New project

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M0XXQ
Posts: 484
Joined: Thu Sep 27, 2018 10:23 pm
Location: Newcastle upon Tyne
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New project

Unread post by M0XXQ »

Following a long break from construction and HAM Radio in general, I was having a long think the other day about how my work has taken over my life and how I am really missing my hobby, I don't have a workshop anymore but I still have all my tools, components and test gear so staring at the small space I do have for a few minutes decided that I could maybe have a go at building something and could just about squeeze a small computer-type desk in my office to provide space for soldering and construction etc.

As operating from the home QTH is not possible due to a zero antenna policy from my neighbours and landlord I decided a small, QRP portable station for 2M SSB was in order, so I set about putting the basic design together for a hoembrew 2M (144MHz) SSB transceiver, still much a work in progress at the design stage so I don't anticipate construction to start this side of Christmas, however the goal is to have a working unit for the summer next year.
To summarise the design, it is a single conversion superhet with a 10.7MHz IF controlled using a simple Arduino micro controller, the choice of oscillator is still up in the air as I have both the SI5351 and AD9959 devices in stock, two very different technologies so some experimentation time will need to be done up front to make sure I choose the best of the two.

The focus at the moment is creating a simple but stable TRX IF stage, I have settled on an SRA-1 as the main mixer for TX/RX and a pair of diode switched ERA-2 MMIC devices as a bi-lateral post-mixer amplifier, using bilateral amps means I can essentially share the crystal filter between TX/RX and keep the overall PCB count down compared to having separate TX and RX IF stages which I have used on previous projects in the past. The receive IF amplifiers will be using three FET devices, first FET will provide initial gain to drive two 3N201 dual-gate MOSFET devices and a simple half-wave product detector using a pair of BAT42 diodes.

The 144MHz front-end will comprise of a 3 pole bi-lateral bandpass filter and a pair of switched PGA-103 low noise amplifiers, the great thing about the PGA-103 is the fact that I get a very good low noise performance on receive and for transmit it will easily give me 100mW output to drive the PA, which I have chosen a 30W Motorola module as they are very easy to obtain and implement.

The overall footprint of this rig needs to be kept small as possible, I am aiming for a size of around that of an old CB Radio like a Cobra 148 or HAM International so roughly 10" X 7" X 3", previous experience of display technology has put me off using OLED-type display modules as they are very noisy, so I have opted to go down the Nextion route as they offer a complete turnkey solution for not much money, plus all of the display controls are handled by a separate micro controller and the design studio is very easy to hook up a rich, functional colour display - Lookup the uBITX Nextion screen as an example.

The front panel controls will be minimalist to reduce mechanical complexity and size, so any non-frequently used controls will be accessible via software control in a simple menu system.

So, watch this space as I will be creating a separate article on the forum dedicated to this project and will upload images, schematics, test results and of course some video too.

Cheers and 73

Rob.