[R-390] PIN Diode Update

[Bob Camp]

Hi

PIN diodes work great for RF because they are "slow" diodes. The  
physics in the junction keeps them from acting like a diode (turning  
on and off with a change in current) faster than some magic number.  
The common name for the number is minority carrier lifetime. Once the  
period of the waveform gets shorter than the carrier lifetime the  
diode starts to act like an open or short circuit. That's what keeps  
their IMD low. It's also what lets you switch a bunch of power with a  
very small (relative to the power) amount of current.

If you run a PIN at a frequency that's "to low" you simply have a very  
normal diode and don't get any of the "neat stuff". You might as well  
use a 1N4001 as a fancy PIN. They have a nice low on resistance and  
only a few hundred pf of capacitance (yikes!!).

Normal switching diodes that have been doped to have good on  
resistance may be your best bet. Gold doping used to be the standard  
way to do that trick.

I hate to go retro on something like the 390 list, but how about  
miniature relays? They are cheap reliable and very low IMD .....

Bob




On Jul 5, 2008, at 10:10 PM, Perry Sandeen wrote:

> GM List,
>
> Thanks to the help of Andy Masters, Collin N4TUA, and Bob Camp I may  
> have come up with some reasonable cost PIN diodes.  Bob said that a  
> lot of what's on the market these days has a fairly small minority  
> carrier lifetime which is OK for VHF but not HF.  I don’t know how  
> to select what would work for my VLF loop.  Amplification of  
> information appreciated.
>
> Below is a list of what I was able to find on the WWW.  Someone will  
> have to tell me what will work.  I was looking for affordable   
> prices as I wanted to use 10 for my loop.
>
> Sources for Motorola MPN3404 PIN Diode.  Prices were from 25 cents  
> to $1 each
>
> Circuit Specialists Inc. WWW.Web-tronics.com
> 220 S Country Club Dr. #2 Mesa, AZ 85210
>
> kitsandparts,com (Also torroids up the wazoo)
>
> Debco electronics.com
>
> Other low cost PIN Diode sources:
> Surplus Sales of Nebraska www.surplussales.com (Surprise-Surprise)
>
> RF Parts Company www.rfparts.com
>
> Down East Microwave Inc.
>
> Tuning Diodes:
> Ocean State Electronics MVAM109.....$2.95
> Dan’s Small Parts (Currently closed due to his severe accident)
>
> Mouser and all the other usual suspects all have the HP axial lead  
> types from $5 to $10
>
> Regards,
>
> Perrier
>
>
>
>
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