AVR-Chat

Yes, that was the "I think" caveat. That suggests being able to use  
smaller limiting resistors.

The high capacitance CAN be a challenge. Sometimes, it will actually  
help. Whether it hurts or helps depends on the "source resistance" of  
the signal source. Logic is typically 80-200 ohms (tending to be a bit  
larger with Vcc of 3.3V compared to 5V). It is THAT resistance, in  
combination with the transient protector capacitance, that sets the  
rise and fall times. If the signal source is the AVR and is "talking"  
through a series current limit resistor such as what we have been  
talking about, you need to add that resistor value to the 100 ohms or  
so output resistance to determine as rise time estimate.

I took a look at the AVX spec sheets. particularly,

//www.avx.com/docs/masterpubs/transgrd.pdf

There, I get the impression that they all have, pretty much, the same  
I-V characteristics.

HF-SSB can be particularly problematic. The high transmitter power,  
the close proximity of the antenna to other wiring, and the fact that  
the frequency CAN be in the range that the micro will respond to,  
makes it difficult. Further, the "real" problem is often signal  
rectification. To solve that, you just have to keep the signal small  
in your circuitry. That often involves shielding, as you have  
observed, good filters, good connections between board ground and  
shields. On particularly problematic lines, you may have to add a  
multi-section L-C lowpass filter to get the levels low enough.

Jim
On Jan 8, 2011, at 10:56 AM, Steve Hodge wrote:

> Thanks, Jim. You have been a good help. I do have a bunch of RF  
> ferrite
> chokes that I put here and there. They didn't do anything to solve  
> the SSB
> radio turning on the horn (!), but a shielded cable did.
>
> The tip about there being no one solution helps actually. I guess I  
> was
> looking for that, and you've made it clear that's impossible.
>
> So I'm going to do more or less what you suggested and use the 3.3 V
> Transguards with a series resistor, at least on low frequency signal  
> uC
> ports. However, I'm still struggling with the numbers.
>
> Poking around a bit on AVRFreaks seems to indicate the internal  
> protection
> diodes can handle up to 1 mA rather than the 0.1 mA you quoted. Is  
> that
> what you meant by your "I think" caveat? Taking this figure and the
> clamping voltage of 12 V for the 3.3 V working voltage Transguards, I
> compute (12-3.3)/1 = 8.7 K for the series resistor. Actually, I  
> suppose one
> could substitute 3.8 V, the abs. max. voltage the port can take, and  
> reduce
> this to 8.2 K. At any rate, being a bit conservative, choose 10 K. Do
> these numbers make sense?
>
> For a max. frequency, the particular Transguard I chose (and happen  
> to have
> 100 or so lying around) has a whopping high capacitance of 5000 pF.  
> If I
> continue to use these, I compute a max. usable frequency of about 10  
> KHz.
> Do you concur? This would be ok for many signals I have.
>
> However, I do have some higher frequency signals. I looked at the
> UltraGuard series, which are for "high speed clock...application".  
> But the
> lowest capacitance for, say, a 5 V one, is only 175 pF, not a heck  
> of an
> improvement by the above numbers. Plus there is no breakdown or  
> clamping
> voltage in the data sheet. So I continue to be confused. Do these work
> differently or something?
>
> Steve
>
> | -----Original Message-----
> | From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On
> | Behalf Of Jim Wagner
> | Sent: Friday, January 07, 2011 9:21 PM
> | To: AVR-Chat@yahoogroups.com
> | Subject: Re: [AVR-Chat] AVX Transguard
> |
> | I missed the part about radios, radar, etc. So, in addition to
> | transient protectors, liberal use of lossy ferrites (beads, etc) are
> | strongly called for.
> |
> | There really is no one technology that will handle the full spectrum
> | of ESD, lighting, general conducted EMI, and RF through microwave. I
> | am sure that I'm not the only one on the list who has been through
> | this commercially. Its NOT a trivial task and its particularly
> | difficult to do with confidence in the absence of testing. Lacking
> | that, you are going to have to rely on your own interpretation of
> | other's experiences and hope that you understand what is offered.
> |
> | Jim Wagner
> | Oregon Research Electronics
> |
> | On Jan 6, 2011, at 9:15 AM, Cat C wrote:
> |
>
>
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