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Old December 30th 05, 07:26 PM posted to rec.radio.amateur.homebrew
Dave Platt
 
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Default Ground Plane construction vs pre-printed "protoboards"

Well, that is certainly a vote for UC. Do you also have an opinion of
the other idea, using pre-patterned, pre-etched boards such as
Veroboard and the like?


My own feeling, for what it's worth, is that these are pretty good for
DC and audio-frequency projects, but can have problems at RF.

A big part of the issue is one of grounding. Many of the pre-etched
proto boards I've seen use most of their real estate for pads and
pad-connection strips. There are usually sets of traces for supplying
power and ground to the components, but these don't use more than a
relatively small amount of the board's copper area and are often
run as long strips coming in from the side of the board.

As a result, if you have two groups of components which are physically
fairly close together, and connect components in these groups to the
physically-closest ground pads/strips (minimizing the lead length),
you may find that there are actually quite a few inches of PC-board
ground trace between one component's "grounded" lead and another.

This is usually adequate at DC and audio frequency. At RF, the
parasitic inductance of those long looping ground traces can have an
adverse effect on the circuit's stability. You can sometimes minimize
this by using a star-grounding approach, but since the connection pads
are pre-etched into clusters and strips there's likely to be a limit
to the number of components that you can connect to a "single point"
ground, and it may not be all that good an approximation of a true
single-point.

One of the recipes for making stable, friendly, and reproducible
designs at RF seems to be to minimize the impact of parasitic
reactances. As you point out, using "ugly" construction of the
Manhattan or free-air (point-to-point) variety minimizes parasitic
capacitance between components, and between components and the board.

It also has the benefit of minimizing parasitic _inductive_ reactance
in the circuit's "ground", if you use a solid copper ground plane as
the basis for your construction. "Ground" is always an
approximation... a perfect ground cannot be achieved at either DC or
RF... but you can get closer if you have lots of copper area to work
with.

Etched PC boards can be used quite successfully for RF projects, of
course, and often are. It's important, when laying out such a board,
to minimize unwanted parasitics... leaving large sections of un-etched
copper for grounding, using wide traces, paying attention to where the
actual ground currents flow, adding ground-shield traces between any
signal-carrying traces that might tend to suffer from cross-coupling,
and so forth.

One usually does not have the luxury of being able to take advantage
of these techniques (at least to their fullest) when using a
pre-etched proto board,

--
Dave Platt AE6EO
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