Friday, October 16, 2009

Designing a downconverter

I've done some looking into it, and it appears that the three parts that would be the most useful would be the SA620 LNA, VCO and mixer, an MC145151-2 DIP switch controlled PLL, and an MC12080 UHF prescaler. From what I can tell, those three parts should be able to work together to downconvert from just about any frequency in the 33 cm band down to VHF channel 3 or 4.

Yeah, I know. In the last post, I made an argument for putting it in UHF instead. But the coax losses are much, much lower at VHF, so if it's reasonable or easy to make it happen, well, why not?

The big problem is, so far as I can tell, the SA620 and MC12080 are only available as surface mount devices. It'd be nice if they were available in DIP packaging. But, alas, no. SparkFun electronics, however, sells SOIC-8 and SSOP-20 to DIP adapter boards.

The concept is that the RF comes in and the LNA built-in to the 620 amplifies it a little and feeds it into the mixer. Meanwhile, the VCO generates the IF of about 850 MHz or thereabouts. A little of the VCO's output is fed through the prescaler, and then out to the PLL. The PLL then generates corrections for the VCO's voltage.

Those 3 chips total are less than $15, from what I can tell, from a combination of DigiKey and Jameco.

And that's all there is to it: 3 chips, a 5 volt regulator, 2 sets of DIP switches and some discrete components to tie it all together. For extra credit, it would be fairly simple to add a bias-T to the output to supply power to the thing to mast-mount it. And if it's going to be mast-mounted, then DIP switches are a completely reasonable way to set the LO frequency, since it'll be something you only set once.

In fact, if you're using a computer peripheral tuner, like the HDHomerun, you'll likely just set it for an LO frequency of 848 MHz - that would put 902 MHz at 54 MHz. You'd then just tell your tuner exactly what frequency it should use. The only reason for making the PLL as adjustable as it is is because TV sets aren't agile enough to hunt around other than on normal TV channels. The HDHomeRun, however, can easily be told to look for 8VSB on a channel center of 64 MHz - which is where the 909-915 MHz ATV channel would be found.

1 comment:

Nick said...

More figuring:

If the reference crystal is 12.8 MHz, and the reference divisor is 1024, then the PLL reference frequency is 12.5 kHz. With an 80x prescaler, that makes the basic channel step size 1 MHz. And that means that you simply set the N dip switch to the desired LO frequency in MHz. Simple!