Kg3v Ham Radio Blog

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IC-7300 "SEND" signal - Be Careful when using it

Posted: 07 Feb 2021 05:22 PM PST
https://kg3v.com/2021/02/07/ic-7300-...when-using-it/



There has been an ongoing discussion for quite some time about a reported
RF Tail on the IC-7300. This report is based upon the fact that the 7300
provides a SEND signal that is supposed to be useful for telling offboard
equipment if the 7300 is currently Transmitting or Receiving. Actually
there are two of these signals, one is connected through the ACC port DIN
connector. The other can be found on a rear-panel phono (or RCA) jack.




It has been shown that these signals do not always accurately indicate if
the 7300 is currently Transmitting RF, as you might think it should. You
can search and find all the detailed data, but the big concern most people
have been discussing is the RF Tail, at the end of a transmission, with the
CW case being used as an example. In this case, the user stops sending a
character and the SEND line returns to the Receive State, while the RF
decays and eventually ends, roughly 2-3 milliseconds later. This is
certainly not what one would expect the SEND signal to do, either
intuitively or from the Manual. One test also shows that the timing is
different between the two SEND signals.




I have been following this discussion which has been off-and-on over many
months. In the past two weeks, it hit a fever pitch, with all kinds of
comments about it, test data being posted, and even some reaching a final
conclusion that it is NOT an issue. I think we may want to slow down a bit,
before claiming that it is not an issue. To me, determining if this is an
issue depends upon several things:



Is there a possibility of damage to the 7300 or interconnected equipment
due to this signal NOT doing what is advertised and expected?Is there a
weakness in the design?Is there a problem with documentation?




It appears to me that the nature of the most recent discussion was to
dismiss this as a non-issue, and be done with it. After much discussion
and many reports from users, it was proclaimed that not a single Amplifier
was damaged and this was the basis of some saying it is thus a non-issue. I
disagree, with the following thoughts on each of the above 3 potential
areas of concern.



Could damage to the 7300 or connected equipment result? It is certainly
possible, depending upon the interconnected equipment and switching times.
In the public discussion, it was pointed out that relays used in Amplifiers
are typically slow and they will likely take longer than this RF Tail to
switch from Transmitting to Receiving paths. That may be likely, but I am
not used to seeing any manufacturer specify the minimum contact closing
time. The claims I have heard are based on a single measured closing time,
or general expectations. Also, someone designing an outboard TX/RX switch
today might very well be using PIN diodes specifically to get fast
switching times.Is there a weakness in the Design? YES. Surely the
Designers did not intend to provide a SEND signal that transitions to the
Transmit State 2-3 mS prior to the end of RF energy Transmission.
Experiments have shown that other ICOM rigs with similar interfaces (I
think the 7600 and 7610 were used as examples) do not have this feature Is
there a problem with Documentation Certainly the User Manual does not do a
good job of explaining the timing of these signals. Vague terms are used
like for use with non-Icom Amplifiers and the pin goes low when the
transceiver Transmits. I have not seen the Service manual, so I can not
comment on that, but most Users probably do not have the Service Manual. As
a System Engineer, would I be satisfied with the documentation I have seen,
in designing an Amplifier interface using these signals? NO




In summary the IC-7300 clearly has an anomaly which results in the SEND
signal sometimes failing to correctly indicate the current TX/RX State, at
least as far as the presence of RF energy on the Antenna connector is
concerned. This seems like something that should be corrected to me. At the
least, it should be clearly documented in the User Manual, with some
worst-case timing diagrams.




I dont think relying on Amplifier relays being slow is a great solution to
this issue. I have heard that some HF Amplifiers have a setting that can be
used to accommodate these types of timing issues. I have used my Elecraft
KPA-500 Amplifier with my 7300 a few times, with apparently no problem. I
will surely look at any KEY-timing related settings before I do that again.