Friday, July 6, 2012

Radio Astronomy Hydrogen Line 1420Mhz RF Front End Testing

I have been building the RF front end for my radio telescope for a few weeks now and am finally at a point where I can sit down and test it. Right now I have the feedhorn assembly completed and the necessary front end electronics mounted to the back of it.

The feedhorn feeds directly into a mini-circuits ZRL-2400LN low noise amplifier. The output of this then feeds into a cavity filter with a center frequency of 1420Mhz. This was a rare but exciting eBay find hat I came across a few years ago, it actually had come off of the VLA in New Mexico. It had been originally tuned to 1430Mhz but with the assistance of a VNA I was able to tune it down to 1420Mhz. This then outputs to my own custom built downconverter which I have mounted in a custom copper enclosure. I will eventually mill a aluminum enclosure for it but this will suffice for now. Finally after the amplified output stage on my downconverter, I pass the IF through a mini-circuits SLP-450 low-pass filter to remove the original source frequencies, LO, and image frequencies. For testing I have my LO set to 1200Mhz which will result in a 220Mhz IF based on the 1420Mhz source. Ideally I will probably downconvert to 70Mhz as there is plenty of 70Mhz detection gear available on the surplus market. I'm still most likely going to build my own I/Q demodulator for the detection side.

Here is a quick video demonstrating the testing setup: