[amsat-bb] Re: Sequencer

[Edward Cole]

At 08:05 AM 3/12/2007, Alan P. Biddle wrote:
> >From the instructions which come with the units:
>
>=====
>Connection of the preamplifier into the system is quite straightforward. The
>coaxial cable from the antenna is connected to the
>preamplifier input terminal (labelled ANT). Preamplifier output (labelled
>XCVR) feeds the coaxial cable that connects to the station
>transceiver or amplifier. Control cable used with the MSPXXXVDG should be
>shielded. Non-shielded cable acts much like an antenna.
>Nearby lightning strikes are capable of inducing sufficient voltage on this
>"antenna" to damage the preamplifier. Shielded cable
>effectively eliminates this problem. Always mount the MSPXXXVDG with the
>connectors facing down to avoid water accumulation
>around the connectors.
>
>Wiring of the power connector is illustrated on the schematic diagram. For
>rf keyed operation only the VDC and GND connections are
>necessary. The preamplifier will sense the presence ofrfand automatically
>switch the preamplifier out of circuit. Hard keying of the
>preamplifier for transmit/receive changeover can also be accomplished. The C
>terrminal of the power connector places the preamplifier
>in transmit when grounded. Also, transmit/receive changeover can be
>controlled with only the +VDC connection. Apply +12VDC for
>receive and remove for transmit. The dropout delay function should be
>defeated for hard keying operation.
>For hard keying operation transmit/receive relay sequencing is mandatory if
>preamplifier failure is to be avoided. A circuit such as the
>ARR TRS04VD will provide sequentially keyed outputs that accurately control
>the timing of all system transmit/receive changeovers.
>Review the TRS04VD instruction sheet for more detailed information.
>=====
>
>I OCRed this, so the formatting is not great.  My understanding is that
>"hard keying" they mean something like QSK where you will beat the relays to
>death.  Certainly CW, and perhaps SSB.  However, there is a "relay delay
>dropout capacitor" which as shipped will protect the unit, and of course
>preclude QSK.  So I suspect the answer to your question is no.  In my
>satellite operations, I have always been tranmitting on a different band
>than receiving, so I just left the pre-amp on the TX side off.  No harm if
>you forget, of course, unless you do it all the time.
>
>Hope this helps,
>
>Alan
>WA4SCA

Alan,

Hard keying is NOT QSK like.  It is like PTT:  You key your radio for 
transmit and it stays in Tx until you un key.  Using the C terminal 
allows a sequencer to switch the unit into Tx-mode without the unit 
needing to sense RF on the coax line.  This is much preferred.  RF 
sensing T/R is more like QSK that you mention (especially when using 
CW/SSB).  The relays in the ARR preamp will chatter back-n-forth with 
each RF peak.  There is a RC time constant to smooth this our 
somewhat, but they cannot hold the relay too long or it will not 
switch fast enough from receive using RF sensing (I've probably 
confused a few of you by now).  The ARR switching preamp apparently 
works just like many brick transmitting amps: you can use auto RF 
sensing or hard keying to control T/R.

Short answer: best method is using hard keying of the C-terminal 
which is controlled by a sequencer.

The bigger question is how you key the sequencer?  You can run the 
PTT line from your microphone to the sequencer.  Then the sequencer 
will have a keying line to your radio's PTT circuit.  For CW it is 
probably easiest to have manual switch T/R controlling the 
sequencer.  If you are running digital you can run a line from the 
computer (usually the software provides this on a RTS line at the 
serial port) to control the sequencer.  I do all three on my station.


73,
Ed - KL7UW
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