[RTTY] RadioShack isolation transformer replacement

[Kok Chen]

To reenforce what Bill said, for quite a while now, K9YC has been advocating the bonding of chassis.  This white paper is as good a place as anything to understand the problems of interfacing to a radio:

http://audiosystemsgroup.com/RFI-Ham.pdf

Jim has posted very often to the Elecraft reflector about this, but for some reason, it keeps falling on deaf ears (as is the Pin-1 problem with commercial interface boxes).

The biggest advantage of what Bill (and Jim) mentioned is the fact that a transformer adds IM distortion.  It is much better, and cheaper to boot, not to even include them in your audio chain.  IMD is not a problem if you use receive filters that only lets through a single signal, but can be detrimental when you open up the passband to multiple signals (for the same reason that you use 24 bit sound cards with wide passbands).  IMD can also be detrimental to modes like PSK31 under single signal condition, but in practice, the transmitters themselves are dirtier than mediocre transformers.

If you really must make do with transformers, K9YC's web site also has white papers about transformers and Jim has many measurements.  

I had already passed along to Don last night a decent $2 transformer (you need 3 transformers  of course for a transceiver that has 2 receive outputs and one AFSK input), together with a slight better but much mode expensive one.

The inexpensive one is the Bourns LM-NP-1001-B1L, which you can pick up from places like Mouser.  I have for example replaced the transformers in my K3 with the Bourns.

If you want to splurge, I feel that the Triad SP-70 is a bit better, but will cost you nearly $50 for a set of three.  You can also get the Triad from places like Mouser and Digikey (heck, 90% of you who build your own stuff probably already have accounts with them :-).  Incidentally, the Triad SP-70 (might be the SP-71) is the transformer that are used in Larry N8LP's LP-PAN.

If memory serves, K9YC has curves for both the Bournes and the Triad.

One other trick that Bill did not mention is the use of balanced audio.  Many halfway decent sound cards come with balanced TRS inputs (the ST-8000 also uses balanced TRS, by the way -- this is not a new thing among hams, perhaps just forgotten).  

You can actually see a picture of this technique used here (with an E-MU sound card) when I measured the IMD from my K3.  

http://www.w7ay.net/site/Technical/K3/Content/imd.html

A short description appears on the second paragraph below the picture.  The actual audio cable was some junk twisted pair CAT-5 UTP that I had hanging around which I soldered to the 1/4" TRS plug.

Speaking of which, K9YC also advocates the use of twisted pair cables for audio connections.  If you have time, go visit Jim's web site. You can have a fun afternoon with various topics, from RFI to Audio to "Pin 1."

73
Chen, W7AY