Replacing S124 (DUAL VCO)

[aleksa]

I have an old schematic with S124 on it.

To my knowledge, only Farnell has it,
and it is in a DIP package.

Is there a replacement part, preferably SMD?

(0 Hz - 10 MHz)

TIA

 

[Michael Black]

On Fri, 23 Jan 2009, aleksa wrote:

I have an old schematic with S124 on it.

To my knowledge, only Farnell has it,
and it is in a DIP package.

Is there a replacement part, preferably SMD?

(0 Hz - 10 MHz)

The 74S124 is over 35 years old, so it's not really a surprise

that it's hard to get.

The 74LS624 through 74LS629 are all TTL VCOs, and more recent.

If you've got a schematic that uses such an old part, you should
be looking at whether the function has been redone in more
recent times. Otherwise, you may find there are other parts
no longer available, or at the very least one can find a better
design that is more recent.

And to properly provide a replacement, one really has to look
at the schematic. RC VCOs have the advantage of wide range, but
they aren't necessarily best for everything. The 74S124 might have
been used in the old schematic because of that range, or because it
has a square wave output, or because it was easiest to use, despite
tradeoffs. An LC VCO will require more fussing, and have limited
range without bandswitching, but it will provide a purer output. But
without knowing what the original circuit was about, one can't decide
whether something better is better, or whether it would add too much
work to update the circuit.

When the 74S124 first came out, there wasn't much choice in ICs. It
actually went up to 50MHz or more, the alternative were discrete
components or the Signetics line of analog PLLs, only one of which
had the same frequency range.

Not much later, the CD4046 CMOS PLL came along, but it's range was
limited to the hundreds of KHz range, not MHz. Much later, variants
came out, I can't recall the specific part numbers, that had much
higher frequency range, and that's a possibility, except depending
on the original circuit it may require work to use a CMOS IC in
a decades old schematic.

There are also function generator ICs that started out mostly for
audio but had later variants that offered up to 10MHz range, though
I suspect that upper range may be stretching things. They had the
advantage of multiple output waveforms.

But it all depends on what the original circuit is supposed to
do, and what the schematic looks like.

Michael

 

[aleksa]

The 74LS624 through 74LS629 are all TTL VCOs, and more recent.

Farnell has 74S628 but also in DIP and with MOQ of 2500!