This issue might best include VOX and full duplex in a round table on
HEO.
Joe K0VTY
====================
On 12 Mar 2007 14:18:32 -0500 tosca005@umn.edu writes:
On Mar 12 2007, Edward Cole wrote:
At 05:15 AM 3/12/2007, Stargate wrote:
Is a sequencer needed when using a 160w switchable preamp from
ARR?
Let's analyze this:
You send a voltage to run the preamp. It needs voltage to switch
to
Tx mode. When RF from your transmitter arrives it switches over to
bypass the preamp. I would guess if you turn off the power to the
unit, it can't operate correctly, so how is a sequencer going to
help? I have not owned one of these units so not familiar with
their
power wiring. If you have two 12 volt power contacts then you
might
run the preamp separately from a sequencer.
Sequencers don't have to only apply/remove DC power in a set
sequence. In
fact, in the general case, a sequencer has a series of switch
contacts that
are enabled in the proper order and disabled in the opposite order.
The
switch contacts could be configured as NO (normally open) or NC
(normally
closed). These can either provide and remove DC power, or short the
pre-amp's own PTT circuit to ground to take it out of receive mode
(or,
depending on the model of preamp, maybe +12 is PTT instead of GND).
The
Down East Microwave sequencer has a DPDT relay for each of the
sequence
stages, so you get two independent NO and NC contact sets to play
with as
you wish.
If (like the ARR models) the preamp has built-in bypass switching,
one
issue that remains is how quickly and completely the preamp gets
switched
out of the transmit path. If slow to switch, and/or if the isolation
is
insufficient, a spike of RF may still enter the very sensitive
preamp and
fry it. With a sequencer, you can (usually) select whatever
switching delay
is needed for the isolation relays to reach a fully switched
position, and
you also get to pick the isolation relays themselves, with attention
to
power dissipation and degree of isolation needed. I believe that ARR
says
that the built-in RF-sensing relay is good for up to 160 watts, but
a
sequencer with a separate isolation relay could basically allow you
to use
1.5 kW, if you had an awfully darn good reason to use so much power,
;)
that is.
On my 144, 222, and 432 bands, I have an RF Concepts "brick"
amplifier with
an internal preamp and RF-sensed automatic switching. On 144 and
432, these
are "soft keyed" (i.e. keyed by sensing RF). On 222, which is done
with a
transverter instead of directly out the back of my FT-847, I went to
the
effort to "hard key" the amp/preamp by using its PTT input (which I
had to
re-configure as PTT to GND instead of PTT to HI). Both "soft" and
"hard"
switching methods work OK, but on 144 and 432 it is best to briefly
pause
after keying the mic before you start talking, to insure that the PA
is
switched in, and when you unkey, you have to hope that the person on
the
other end pauses briefly before responding because it takes a
noticeable
amount of time for the amp/preamp relay to switch back to receive
mode.
(The delay is intentional, to prevent relay chattering.) On 222,
since the
radio, transverter, and external PA/preamp are all hard-switched by
their
own PTT signal, the send/receive cycle is much smoother, no excess
delays,
no chance of relay chattering if the loudness of your voice drops
low for a
moment, etc. So I definitely prefer the "hard keyed" method of
operation.
One of these days, I will get around to modifying the 144 and 432
amps for
hard keying also...
On 902, 1296, 2305, and 3456 MHz, my transverters were built with
split
Tx/Rx, which makes it simpler to connect a preamp (if needed) to the
Rx and
a power amp to the Tx, and of course you need a T/R coax relay
external to
the transverter. So far, because my PA's on these bands are low
power, I
hard-key the T/R relay but it is not sequenced -- radio PTT =
transverter
PTT = T/R relay PTT, all at the "same" time. When I move up to the
high-power class of operation, I will install a sequencer on each.
We have
local folks here who have had the experience I am trying to avoid,
namely
simultaneous switching works ok at low power, but add in some REAL
output
power and things fry if not sequenced. These bands have PA's of
40-120
watts waiting to be integrated. (This is for terrestrial work, not
satellites, by the way, as you can probably figure from the high
powers and
non-satellite bands/sub-bands mentioned...)
73 de W0JT
AMSAT-NA LM#2292
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joseph Murray