[time-nuts] PCB design questions

David Forbes dforbes at dakotacom.net
Mon Jun 2 14:38:11 EDT 2008


I do that sort of work for a living and a hobby. I'd be happy to 
generate PC board artwork for you in exchange for one unit of the 
finished product.

Perhaps you could post a schematic diagram, and I could evaluate it further.

Answers to your specific questions below.

At 7:19 PM +0100 6/2/08, David C. Partridge wrote:
>I've been working on the design for a frequency divider to complement the
>Thunderbolt I recently bought from TVB (thank you Tom, it's working very
>well as far as I can tell, though of course I've no other standard to
>compare against).
>Now to questions:
>1. Surface mount or through hole?  I don't have a re-flow oven (or even a
>hot air soldering system), so my inclination is to use through hole CMOS
>(74HC163s with 74AC glue logic and flip-flops), with the surface mount
>restricted to the clock shaper using a BAV99 and either an ADCMP600 or
>MAX999 and surrounding components.   Will using through hole cause me grief?

I'd recommend SMT because the parts have much lower parasitics than 
thru-hole parts, and they're much more widely available.

It is not at all difficult to solder SOIC, QFPs, chip resistors and 
capacitors, etc. to a PC board with a microscope, a soldering iron 
and the right type of solder and liquid flux. I do it all the time, 
even at home, QFPs down to 0.5mm pitch and components down to 0603 

>2. How many layers?   In an ideal world with money no object, if I
>understand the current art correctly, I think I'd probably aim for a five
>layer board with Vcc, Digital Ground and Power Ground being separate
>internal planes, and trace routing on the top and bottom of the board with
>as few vias between top and bottom as possible.  Does that sound right?
>Do you think I can safely restrict myself to two layers, and if so does it
>make most sense to make one side of the board digital ground, and route
>everything else (Vcc, Power/Analogue Ground, and signals) on the other side.
>Or is there a better approach (always assuming that a two layer board is a
>viable option).

Two layers is reasonable for most analog-type circuits, since with 
proper component placement there are hardly any vias. A ground plane 
is used on the bottom of the board.

If you have need for high density (small PC board size requirements), 
then a four-layer board is a good idea, otherwise a 2-layer will work 
in many cases. It really depends on the circuit requirements.

There is absolutely no reason to have a separate layer for digital 
ground versus analog ground. One ground plane is plenty. Just 
segregate the noisy stuff from the sensitive stuff by putting them in 
different parts of the board.

>Dave Partridge


--David Forbes, Tucson, AZ

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