EMRFD Message Archive 5259

Message Date From Subject
5259 2010-09-27 07:46:05 wa9cgz 74G74 Patato IC circui help needed
I purchased some 74G74's to replace a 74s74 divider used in a home brew
SRD Flex clone. I need to drive a short coax jumper (50 ohms) about
3 inches, after looking at the 74G74 data it's unable to drive
much of a capacitive load. I plan on using the 74G74 as a divider from 200mhz to 50 in order to add 6 meters to my radio. I'm looking at some
TTL 50 ohm line drivers such as the 74s140 or 75128, will the 74G74
switch at 200mhz driving the above or could you suggest a better circuit
or chip.

Joe wa9cgz
5260 2010-09-27 08:33:55 Andy Re: 74G74 Patato IC circui help needed
> ... I need to drive a short coax jumper (50 ohms) about
> 3 inches, after looking at the 74G74 data it's unable to drive
> much of a capacitive load.

I'd be really surprised if a part can't drive 3 inches of t-line at 50
MHz. Are you sure it can't?

If it can't, there ought to be other flip-flop parts that can (though
I admit to being somewhat out of regular touch with those kinds of
parts for a while now). It seems like overkill to add line drivers
just to move a signal 3 inches!

Since you've got the parts already, why not breadboard it and see what
it can do?

Andy
5262 2010-09-27 09:43:19 ehydra Re: 74G74 Patato IC circui help needed
If it is only the divide-by-two thing, why not try this:
http://www.analog-innovations.com/SED/ToggleFlopAncient.pdf

Faster transistor -> faster T-FF !

- Henry

--
ehydra.dyndns.info


wa9cgz schrieb:
> I purchased some 74G74's to replace a 74s74 divider used in a home brew
> SRD Flex clone. I need to drive a short coax jumper (50 ohms) about
> 3 inches, after looking at the 74G74 data it's unable to drive
> much of a capacitive load. I plan on using the 74G74 as a divider from 200mhz to 50 in order to add 6 meters to my radio. I'm looking at some
> TTL 50 ohm line drivers such as the 74s140 or 75128, will the 74G74
> switch at 200mhz driving the above or could you suggest a better circuit
> or chip.
5266 2010-09-27 14:47:00 Neil Douglas Re: 74G74 Patato IC circui help needed
From my university days a long time ago I was taught that it is bad form to
drive coax directly from the output of a flip flop.



If the line is not terminated correctly, then any reflections can toggle the
flip flop back again.



I would also check the logic diagram for any f/f IC to make sure there is a
buffer between the f/f and the output pin.



NeilD

G4SHJ



_____

5327 2010-10-03 00:43:29 Tim Re: 74G74 Patato IC circui help needed
I'm at a loss because I don't even know what the 74G logic family is.

I do find that 74AC makes great drivers for all sorts of things. The symmetrical drive abilities are just wonderful and beat the pants off the old bipolar 74S (where sink had all the muscle and source was not so hot) as a driver.

Do you mean, maybe, 74C ? That'd be quite wimpy as a driver. But 74AC is great.

3 inches of coax is nothing, BTW.

5328 2010-10-03 01:59:09 Dave - WB6DHW Re: 74G74 Patato IC circui help needed
Tim:
The 74G series is made by potato semi (Potato Chips!!). The G stands for GHz
logic(although the 74G74 is rated at 600 MHz). You can get data sheets and chips at
<http://www.potatosemi.com/>

Dave - WB6DHW
<http://wb6dhw.com>

5345 2010-10-05 07:00:16 wa9cgz Re: 74G74 Patato IC circui help needed
I replaced the 74ac74 divide by 4 quadrature generator IC with a Potato Semi. 74G74 my circuit now works reliably past 200MHz which is necessary for 6 meter operation.
The final configuration is SI570 oscillator, DS90LV028 differential line receiver,74AC00 buffer. I operate the 74G74 0n 3.3 volts everything else on 5. I realize that the line receiver is running at it's upper frequency range but it was in place in the original layout.
Thank to all who responded on list or my mail.

Joe wa9cgz