[Elecraft] Transmit Inhibit

[Clay Autery]

OK....  I AM making progress.

What I have learned thusfar...  (Thanks for the patience of Don, WFPR 
and others.)

... and correct me if I am wrong, PLEASE.

1)  Pin #7 is TTL logic...  And logic pins need to NOT be left 
floating.  They need to see discrete logic high (+ 5vdc) or low (0 vdc) 
levels at all times. (Playing with the TX-INH settings on the K3s with 
nothing connected, I have seen what happens when the pin is left 
floating... the MCU doesn't know what it's going to get...  I've 
successfully gotten it to inhibit AND enable on both settings.)

2) The pull-up resistor basically acts sort of like a current limiter on 
the input pin and a load when the pin is pulled to ground.

3) Lower pull-up values improve switching speed.  Smaller is faster, but 
too small and you get too much current and power consumption/heating 
become issues.  Too high, switching too slow and voltage level may not 
be high enough to clearly be taken as a "high" logic state.

4) The "switch" needs to be between Pin #7 and +5VDC....  and Pin #7

So now, it's just a matter of  putting  together a switching circuit 
that feeds Pin #7 with +5VDC when the SPE TX-INH signal is 12 vdc, and 
when the +12 vdc is removed, Pin #7 sees 0 vdc.  Seems simple, but...

Here is a link to a draft circuit I've drawn up based on a suggestion as 
to a particular MOSFET to use as a switching device:

http://www.montac.com/images/circuits/TX-INH_circuit.jpg

Notes/Questions:

Q1 = 2N7000G Small Signal MOSFET (N-channel)

R1 = Pull-up resistor, as per manual (2.2-10 kOhms)

?? R2 = Pull-down resistor, value unknown.  Needed?  I THINK it might be 
needed in view of the "requirement" for R3 below, and to potentially 
avoid using D1 below.

R3 = Rgs resistor, multiple references to the gate to source as a 
non-polarized capacitor and the need for a bleeder resistor to ground to 
aid in reliability of switch turn-off.  Value undetermined as yet.  My 
gut tells me R3 < R2, but I can't understand why I think that yet, exactly.

?? D1 = Protection for Pin #7 from 12 Vdc via bleeder resistor path.  
Yes, I realize I put it in the circuit the wrong way... <smile>  I 
really don't want to give up the forward voltage drop, but I can reduce 
the drop with a Schottkey, et al.  Reckon this extra protection is 
actually necessary enough to trade the 0.3+ volts or so?

+5 VDC....  Don't know where I'm going to get this yet....  Don't really 
want to steal the FSK pin...  maybe the amp has a line 5 volt line I can 
steal.

This device is getting bigger by the minute... the idea is to shrink it 
into the cable right behind the connector housing... IDEALLY, it would 
be better INSIDE the connector shell, but that's not going to happen.  
May have to demo the circuit and then recreate it in SMD.


73,

______________________
Clay Autery, KY5G
(318) 518-1389

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