All SMD Radio Transmitter

In a current personal(1), i.e., not for sale or distribution, project I have been using a Chinese ASK radio transmitter module – the STX-882, which I posted about back in February of this year. The STX-882 works well BUT being a module, it is a through-hole device and my goal is for my current project to be 100% Surface Mount Technology (SMT). While  this project will not be manufactured for sale, I am curious as to what such a device would cost IF it were turn-key manufactured and what the final cost would be. It appears that the cost would be greatly reduced if there are no through-hole devices. Through-hole devices in a low volume product cause a two step manufacturing process where the board is assembled using Pick aNd Place (PNP) fabrication but must be finished by hand. As a result, I needed to replace the three through-hole parts in my design with SMD parts. One part, a large capacitor, I will replace with several, smaller capacity SMD capacitors. However, two modules, a Temperature/Humidity sensor and the aforementioned STX-882 433.92MHz RF transmitter module would need to be replaced with SMT designs that would be incorporated on the same board.

I decided to build three small personal prototype boards solely to test my design and fabrication. To replace the 433.92MHz transmitter I selected Microchip’s MICRF113YM6 single-chip ASK Transmitter IC. Quick calculation indicates an ERP of -10.005 dBW with a λ/4 antenna. Transmission time is a few milliseconds. Utilizing a slightly modified version of their reference design I used pcb-rnd PCB design software to design a small prototype board which I then had made by OSHPark. This board was designed VERY quickly – I didn’t spend over 30 minutes designing the schematic and laying out the board. Such is the case with pcb-rnd, which was forked from geda PCB in order to add a number of innovative features.


Fig 1: 433.92MHz Prototype Board Design


Fig 2:  Prototype Board pcb-rnd view

Today, I broke out my 858D hot air workstation and built up one of the prototype RF Transmitter boards. Next I cranked up a breadboard’ed test receiver for this project and on a transmit breadboard for this project I switched out its STX-882 transmitter and substituted my newly designed board. See Fig 3, below.


Fig 3: New Prototype Substituted for STX-882

To my great joy, the instant that I powered it up error free data was being delivered over 433.92MHz. Refer to Fig 4, below, and you’ll see the received temperature and humidity data. Also, Fig 4 is a spectrum scan while the radio periodically transmitted data (once per minute). The measured frequency is 433.916 – not quite the 433.92 expected but certainly well within tolerance. I’ll accept it because I didn’t use precision parts for C5 and L2. They are 10% tolerance parts.


Fig 4: Data received from Prototype RF Module.


Fig 5: Spectrum Scan

Now I’ll move on to testing my prototype temperature/Humidity sensor module. More on that later.

1. This is a personal use home-built device (not kit-constructed) that is assembled in a quantity of five or less. BTW – this odd verbiage is in case anyone from the FCC reads this post.

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One Response to All SMD Radio Transmitter

  1. Doug Drye says:

    Wow Integration has certainly come a long way, Ed… I remember when I helped Dale distribute the 56K RF modem in 1988, it was huge and expensive compared to this design… Nice article… Looks like Fun!..


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