AoE question

[garyr]

On page 654 is a schematic of a varactor-tuned PLL. Can someone please
tell me why two 'HC04s are used to square up the reference signal input
to the 'HC390. Wouldn't one be enough?

 

[Jim Thompson]

On page 654 is a schematic of a varactor-tuned PLL. Can someone please
tell me why two 'HC04s are used to square up the reference signal input
to the 'HC390. Wouldn't one be enough?

The first 'HC04 is biased with the 1Meg resistor to the "sweet spot"
(threshold) so that it is sensitive to small input signals.

...Jim Thompson

 

[Joerg]

Hello Gary,

On page 654 is a schematic of a varactor-tuned PLL. Can someone please
tell me why two 'HC04s are used to square up the reference signal input
to the 'HC390. Wouldn't one be enough?

The first one is used in "semi-analog" fashion with it's input biased at
threshold. So there was probably concern that the output signal wouldn't
look quite so kosher and it can't hurt to further square it up.

BTW, if you take the Ref input signal away the HC04 will become a bit
toasty because the first stage in the schematic will be sitting at
threshold and thus will be drawing quite some quiescent current. It's
not advised to leave them in that condition (meaning without ref input)
when that HC04 is supplied near its max rated VCC.

 

[Jim Thompson]

Hello Gary,



The first one is used in "semi-analog" fashion with it's input biased at
threshold. So there was probably concern that the output signal wouldn't
look quite so kosher and it can't hurt to further square it up.

BTW, if you take the Ref input signal away the HC04 will become a bit
toasty because the first stage in the schematic will be sitting at
threshold and thus will be drawing quite some quiescent current. It's
not advised to leave them in that condition (meaning without ref input)
when that HC04 is supplied near its max rated VCC.

I'd use an 'HCU04 myself ;-)

...Jim Thompson

 

[Joerg]

Hello Jim,

I'd use an 'HCU04 myself ;-)

Yeah, most folks these days won't even know what unbuffered logic is. In
this case the designers probably wanted all the gain they could get.
Logic inverters of the buffered and unbuffered kind are probably still
the lowest cost analog amps money can buy if you don't count "time
shares" on an ASIC.

I remember when I was a kid and needed a frequency counter it took me
days to figure out how to build a nice transistor amp. Wasn't allowed to
the library because I had some kind of contagious flu. Then another few
days, cuts and bruises to scavenge AF139 transistors out of old TV sets.
Had I only known this trick of using inverters back then. But at least I
learned something by building that transistor pre-amp.

 

[garyr]

I'd use an 'HCU04 myself ;-)

...Jim Thompson

Why? In both places?

 

[Jim Thompson]

Why? In both places?

'HCU04 is STABLE with DC feedback... use the remainder (as needed) of
the package to square-up.

...Jim Thompson