[Lowfer] Re: Downconverter for Software Defined Radio

[Alberto di Bene]

"Lyle Koehler" <lyle@mlecmn.net> wrote:

> By the way, I believe the programmable logic device (PLD)
> that is used to generate the quadrature LO signals in the 
> Time Machine can be replaced by a 74AC74 or 74HC74 Johnson 
> counter as shown in the QEX software defined radio articles 
> (also available via the FlexRadio web site). The PLD sounds 
> like a neat way to go, but I don't know what it takes to program one.

I also used a 74HCT74 Johnson counter in my front end. The problem
with this configuration is that you need a LO at four times the
wanted frequency if you use the Tayloe mixer, or two times if you
use the H mixer. For LF this may not be a problem, but it starts to
be when you want to reach the upper HF range.
The ideal circuit would be one that, fed with a given frequency, 
could spit out four square waves at the same frequency, but 
progressively delayed 90 degrees. Not an easy task, I bet...

73  Alberto  I2PHD