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Old 13th Feb 2021, 11:23 pm   #3
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: PIN diode band switching yaesu ft-767

A PIN diode is a deliberately spoiled diode with a layer of undoped silicon in between the P type and N type doped parts. They are slow. Very slow.

At low frequencies, they act like a diode, though not a terribly good one. At higher frequencies they look like a resistor. What makes them valuable is that the resistance value can be controlled by varying the DC forwards current through the diode.

Some are optimised for use as RF variable resistors. PIN diodes have a parameter 'Carrier lifetime' which essentially says how long it keeps on conducting after you take the forwards voltage away - Yes! PIN diodes do carry current in the reverse direction, for a little while. The carrier lifetime is a marker sor there the bounadry between diode and variable resistor action can be found. It isn't a hard boundary. For use on the HF bands, you need a diode with a carrier lifetime of at least a few microseconds for it to be reasonably linear. Once you are into the variable resistance frequency band, DC forwards current will program the effective RF resistance for you. You can get values from 0.1 Ohm to kilo Ohms. The more DC bias current, the lower the resistance.

Other PINs are optimised as switches A typical small signal one will look like an Ohm with 20mA forward current and you'll get a couple of nH stray from the package. Reverse bias it with 20v and it'll look like a couple of pF.

They have no moving parts, don't wear out and are quick in operation.

In that receiver, you could see if they are the cause of your deafness by lifting one end of each diode for each filter you don't want in, and fitting a short in place of each diode for the signal path of the filter you do want in.

You will see DC bias current paths switched by transistors or switch contacts supplying DC current to the diodes through resistors and sometimes chokes. At the same time you will see the RF paths connected in and out via DC blocking capacitors.

Tedious, but it gives a definite result.

But you don't just want to make sure the wanted filter is in, you need to be sure the unwanted ones aren't shunting it.

David
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