[amsat-bb] Re: Sequencer

[tosca005@umn.edu]

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