My Latest Radio Design Project
Here's a photo of my most recent amateur radio project. After over two years of working in my spare time, it's almost ready to put on the air for the first time. I'll eventually get around to writing a full technical description, but in the meantime, here are some of the highlights of the design -
Single 13.8 volt power supply
Modular design using 3.8 x 2.5 inch custom-designed printed circuit boards
20- and 40-meter CW operation
High-quality Sabin bandpass filters, selectable for each band in receive mode
400 Hz 5-crystal filter with a Gaussian roll-off to reduce IF ringing
5-watt audio driver to allow plenty of speaker driver capability
16-character electroluminescent display for frequency readout
Local oscillator and transmit frequency generated by Analog Devices DDS chip
Display and DDS controlled with a PIC microcontroller
Digital volume control managed with a PIC microcontroller
High-quality 7-element Chebyshev lowpass filters, selectable for each band in transmit mode
Consistent and stable 10-watt RMS signal output drive to 50 ohms on either band
Back-to-back comparison tests with this radio and my Icom 735 have shown that the receiver is very good, often surpassing that of the Icom because of the advanced receiver filters used in the homebrew. I would expect the performance to improve even further after applying RF shields to the critical areas of the receiver such as the crystal IF filter.
The transmitter is currently putting out 10 watts RMS to a 50-ohm resistor load. This could probably be increased further, but the radio is currently limited by the size of the drive transistor's heatsink and small fan. Since this was my original design goal anyway, on-the-air testing will be done at this power level.
The photo below shows the test bench that I have constructed for the radio. The receiver is mounted on the metal sheet to the left, and the transmitter section is on the right.