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ZL2BSJ > TECH 24.08.05 13:26l 179 Lines 5494 Bytes #999 (0) @ WW
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Subj: Sequencer - circuit discription
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What follows is the description for a home-brew sequencer.
The circuit diagram is to follow in 3 parts.
Wilbert, ZL2BSJ
--
Introduction
------------
In ZL, the legal power limit was recently raised to 500 W PEP and CW
carrier. As a result, RF power amplifiers have become more popular.
The following circuit may be of interest to people who use outboard
amplifiers or masthead preamps.
The circuit is a gizmo called a 'sequencer'. It controls the rig and
the amplifier.
The schematic is a Postscript image, zipped up and 7-plussed. You'll
need a Postscript viewer (e.g. Ghostscript) to view and print it.
Purpose
-------
The purpose of the sequencer is to avoid hot-switching of amplifier
and rig relays. In switching rig and amplifier from receive to
transmit - and vice versa - the sequencer applies small pre-set time
delays. This ensures cold-switching.
Hot-switching is highly undesirable: it welds relay contacts, may
cause transient oscillations, arcing and QRM, and it can stress or
even destroy amplifier and rig finals.
Some rig/amplifier combinations may not need an outboard sequencer -
see below.
Limitations
-----------
The circuit is experimental. It looks complicated but uses cheap,
easily obtained off-the-shelf CMOS logic and can be built on
vero-board in a die-cast box.
The sequencer caters for semi-QSK in CW and digi modes only and
requires a key signal.
Description
-----------
Input signal: /KEY
Outputs: /AMP PTT
/RIG PTT
/Rig CWKEY
All these signals are active-low.
The '/KEY' signal is taken from the outboard electronic keyer or from
the up-and-down morse key. The output signals go as their names
indicate. Do not use the rig VOX: the sequencer has a VOX timer.
How the sequencer works:
------------------------
Receive to transmit:
--------------------
When you push the morse key down, the sequencer puts the amplifier
in-line immediately with the '/AMP PTT' signal. It also triggers a
'rig turn-on delay' timer. This timer holds off the '/RIG PTT' and
'/RIG CWKEY' signals until the amplifier relays have settled.
Once the amplifier relays have bounced out, the '/KEY' signal passes
through to the rig as RIG CWKEY, and the rig is turned on with the
'/RIG PTT' (assuming the rig VOX is disabled).
Transmit to receive:
--------------------
When you let go of the CW key, the '/RIG CWKEY' signal goes
inactive. Also, the VOX timer inside the sequencer is triggered. The
VOX will hold the state of '/AMP PTT' and '/RIG PTT' until it
expires. This interval is called the 'VOX hang time'.
When the VOX expires, the rig is switched to receive as '/RIG PTT'
goes inactive. At this moment, another timer is triggered, called the
'amp turn-off delay' timer. This timer delays the switching off-line
of the amplifier until the rig has settled on receive.
Upon expiry of the amp turn-off delay' timer, with the rig settled on
receive, the '/AMP PTT' signal goes inactive, taking the amplifier
off-line.
Adjustments
-----------
1. VOX hang time:
-----------------
The VOX hang time, rig turn-on delay and amp turn-off delay can all be
set independently. The VOX hang time is a personal preference. You
should make this easily adjustable if your gear is to be used for
multi-op contesting.
2. rig turn-on delay:
---------------------
You should use an digital oscilloscope to measure (or verify) how long
it takes for your amplifier to fully switch in-line from the time of
'/AMP PTT'. This can be tricky to measure. Sometimes it can be done by
using no RF drive and feeding current-limited DC voltages into the
amplifier in- and output sockets while observing DC levels across a
dummy load at the output.
Add a safety margin to the measured worst-case time and set the 'rig
turn-on delay' timer accordingly.
3. amp turn-off delay:
----------------------
Similarly, use a scope to measure or verify how long it takes for the
rig to drop RF from the moment '/RIG PTT' and '/RIG CWKEY' go
inactive. Use this time plus a generous safety margin to set the 'amp
turn-off delay' timer.
Closing comments
----------------
Some fast 'full-QSK' rated amplifiers with vacuum relays can switch faster
that the rig is able to produce RF.
Certain radios can be programmed to delay delivery of RF. This avoids
hot switching at the start of the transmission.
In addition, the rig's VOX should be used to ensure there is no RF at
the tail end of the transmission when the rig goes back to
receive. When this is done, no sequencer is needed.
Other rig/amplifier combinations use handshaking to ensure hot-switch
free operation.
The FT-1000-MP has been observed to intermittently truncate the first
dot/dash when '/RIG-PTT' and '/RIG-CWKEY' are applied simultanously
AND the rig's VOX is off. This is an annoying problem. Adding a slight
one-shot delay to '/RIG-CWKEY' is suggested as a work-around.
The sequencer has been used in several contests and has performed
well. Avoid ingress of RF into the circuit by RF-decoupling all leads
(in particular the power supply). Avoid the presence of RF on the
chassis of your rig and amplifier. RFI guarantees hot-switching and
may cause latch-up.
Disclaimer
----------
Although I have used the circuit extensively, I do not claim that it
will work correctly in all cases. Use this circuit entirely at your
own risk.
Wilbert, ZL2BSJ
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