SDR to see as much signal as possible so decided to tap off just before the band pass filter network. Remember the LNA used to tap the signal off has a high impedance input and so the effect on the radio signal path will be minimal.

The other area to be considered is where the LNA board is going to get its power. Fortunately the FT-450D schematic is quite clear and well annotated and so a suitable power point can be located. The power point chosen powers the LNA when receiving but powers down the LNA during transmit. This isolates the SDR input by >30dB during transmit. The powered LNA current consumption is ~9mA.

The attached images show the following:

- The RF tap point shown on the block diagram

- The RF tap point shown on the schematic

- The RF tap point shown on the board layout

- The LNA power tap point shown on the schematic

- The LNA power tap point shown on the board layout

Step 6: Fitting the LNA Board to the Yaesu FT-450D

Now the LNA board has been fabricated, characterised and a suitable tap point identified the time has come to actually fit the board to the FT-450D.

At this point it is customary to point out that you perform this modification at your own risk. It is not complicated but there is always a risk of damage and I personally wouldn't perform this modification on a radio that was still under warranty - I am sure the warranty will be void after the modification. I bought my FT-450D second hand from ebay so there is no warranty to worry about in my case.

If you do decide to perform such a modification just go carefully and methodically - use the wise old saying which applies to most delicate situations ...... measure twice and cut once :)

I decided not to drill any holes in the FT-450D casing but instead to mount the SDR to the side of the FT-450D and run out a SMA terminated fly lead to screw directly into the SDR antenna input. The fly lead is secured at the radio exit point to provide strain relief.

See attached images ....

Step 7: The SDR in Action Sourced From the RF Tap Via LNA Board

At this step there is a short video showing the SDR radio in operation with it's antenna source being the FT-450D antenna tap via the LNA board. This test was conducted late(ish) at night and the band is a little dead but the response of the SDR is as expected. When the FT-450D is transmitting the input to the SDR is effectively muted due to the LNA board isolation when not powered.

Step 8: Conclusion

Well above all this instructable has been a great deal of fun and I am very pleased with the result. Like all good projects there are three primary goals .... to learn new skills, to make the project a success and to share knowledge with anyone who cares to read this far.

I at this point doff my cap to the late Dave G4HUP. If it were not for the work of Dave this project may not have materialised. I cannot claim the original LNA design as my own but only to have taken a design and attempted to make it in my own way. I can only hope Dave would approve of his work being developed and shared with others.

In conclusion the project has been a success.

Please feel free to fire off any questions and I will do my best to answer them.

Best regards,

Dave (G7IYK)