Sunday, June 17, 2012

21cm Hydrogen Line Feedhorn Assembly for Radio Astronomy

I am excited to finally be able to say that my radio telescope is starting to come together. This morning I worked on another major component of my receiving system, a 1.420Ghz hydrogen line feedhorn. This has been a project that I have been wanting to build for roughly 16 years and now that things are moving along I am hoping to have a system together ready for testing within a month or so.

Now for the feed, I decided to go with a rectangular design for my feed instead of a circular one for simplicity of assembly. I went back and forth many times on which design I should use but ultimately ended up deciding on the rectangular feed for several reasons. It is easier to assemble (90 degree cuts are easy to mill), it's based on a standard size, and the material was cheap. Rectangular waveguides are polarized, although for radio astronomy purposes this should not matter as any natural occurring signals would in theory have random polarization. Here is the final assembled version:

While not really a feedhorn as of yet (I have not built the horn) it is a nice waveguide to coax adapter that will be used as a feed at the focal point of my dish. As for the horn, I will have to check to see if I will have any benefit of using one. A horn can provide additional gain from the dish, but it also blocks off surface area of the dish in its shadow. A choke ring on a circular feed would have the same effect in blocking the signal, this is just something I need to research more.

The feed itself is assembled out of 1/4" 6061 aluminum stock that I cut and milled down to size. The dimensions of the opening are 6.5" x 3.25" which is the exact spec of the industry standard WR-650 waveguide which is designed for frequencies between 1.12Ghz and 1.70Ghz. The Hydrogen line of 1.420Ghz fits nearly perfect between these two limits which makes this specific size ideal for radio astronomy. I drilled and tapped 22 holes which have stainless steel hex head screws holding it together. I was very pleased with the final assembly as it has a nice tight fit.

The probe consists of a 4mm section of copper wire which is exactly 1/4 wavelength of 1.420Ghz long and positioned 1/4 wavelength from the back of the feed. The probe terminates to an SMA connector mounted to the top of the feed. I had to mill a small slot into the top of the feed to allow the bottom section of the SMA panel mount jack to lie flush with the inside of the feed.

One note on the WR-650 standard itself. There are commercial feeds available as it is a standard waveguide size, but the cost is extremely high since this this specific size of waveguide does not show up in the surplus market very often. Smaller waveguide standards for higher frequencies like WR-90, WR-42, etc, do show up but it has been extremely hard to find anything WR-650 available for cheap. My total cost to build this feed is about $50.

I have already tested this with my HP 8614A signal generator set at 1.420Ghz and have verified it does indeed work very well. Next steps are to add the mounting brackets to it which will allow me to mount it at the focal point of my dish and also add the additional RF amps, filters, and my downconverter to the back section of the feed. I still also will need to calculate total system gain and noise once completed.

Monday, June 11, 2012

Altera EPM3256A CPLD Breakout

A few months ago I received my newly designed breakout boards from Laen's OSH Park PCB service and finally had time to assemble my new boards. Stepping up from my original EPM3032A board I went to the EPM3256A here in its final assembled form:

With this new board I only broke out about 100 or so pins of the 208 pin package, this will be more than enough for the projects I have planned with it. I added a few features to it including an on-board socketed crystal with options for 3.3v or 5v operation. I also have options to route the clock to the global clock input or a specified general purpose io pin depending on the application.

A couple nice features about the MAX3000 series specifically to the EPM3256A:

- 3.3v and 5v compatibility. Makes interfacing existing logic very easy.
- 256 macrocells available. Small compared to modern FPGAs, but is enough to be very useful.
- 158 Usable i/o pins.

I will be posting some projects based on this board very soon.

New Spectrum Analyzer: Anritsu MT8801C

The Spectrum Analyzer is by far the most important piece of gear for any RF design work, unfortunately spectrum analyzers are also one of the most costly pieces of gear you can buy (A VNA is right up there too, but that's a different post). I have had access to spectrum analyzers at a few previous jobs which is great whenever you need to test your latest RF design. The problem is when you are at home working at your own bench at 2am,  it's annoying to not be able to have access to this gear all of the time. Buying an analyzer is ideal, but costly. Anything new is pretty much out of the question, so the usual source of eBay is the place to go. Both Tektronix and HP/Agilent have some amazing pieces of gear for an 'affordable' amount (the Tek 49N series and HP 85NN series come to mind), the problem is any of these models can easily cost over $1000 in good working condition. The other issue with this equipment (like all older gear) is their age. Since most were used in a production or lab environment, they have been powered on for 8 hours a day for years. This can result in some crt burn in, the devices being way out of calibration, instabilities and other problems as most of the equipment in this class is 15+ years old (note that makes it affordable). I have had a good run with all my HP / Tek gear as this equipment is really built extremely well. As an example, my HP8614A signal generator was built in the 1960s and still works perfect today. So what happens if you want a spectrum analyzer but don't want to spend $1000+? As I found, there are a few options:

1. Buy a really old analyzer. Some of the older HP models will go for under $500. Keep in mind that these models usually have a max frequency range of no more than a few hundred Mhz.

2. Buy a lesser known brand. There are a handful of analyzers by Chinese companies that go for cheap. They may be perfectly fine, I just prefer to go with a good established brand if I'm going to invest in one.

3. Watch local auctions. There are a ton of company liquidation auction houses such as Dovebid that sell off large companies test equipment assets. These are great places to pick up gear. The issue with this is that there is no guarantee that the gear works (no one tests it) and if it is a valuable piece it will most likely get bid up pretty high. Packing and shipping can cost hundreds of dollars as well if you are not able to pick up the gear locally.

4. My favorite option. Buy gear whose primary purpose is not a spectrum analyzer, but has an analyzer hiding inside it. A lot of communication analyzers and cell phone test sets have an available spectrum analyzer option. I will look up unusual gear on eBay that seems to be selling for cheap and read the product literature on them. You will be surprised on what you will find. I have purchased both of my spectrum analyzers this way.

My first spectrum analyzer that I bought a few years ago is an HP8922H GSM test set. It is designed to replicate a GSM cell station to test GSM cell phones. It also has a bunch of options included one of which is option 006, a 10Mhz to 1Ghz spectrum Analyzer. You can find these for around $500 or less. Now 1Ghz is great, but you eventually reach the limits of what you can do with it. One of my current projects is building a hydrogen line radio telescope which at 1420Mhz is outside of my analyzers reach. I needed something to at least 2Ghz at this point to test my down converter so I began my search again.

While recently looking at more gear that was available I came across an Anritsu MT8801C radio communication analyzer. Not being familiar with Anritsu as most of my gear is HP/ Agilent and Tektronix, I did a bit of research into this particular model and discovered that not only was it an amazing piece of gear, but much like my HP8922H, it has an option for a 300Khz to 3Ghz spectrum analyzer (Option 07):

Being a communications analyzer it has a bunch of other nice features such as a 300Khz to 3Ghz RF frequency generator, and an RF power meter:


Here is a full span of 300Khz to 3Ghz to my outside wideband antenna:

A couple nice things about this analyzer is that it has a large LCD screen, is capable of displaying a full frequency span, it has a resolution of 1Hz, and a very fast interface. I checked its calibrated accuracy with my HP 8656B RF generator and it was spot on which made me very happy as well:

The additional 3dBm loss above is from the mini-circuits splitter I was using between the generator and analyzer.

Included with this unit was a nice shielded RF test chamber for no extra cost. I can only guess what this had cost new:


It will be a great tool for testing devices within a completely shielded environment from external interference.


So all said and done this complete setup cost less than $600. Quite a steal considering new this was a $30K+ piece of gear. With that being said I'm finally excited to test my down converter further and also get my RF front end for my radio telescope underway.