Monday, October 26, 2009

PLL design

The North Country Radio downconverter kit arrived today. It's a bit daunting, but they did supply some suggestions on how to tap off the RF sample for the PLL and put in the VCO control voltage. That was nice of them.

It's hard to find PLL components anywhere - certainly there aren't any one stop shops anymore. Makes me wonder how companies prototype stuff like this in this day and age.

Jameco still sells the MC145151 in DIP packaging, which is convenient.

The North Country Radio downconverter schematic says that the VCO control voltage is 2 to 8 volts. That exceeds the 5 volts that run the MC145151 PLL, which means that the PLL loop filter will need to be an active one. Given that the reference frequency in this case is 15,625 Hz, the op amp for the active loop filter isn't going to have a lot asked of it - an LM741 would do just fine.

That just leaves a divide by 64 prescaler. That, it turned out, was hard to find, and impossible in DIP packaging. I wound up buying a µPB1507GV from Mouser, but I will need to solder it down to an SSOP-8 to DIP adapter board for prototyping.

The nice thing about working on PLLs is that only on the input side of the prescaler are you dealing with real RF - the entire rest of the circuit is dealing with low enough frequencies that it can all be breadboarded.

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