TenTec builds a great amateur radio and obviously to give you a 'million dollar
radio' that cost the user five bucks, economics really does have to enter the
picture. PIN diodes have been around for many years, however they were
initially invented, designed, manufactured, and sold to be microwave switching
devices, obviously because you can not use mechanical relays for internal
switching within a microwave RF circuit. It has not been developed as a 'low
frequency' device until within the last 20 years or so.
Dont quote me on this because I've been out of microwave design for too long. A
regular diode is a piece of silicon (or germanium) that has a junction. One
side of the junction is doped, during mfg process, to have an excess of
electrons (+P) and the other side of the diode is doped to have an excess of
holes (-N), therefore the term PN junction. And of course a PN junction diode
will pass current (a signal) in only one direction. A PIN diode has these
(almost) same characteristics (but each side is doped differently than than a
standard diode), and in addition, between the +P and the -N sections is a third
section that is doped specifically to allow the diode to switch VERY rapidly,
and this region is called the Intrensic junction, thus the name PIN. The speed
that a PIN diode can switch is basically determined by the width of this
Intrensic area, the narrower the I junction, the faster it will switch and
therefore the higher the rf frequency that you can pass throught it.
Unfortunately, to build a PIN diode that will switch at HF freqs (below 30
MHz), the I junction has to be made wider, or the diode will switch too fast to
allow one cycle of an HF signal to pass through it.
What all of this means is that PIN diodes are relatively expensive. A regular
PN junction diode (typically a 1N4148 for instance) may cost 5 cent apiece,
however a 'cheap' true PIN diode will cost between one and three dollars
apiece; and thats why you do not see them in very many amateur radios.
ECONOMICS. (I'm not sure why it took that much verbage to explain, but it did.)
Corsair II's used a regular silicon switching diode, 1N4148 to switch the
filters. The Omni 6 does use PIN diodes, but probably because of the above
mentioned economics, TenTec uses diodes that 'will do the job' verses expensive
PIN diodes.
I read Rhode's article on PIN diodes and decided I could improve the IM
performance of my Omni 6 (it didn't need it!!), so I bought the expensive
Hewlett-Packard PIN diodes that Rhode stated were the best, and installed them
in my Omni 6. Over the past five years using my Omni, chasing DX and
participating in some serious DX contest, I have yet failed to see where these
expensive HP PIN diodes made any substantial improvement.
TenTec runs about 10ma of current through their production PIN diodes, in order
to gain the full IM advantage of the HP PIN diodes, you must run 80ma through
the HP PIN diodes!
So the bottom line is if you replace the PIN or silicon diodes in a rig, you
will see (hear) practically no improvement, UNLESS you redesign the circuitry
to utilize the diodes operating at their optimum design specifications.
Probably if you find the filter switching diodes in your rig are running
'hot' to your touch, it probably means that someone has taken the time to
change the current running through the switching diodes to really improve the
IMD.
At 01:32 PM 9/17/97 -0400, you wrote:
>H. M. 'Puck' Motley W4PM wrote:
>>
>> I have the feeling that the pin diodes in question are a modification
>> suggested in an article by Ulrich Rhode (not sure of the spelling of his
>> name) a few years back concerning 2nd order IMD in modern rigs. One of
>> the rigs mentioned was the Paragon. The article stated that by replacing
>> the common switching diodes used to switch the receiver front end band
>> pass filters with a certain type of pin diode, 2nd order IMD could be
>> improved. Maybe some of our more technically oriented folks remember this
>> article and can comment in greater detail. This is all I remember so if
>> you have additional questions don't ask me!
>>
>Thanks, Puck. I was certain it was something Rohde said, just wasn't
>quite sure when or what the exact reason was. I just spoke to Ten Tec
>about this, and they actually said they had tested different types of
>diodes to switch the Paragon's receiver filters, and settled on regular
>switching diodes because there wasn't much difference with other types.
>So, I guess replacing the receiver filter switching diodes with PIN or
>other (hot carrier, etc.) types is probably a mod that some users have
>done themselves. At least I know for sure it's not a factory
>modification.
>
>Is there anyone out there who knows this for sure? Has anyone done the
>aforementioned mod? I know one fellow recently mentioned in a message
>that a rig he had for sale had the mod. Now I'll go search for the Rohde
>article. 8^)
>
>73, KE3KR
>--
Chester Alderman
<color><param>0000,0000,ffff</param>W4BQF -- Tom
</color><color><param>ffff,0000,0000</param>chestert@pressroom.com
</color>
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