Jim,
You've asked a very reasonable question. In many cases, when one is certain
RF will not arrive at the input of an amplifier until the amplifier is ready
to receive it, then relay sequencing is generally not necessary. Even in
that situation, however, I think it is a good idea to make certain the
ampifier has a proper load before it is biased on, since one could
potentially have an instability or parasitic in some situations if the amp
faces a momentary open circuit before its output relay closes. Relay
sequencing eliminates that risk, however remote.
More generally, however, good design practice suggests that an amplifier
should be protected no matter what transceiver is connected to its input.
One shouldn't have to depend on the operator always choosing a correct menu
setting on a transceiver, or never transmitting accidentally at the wrong
time, or using a transceiver ill-suited to the amplifier's timing circuits.
Also, the point of QSK circuitry is to speed up the amplifier's turnaround
time, not slow it down. Adding a delay (with a menu setting) in a
transceiver may prevent hot-switching, but it does so at the expense of
making the amplifier response more sluggish..
A single "dit" at 40 wpm is about 50 mSec long, so a reasonable objective
for QSK is to make the turnaround about 10% that length, or 5 mSec.
Obviously, a transmit delay of 10 or 15 mSec is too pokey to meet that
requirement, as are most open frame T/R relays To speed up the T/R
turnaround not only requires fast switching in the amplifer relay circuits,
but also optimizing the coordination between the transceiver and the
amplifier. Basically, one needs the shortest possible delay time (between a
key closing and the arrival of RF at the amplifier input), consistent with
the intrinsic turnaround time of the amplifer relays. If the entire
turnaround time is to be completed within 4 or 5 mSec, then very careful
attention to relay timimg, contact bounce, etc., is necessary. because one
is pushing the limits of what the equipment is capable of. When operating in
that regime, my view is that relay sequencing is a desirable feature.
My last point is that building proper relay sequencing into a QSK circuit is
easy and inexpensive, so why not do it? The component cost (not counting the
vacuum relays) is only about $10 for my preferred circuit, which is pretty
cheap protection!
73,
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim W7RY
> Sent: Sunday, November 10, 2013 10:50 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Best source for Gigavac GH-1 ?
>
> Jim
> Can you tell me why sequencing is a requirement? I have found that the
> RF never flows (6 ms after the tranciever control circuts energise)until
> way after the relays and cathode bias has switched which as you have
> pointed out is 3-4 ms.
>
> Thanks
> 73
> Jim W7RY
>
>
> >> In researching the subject during the past year, I've looked at the
> >> relay
> >> circuits of a number of commercial amplifiers. Most seem to treat T/R
> >> switching pretty cavaliearly, without adequate protection for the
> >> relays. In
> >> my own design (the circuit mounts on a 3"x5" PCB), I've tried to
> >> prevent any
> >> situation that can damage the contacts. This involves several
> >> precautions:
> >> (1) sequencing the amp so the output relay, bias switch, and input
relay
> >> operate sequentially in that order, after allowing for the relay
closing
> >> times and contact bounce. (2) Having an RF "lockout" sensor that
> >> prevents
> >> the amplifier switching sequence from beginning if RF is present on the
> >> input line. (3) Having an RF "lockon" sensor that keeps the relays
> >> closed so
> >> long as RF is present. My new amplifier completes its T/R switching
> >> in 4 mS,
> >> which is suitable for QSK speeds up to about 60 wpm, far beyond it's
> >> builder's capabilities.
> >> 73,
> >> Jim W8ZR
> >>
> >>
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