[amsat-bb] Re: Sequencer

[joseph Murray]

This issue might best include VOX and full duplex in a round table on
HEO.

Just in case
 
Joe  K0VTY
====================
On 12 Mar 2007 14:18:32 -0500 tosca005 at 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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