[hpsdr] Driving Altera FPGAs with a VCTCXO

[J.D. Bakker]

[Forgot to cc: the list]

At 18:24 +0100 06-07-2008, Charles Brain wrote:
>I am trying to drive an Altera FPGA from an ABRACON VCTCXO
>which has a 0.8v clipped sinewave output. I have done a search of 
>the internet and the only technique I can find is to use a 74HCT04 
>inverter with a 1M feedback resistor.

That's the canonical cheapest solution. A HCT part may not be the 
best choice, though, due to its nonstandard Vih/Vil. More common is 
to use a non-buffered gate, such as the HCU04, possibly with a second 
inverter (sans feedback R) as a buffer. Come to think of it, 20MHz 
may be pushing it on HCx logic, especially at lower voltages.

>  Is there a better way of doing this?

That's hard to say without qualifying 'better'. What is important to 
you? Jitter? Power consumption? Board area? Cost? Parts count? Parts 
availability (from DigiKey/your junk box)? Total height? 
Hand-solderabilty? Available supply voltages? Noise immunity?

Options you might consider include:

- the abovementioned unbuffered gate + resistor
- a fast comparator like the LT1720
- a limiting amplifier (either integrated or discrete)
- AC-coupling into an LVDS receiver
- transformer coupling into an (LVP)ECL receiver

HTH,

JDB.
-- 
Jan-Derk Bakker, jdb at lartmaker.nl

The lazy man's proverb:
     'There's no business like slow business !'

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