[RTTY] afsk to fsk

[Ian White G3SEK]

ws7i wrote:
>
>Well whoever sez that an optoisolater won't work with a FT-1000 is 
>wrong. Works just fine. I don't have a pro3 but they will work fine 
>with any rig.
>
>Its just a matter of getting the proper amount of resister value.  In 
>fact I have done two FT-1000d's and what is odd is that they each took 
>a different value of resister.
>
It is much more important to use the right optoisolator. The most common 
kinds are not suitable for these keying applications, which involve very 
low currents.

All the most common kinds of opto are specified at an input current of 
10mA, but at low currents the current transfer ratio (CTR) will be much 
lower than specified... and also very uncertain, and variable from batch 
to batch. If the resistor value is critical, it is a sure sign that the 
performance of the opto is marginal.

For these applications, we need a sensitive type with a CTR greater than 
100%, and most importantly the CTR must be specified at a low current 
(1mA typically).  These are easy to find in component catalogs, but 
you're unlikely to have one in the junk-box.

With the right kind of opto, the resistor value should not be critical 
-anywhere around the typical 2.2K should be fine.

Pete: for FSK with the FT-1000 I have used both the MCT5211 (obtainable 
from Farnell in the UK) and the SFH618-A4 (Rapid Electronics). Either 
one, with its 2.2K resistor, will fit inside a 9-way D connector shell.

An archive search for "opto" should pull out a lot more information from 
this list, a few months back.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek