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Raspberry Pi and RTL_SDR March 21, 2013

Posted by dhtaylor in Uncategorized.

I’ve recently been experimenting with the Raspberry Pi and the popular Realtek DVB receiver turned general purpose Software Defined Radio. My main goal is to have an SDR receiver up on the roof, close to the antenna to spare me the 200′ of coax required to get down into my basement lab. The antenna feed line would be approximately 5 feet while the remaining 195′ would be covered by an Ethernet link.

The RTL_SDR project provides open source tools for controlling the Realtek USB DVB receiver. The project also contains a package for incapsulating the I/Q samples generated by the RTL_SDR’s A/D converter into TCP packets and sending them over a network. The Raspberry Pi can be easily turned into a server for the RTL_SDR’s samples and connected to from any SDR application that supports acquiring sampled data from a TCP source. To top it off, a PoE splitter was installed in the weathertight enclosure to power the Pi.


Raspberry Pi and RTL_SDR card installed in weathertight enclosure. The RF input of the RTL_SDR is cabled into a bulkhead N-type connector for antenna connection.


Raspberry Pi being powered by a PoE splitter. The splitter has a selectable voltage to allow it to provide 5, 9 or 12v output. The 5v output from the splitter is wired into the Pi’s USB power jack.


Rpi, PoE splitter and RTL_SDR all together and mounted on antenna mast


Antenna mast on roof


The antenna is a 144/440 MHz Comet GP3 with LMR-400 feedline.


Software Defined Radio February 21, 2012

Posted by dhtaylor in Uncategorized.
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Currently I’m experimenting with designing a software defined radio (SDR) architecture for high sensitivity receiver applications. Various projects are in the works by others but information and solid designs seem to be scarce. The requirements of this project should be:

  • Direct Sampling. The SDR should have a direct sampling analog to digital converter (ADC) on the front-end. In other words, the ADC should directly sample the RF spectrum and provide I and Q samples to the DSP hardware.
  • FPGA controlled. Ideally a field programmable gate array (FPGA) will be used to interface the ADC front end to a USB bus for transport to a computer. The FPGA code will be simple in that it is only moving samples between the ADC and the USB interface.
  • Limited amount of discrete analog components. The analog circuitry should be as minimal and universal as possible, consisting mostly of bandpass filter


1. In search of the perfect ADC

More to be posted soon!


WiFi Enabled Thermostat February 20, 2012

Posted by dhtaylor in Electronics Projects.
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One feature missing from many off the shelf programmable thermostats is the ability to control the operation remotely. A solution to this is to give the thermostat the ability to adopt to a WiFi network and act as a simple web server with a basic GUI.

This project consists of a general purpose I/O board with an onboard WiFi module. The design is based on an Atmel ATMEGA1284P microcontroller and a WiFly GSX WLAN module from Roving Networks. Supporting peripherals include a real-time-clock module and two output relays. Inputs include 4 general purpose ADCs and a OneWire interface for remote temperature sensors. Since the system is designed for general purpose I/O control, the thermostat logic is completely software based.

The web based GUI allows the user to edit the output relay’s on/off schedule using an easy HTML table. All other parameters can be configured through the web GUI. The real time clock module is synced through an NTP server as defined in the configuration.

The WiFi module is a simple serial to WLAN converter which contains all physical layer hardware/software as well as a full TCP/IP stack. This leaves the microcontroller free for other tasks.

Improvements in North Carolina Railroad Infrastructure August 9, 2009

Posted by dhtaylor in Railroad.
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Within the last few years, the North Carolina Railroad (NCRR), currently leased to Norfolk Southern, has been improving rail capacity by adding a second mainline track between Greensboro and High Point. The second track was originally removed in the 1960s as part of a CTC project and has become a major choke point in the present day’s passenger and freight operations on the line. Approximately 30 NS and 6 Amtrak trains use this portion of the NS Danville District on a daily basis. The construction has been ongoing for the last 2 years and is nearing completion.


NCDOT map of corridor improvements


Handheld APRS Tracker April 2, 2009

Posted by dhtaylor in Electronics Projects.
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One of my larger projects over the last year has been the design of a handheld GPS tracker for use in the Amateur Radio VHF APRS system. Since I walk/bike more than I drive, I wanted to experiment with a battery powered self-contained device. The goal was to have a complete station in a single box: GPS receiver, radio, modem, battery and antenna. Ultimately the device should have the following specifications:

1. As small as a typical HT radio.

2. Fully bidirectional, packet encoding and decoding… or at least channel carrier detection.

3. Powerful enough to reach digipeaters within a few miles, yet have a long enough battery life to operate for a day.


To construct a prototype, I used a device from the TinyTrack project to handle the modem and packet formatting functions.

The most difficult solution in this project is that of the radio. Ideally an embedded module would solve this problem yet they are very expensive and hard to come by in small quantities. Luckily, the TinyTrak developer has also started a project that includes an embedded VHF transmitter module. Problem solved! Only half of the problem that is, since it’s only a transmitter and not a full transceiver.


A GPS receiver is required to provide coordinates to the packet encoder. These days small OEM GPS receivers are a dime a dozen and the modern chipsets perform very well in most conditions. I chose USGlobalSat’s EM-408 with embedded patch antenna and TTL serial output. The TTL serial can be piped into the TinyTrak module directly and the EM-408 takes care of the rest.

em408-01-lMore to come soon!

RF Module Woes April 2, 2009

Posted by dhtaylor in Electronics Projects.

Recently my Icom 2100h VHF mobile transciever’s final amplifier stage failed, leaving only a few milliwatts from the exiter making it to the antenna. I determined that the exciter was operating correctly and found that the PA (power amplifier) stage was being provided with it’s needed power. The only remaining culpret was the PA stage itself, mostly contained within an SC-1091 (or Toshiba S-AV17) RF module, shown below in the radio.


Upon extracting the module from the radio and opening it, I noticed a small hairline crack in the beryllia ceramic substrate that the traces and components sit on. The crack was just large enough to prevent one of the traces from conducting and thus rendering the module inoperative. The repair was a simple matter of reflowing the solder on the trace to complete the circuit. Shown below is the inside of the RF module.


This seems to be the case with a number of other RF modules. In all cases they were easily repaired by the same method. The cracking is probably due to heat damage over time.