Thursday, February 17, 2011

Hacking Cobra Microtalk FRS radios: Intro

Are the ubiquitous and incredibly cheap FRS radios of any use in ham radio?

I have always been really curious about that. But a Google search turns up very little in the way of hams hacking FRS radios for amateur radio uses. Well, I'm working with Dr. Cool (who prefers to remain anonymous) to do just that. We got some Cobra Microtalk FRS radios for just $2.50 after rebate. Cool opened them up to see just what's inside.

Turns out that the PLL chip is clearly marked (it's the MCD2926) and the datasheet is available. Reprogramming the PLL for different freqs is pretty simple. I even created a spreadsheet that lets you plug in a frequency and it comes up with the various command values you need to send to the chip. So theoretically, a guy should be able to push these dudes down into the ham bands by 1) retuning the VCO that's in the radio to go down about 20 MHz (from the FRS allocations, into the 430 - 450 MHz ham band) and 2) hijacking the control pins to the PLL to tell it where to go. Maybe just the first would actually get you into the ham band.

The other thing I'd like to do is to replace the VCO that's on the radio with one of the Maxim VCO parts (MAX2605/2606/2607/2608/2609, costing about $1 each). That way you'd get a nice programmable oscillator for the range of the particular VCO you select. Seems like it would be great for a small bench-top programmable oscillator or for the basis of a cheap beacon for 50/144/222/432 MHz.

We're going to keep hacking at these things, so I'll keep you posted.

[Here's the data stream being sent to the PLL chip. Won't be hard to program a little PIC processor to send that out.]

73 de W9HQ

Monday, February 14, 2011

W9HQ on Transverters in QST Magazine

I recently worked with Paul Wade, W1GHZ, to have a little blurb in his Microwavelengths section of QST magazine. This year I have resolved to get serious about finishing up at least a few of my W1GHZ transverters, hopefully in time for the August ARRL UHF contest. Working on these projects is exciting and educational. I started a Yahoo! group for other builders and it has almost 100 members now, with a nice boost from the QST article. Here's what I wrote:

UHF and SHF weak-signal operation is an aspect of ham radio that held no interest for me, until recently. Now, however, I’ve been totally hooked by this new frontier in radio. The circuits needed to generate such high frequencies have always been mysterious to me and the technical challenges and sometimes esoteric materials involved discouraged me from seriously pursuing any sort of equipment building.

Fortunately, the levels of integration available in individual RF parts capable of functioning way up into the GHz levels is such that the really hard work has been done for us. The amplifier, mixer, and filtering blocks are cheap, readily available, and highly optimized. What’s left is combining these building blocks into a design capable of operating in the ham bands.

The transverter circuits created by W1GHZ are delightfully simple and just examining the schematics and reading his description of how he designed them is an excellent tutorial in transverter operation. Fundamental operations such as frequency multiplication, filtering, and IF/LO mixing suddenly become clear when confronted with such a clear and basic design. Once you understand Paul’s simple designs, more complex transverters suddenly become clear. They all work the same way!

Aspiring builders can get the PC boards from W1GHZ, but then you’re on your own to acquire the necessary parts from various sources. A fine pointed soldering iron, a little magnification, and a steady hand are all that are needed to stuff the small surface mount parts to the boards. But this is not kit building, so definitely not for the beginning builder. There are no part by part instructions on how to assemble these boards and the means of final check-out is entirely up to the individual. That brings up a potential obstacle for some builders. At least in my opinion, check-out of the modules would be very difficult without at least some access to a spectrum analyzer. With an analyzer I was able to bring up the 1152 MHz LO for the 1296 MHz transverter and found immediately that, although it was oscillating, its output was extremely low. Working back over my board revealed that I had misconnected one end of the bias resistor to the final amplifier stage. A quick rework brought the LO up to its desired output. I can’t really imagine how I would have found something like that without a spectrum analyzer.

I’m learning incredible amount both in theory and practice by building my W1GHZ transverter boards. I can’t wait to get one on the air—that first contact with a SHF transverter that I built myself will be thrilling indeed.

Those interested in joining an on-line group dedicated to building W1GHZ transverters are encouraged to visit