Simple RF circuit board design

[James Wolf]

This was far too simplified, and would likely get someone in trouble if they take statements at face value without understanding the deeper reasons why or why not.

Using four layers (or more) is only required when necessary, such as having components on both sides of the board, but in other cases it is necessary just to keep crosstalk from traces low.

50 ohms everywhere?   A brash statement that one wishes were always the case, but not always practical.  

In today’s world, depending on the application, you need to think of digital layout the same as RF.  

Good observations Martin.  There is no black magic involved.

 

Jim – KR9U

 

 

From: Tacos [mailto:tacos-bounces+jbwolf=comcast.net at amrad.org] On Behalf Of Martin
Sent: Thursday, March 24, 2016 9:18 PM
To: Maitland Bottoms
Cc: Tacos
Subject: Re: Simple RF circuit board design

 

I've recently been tinkering with making HF filter PCBs so I was really interested in it.  It is aimed at working in microwave which hams should be doing more of.   I would like to hear more of the "black magic" that was left out.

 

I'm a cheapskate and appreciate the faster turn around with two layer boards.  I'm willing to learn how to make two layer work for simple things.  I'm going to break rule #1 - "use four layers" for now.  A Hackaday comment was the reference designs are frequently on two layers.

 

Some of it doesn't seem to be applicable to HF/VHF/UHF.  I tried rule # 2 - "integrated components" and opened up the Digikey catalog for filter chips.  Just a handful in the sub-microwave region and not available in small quantities.  I know you can ask for samples but most of the parts didn't seem that useful.

 

I've been working on some filters for RTL-SDRs - input 75 ohms.  I looked up the recommended trace width for OSHPARK two layer stackup at 14 MHz and it was over a 100 mils wide for 50 ohms.  That would take up a lot of board space and may be impossible for complex circuits.  So sometimes rule # 3 - "50 ohms everywhere" is hard to do.

 

I can't argue with 4 - "follow manufacturer's recommendations" .

 

I was surprised by rule # 5 - "route RF first".  Sounds like a good idea but not nearly enough.  I though that totally segregating RF and digital was the goal and where possible ground via walls are a good idea.

 

Any thoughts or black magic lessons welcome.

 

73 Martin W6MRR

 

On Wed, Mar 23, 2016 at 4:11 PM, Maitland Bottoms <aa4hs at amrad.org> wrote:

This showed up today:
http://hackaday.com/2016/03/23/michael-ossmann-makes-you-an-rf-design-hero/

A few rules and a video describing example projects.
Discuss.

73 de aa4hs
-Maitland
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