[TenTec] Slaving radios

[Dr. Gerald N. Johnson]

On Sun, 2007-04-22 at 09:22 -0700, Jim Brown K9YC wrote:
> On Fri, 20 Apr 2007 10:36:31 -0400, Carl Moreschi wrote:
> 
> >I think the limit on a serial cable is about 50 feet.  

The rules, RS-232 say 50'. Those rules also limit the drive current to
more limit the distance to that 50'. I had a commercial modem once that
for RF filtering had a .01 across the signal lines. An RS-232 card
(Apple II+ as I recall) that really did limit the drive current would
not work with that modem at 1200 baud only worked at 300 baud because of
that capacitor.
> 
> Not in the real world. The REAL limits on serial cable are 1) 
> capacitance and 2) noise pickup.

Most PC serial ports use a variation on the LM1488 as a driver and it
will put out a great deal more current at transitions than RS-232 rules
allow and so will drive much larger capacitance loads.

> If you use low capacitance cable, 
> and if the cable has good noise rejection, you can go hundreds of 
> feet at decent data speeds. Plain ordinary CAT5/6/7 cable satisfies 
> these requirements quite well, and is an excellent serial cable if 
> you use it properly. 
> 
> See Chapter 8 of the RFI Tutorial that is on my website for detailed 
> advice on this issue.
> 
> http://audiosystemsgroup.com/RFI-Ham.pdf
> 
> BTW -- another excellent reason for slaving radios is diversity 
> reception. You can play a lot more tricks with signal processing to 
> pull signals out of the noise if the radios are on precisely the 
> same frequency with their oscillators in phase and their audio in 
> polarity. W8JI has talked about this. 
> 
> 73,
> 
> Jim Brown K9YC
> 
73, Jerry, K0CQ