[TenTec] First dit shortening

[Ken Brown]

Hi Don,

I was not referring to a specific rig. Although my comment was sort of a 
response to some of the other discussion about the Orion. I have a 
Ten-Tec Omni VI. I have never looked at it's keying envelope with an 
o'scope. So I hope it is really good. From the perspective of the 
operator of this radio the QSK is fantastic. I hope that this is not at 
the expense of funny sounding keying to the other operators I QSO with. 
I have never heard any such comment about the Omni VI, so I'm not concerned.

I have seen first dit shortening in a lot of QST rig reviews. My reason 
for posting the comment was to try to gain some more understanding.

Here is my understanding of what has to happen at the beginning of key 
down on a rig:

The receiver has to be muted,

The local oscillator has to get changed to the right frequency for 
transmit. There may also be a BFO frequency that needs to get changed. 
If these oscillators are digitally controlled, then that means a new 
divide by N number has to get loaded into the PLL, and then the PLL has 
to settle at the new frequency. This process may require differing 
amounts of time depending on whether you are operating a large TX/RX 
frequency split, or even cross band. It would also be possible to switch 
between different oscillators, instead of shifting one oscillator. I 
don't think any modern rigs do that though.

The T/R antenna switching has to change. This could be pin diodes or relay.

Then the transmit carrier can be started, perhaps by putting a DC bias 
on a balanced modulator, unbalancing it and just letting carrier 
through. Somewhere there should be a stage that has a controlled gain, 
that can control the shape of the leading edge of the keyed envelope, so 
we don't have keyclicks.

Now we are finally transmitting. All of the above steps require a 
certain amount of time. If the last step starts before the other steps 
are finished you can get chirp in your TX, a blown up receiver front 
end, permanent hearing loss, or other undesirable and damaging effects, 
depending upon which steps have not been completed.

Now when we let the key up, pretty much all the same things mentioned 
above have to revert back to their receive states. Switching everything 
back to the receive mode may take a different (less?) amount of time. 
 Since there were X milliseconds delay between key down and  beginning 
of carrier output, it only makes sense at this point to add an 
appropriate Y milliseconds of delay time to make an equal delay of X 
milliseconds between key up and end of carrier output. In this way this 
dits and dahs transmitted are just as long as the key down times, 
however they begin and end  X milliseconds after key down and key up. 
 If X is a pretty big portion of the between element time, then the time 
you can actually receive between elements could be very small. So it is 
good to design the radio so that all the processes that take time and 
add up to X are as fast as possible.
 Some designs may fudge a little, shortening  the added Y delay  at key 
up, and therefore shortening the actual carrier on time, so that there 
is more receive time between dits.

So my question is, if all this has to happen on the first dit and the 
second dit and the third dit and on and on, why is the first dit any 
different? I am sure I have missed a few subtleties (like how to spell 
subtle) that others will comment on.

Thanks,

Ken N6KB


VE1BN at aol.com wrote:

>Ken -
>
>
>What rig are you referring to?  I use Ten-Tec rigs, and detect 
>no first dit shortening on any of them,  Paragon, Paragon II, or 
>OMNI 6+.  I use a 120 mHz scope and an RF pickoff to observe 
>the waveforms.   
>
>If you are talking about IC rigs, then yes I agree.  Have no idea 
>why the JA manufacturers can't correct the problem.  I know 
>my IC706MK2G has a shortened first dit, noticable on full break-in 
>only.  
>
>Just curious, Ken.  Comments?
>
>73, and a very good 2004 coming up for you and yours,
>
>Don,   VE1BN at aol.com
>
>  
>