to select a solid-state HF amplifier ? (II)
ALC circuit (ALC standing for Automatic Level Control) is similar to
the AGC. For short, it maintains maximum "talk power" in
limiting drawbacks like distorsions and reduces drive when
"out-of-limit" condition is detected.
ALC usually consists of three parts : an amplifier, a rectifier, and
a controlled impedance. In this circuit the functions of the
amplifier and rectifier are performed by a single operational amplifier.
This makes the system simple and cheap. The rectifier is made with
the output push-pull cascade of the op amplifier and other
components, while a transistor and a diode are used as a
and internal view of the Tokyo
Hy-Power HL-1KFX amplifier. It develops 600W PEP output in SSB for
1.2 kW DC input.
an amplifier, the ALC output sends the DC control
voltage to the IF stages of the exciter which is thus gain-controlled. This voltage is
proportional to the amplifier power output but negative-going
: that means that the higher the voltage, the lower the
exciter output. The reason is next.
usual conditions, the signal (forward-power) is sent by the reflectometer
to the controller that derives the ALC voltage. As soon as a
anomalous conditions is met (e.g. the reflectometer reports a load
mistmach), the controller develops a positive ALC voltage to reduce the drive to a safe level.
At this instant the exciter reduces its RF output power.
we explained in the page dealing with amplification
classes, a good linearity means also a low IMD. But if the power
rating is exceeded, what might occur in RTTY or CW mode, 3rd-order
IMD degrades rapidly with for consequence to overdrive your RF power
devices. To avoid this overdrive, you have to set the exciter’s RF
output control at 100% and manually adjust the
ALC to reduce the ouput to the nominal value (the rated level).
such an adjustment can still be "forgotten" on a tube
amplifier, the ALC control is mandatory when
using a solid-state amplifier. The ALC circuit constitutes the main
amplifier's protection against overdrive. Remind you that in setting
not properly the ALC, you risk to damage the RF power devices, and
these MOSFETs are among the most expensive components of your
to prevent overdrive ? Depending on the controls available of your
amplifier, usually the ALC control has to be adjusted as indicated
in the manufacturer's instructions manual. Then, to ensure the maximum
security, the exciter's drive power control should be set just
above the point where the amplifier delivers nominal power output
(e.g. 60W PEP), rather than at maximum (100W PEP). In this position
you will prevent overdrive even in the unlikely event of an ALC failure.
in the RF compartment, the low-pass filters (LPF) box is designed to suppress harmonics and spurious
emissions to a level of about –46 dBc or lower.
box contains a bank of bandswitched low-pass LC and crystals filters. These filters are usually Chebyshev
networks. This approximation (the blue curve at right)
produces ripple within the passband and an improved roll-off
rate compared to the Butterworth response (red curve).
obtained by this approximation are the most easily realized. They offer
a high attenuation and a steep roll-off near the cutoff frequency.
all amplifiers we find one filter per frequency band, and the filters
are switched in and out of the signal path using fast
sealed relays. As these circuits generate RF losses, the cooling
system passes through the LPF box too. The filter insertion loss is typically 0.5
to 1 dB in the passband.
high-ends solid-state HF amplifiers like Icom IC-PW1 or Yaesu
VL-1000 Quadra are fitted with an internal automatic antenna tuner
(auto-tuner for short). Like in any transceiver or amplifier, this
circuit is made of a T-network, variable capacitors forming the arms
of a " P"
and two parallel inductors forming the top (also designed as a
"T" network, hence its name). Its role is to attenuate or
suppress harmonic distortions to present a 50-ohm load to the
exciter, the tuning offering the advantage to get the lowest SWR and
loss as possible.
Close-up on the ultra high-end
Icom IC-PW1 amplifier main unit and control head.
Inserted in a cube of about 38 cm wide, this is the Rolls of HF amplifier, a true
"full automatic" blackbox : no knob, no tune to adjust but LEDs and push buttons.
It includes an auto band-changing, auto antenna tuner, auto AC input voltage selector, and CPU memories for
automatic antenna selection and band switching. Powered on 220V AC its SSB power ouput is 1 kW PEP (500 W PEP
output on 110V AC). By the way its price is... $4700. All amplifiers fitted
with auto-tuner are very expensive.
A reflectometer at the tuner input informs
the controller when an optimum match condition is met; this happens when reflected
power is minimal and the input/output phase shift is exactly 180º. This effect stops
the capacitor drive motors.
is able to match load impedances in the range
what represents a load VSWR < 3:1. This is a quite high threshold
because most transceivers refuse to match the load when VSWR >
1.5 because they impose that insertion loss does not exceed 0.5
dB when matched to VSWR < 1.5:1 at the tuner input.
Of course an
auto-tuner is not intended to match
non-resonant antennas but if there is no manual intervention from
the operator to reduce the VSWR (imagine that you try to emit on a
non-resonant band), some auto-tuners try to match the system in
reducing automatically by half the output power to allow the user to
work his QSO
In worst cases an external tuner
is requested behind the amplifier output to match the load. We will
discuss about this option later.
prevent high RF voltages and feedback into the auto-tuner and/or
low-pass filters, the auto-tuner has to be disengaged when
using an external antenna tuner. The problem is that cascading tuners
can reflect high reactance values back into the system, what might generate
high RF voltages across capacitors in T-networks,
leading to the destruction of components.
Most auto-tuners store and hold
in memory settings previously used during a tuning cycle. The best are able to track frequency
and load-impedance changes dynamically in less than 10 seconds.
last, after the power transistors, the internal antenna tuner of a
kW-class amplifier is the second module that requires a cooling, losses in the inductors and fixed
capacitors generating heat (about 125W for 0.5 dB insertion loss).
To recycle this heat usually a small fan is installed in the auto-tuner
compartment. Some designers however prefer to divert a part of the amplifier
cooling air through the tuner area.
but still too few solid-state amplifiers are today equipped with an
automatic band-switcher. Usually this action is handled by the controller board
that counts the excitation frequency and determines the correct band. Others
use the exciter that supplies coded band information to the controller. Once
this last known the working band, it selects the correct low-pass filters set suited for that band.
External interfaces include the power input, RF input and a special
one called "Band data" input. The function of this latter
is to set the correct frequency range. This input enables the exciter to send band-selection
information to the amplifier controller. In some proprietary
designs, a reverse line allows the amplifier to key the exciter to request a
carrier to set the frequency. This feature preserves automatic bandswitching
when the amplifier and exciter are of different makes.
be surprised to find in solid-state amplifiers a dual RF input, keying and ALC
interfaces (usually brought out to RCA jacks). These parallel
circuits allow the operator to switch between two exciters and separate ALC adjustments
for each input. In some amplifiers up to four selectable RF outputs
are provided (for four antennas). Sometimes programmable, they also permit automatic antenna selection by
frequency range or band.
PA stage performances of a solid-state amplifier are closely
monitored by a dozen of sensors that measure a variety of
operating parameters. These data are then forwarded to the system
controller board. These parameters include
DC supply voltage and DC PA input current (total and per-module), RF
drive power, RF power-device case temperature, per-module output
power and total forward and reflected power (at the combiner
output, the LPF output and the auto-tuner output), without to forget
that the controller drives metering functions too. At a minimum,
these should include DC supply voltage, DC PA input current, RF
power output, load SWR, and ALC level.
other devices send forward and reflected power signals to the controller : reflectometers, the LPF
output and the automatic antenna tuner input and output. These
signals control auto-tuner settings. They also drive power-output,
and SWR metering and monitoring functions.
practice, if the load SWR exceeds the matching range of the auto-tuner (say VSWR > 3:1) the
reflectometer located between the PA combiner output and the LPF input reports high reflected power
and signals the controller to reduce or to lock the amplifier output. This feature
protects the antenna tuner and all the amplifier against possible damage due to
a failure of the antenna system.
a good amplifier the security system should detect at minimum the following
anomalous conditions : over-current, over-voltage, over-drive, over-temperature, insufficient
power gain, power gain imbalance between PA modules, wrong band (exciter and amplifier
not set to the same band), excess forward power, excess reflected power, auto-tuner
out of range (load SWR > 3:1).
are two stages of protective action and, depending designs, there
are several cascading actions : either there is an automatic drive fold-back via
the ALC line when for example SWR > 1.5:1, followed by amplifier shutdown or
lockout at SWR > 3:1, or the amplifier protection reduces itself
the ouput power by half and try again to match with the load. If the
SWR stays over 3:1 it locks. But in some simpler amplifiers there is
only a "brute-force" mechanism; a drive-limiter
circuit fitted in the RF input signal path absorbs initial
RF power spikes generated by exciters. However the use of a smart
interface based on sensors is by far preferable.
sections of an amplifer, from the PA stage to the PSU should be totally insulated and enclosed in individual
shielded compartment. Power lines and control leads should enter these shielded enclosures via
feed-through capacitors, while everywhere lead dress should be used.
the RF compartment, the most sensitive to RFI, all interconnect wiring should be made of coaxial or
shielded wire. "RF" and "non-RF" wires
have also to be separate and never be found side by side in the same
area. "RF" wiring for example should be isolated in its
unit away from the other wires to avoid side effects due to RFI. RF
interconnecting cables should have their braiding grounded at shield
protecting measures will maintain RF integrity, minimise internal RF
feedback, and ensure regulatory electromagnetic compatibility (EMC)
is naturally attracted by pleasant and good looking things, leaving ugly ones on the side.
However a heavy and cumbersome amplifier, showing thick
printed-circuit boards and large solder-plate tracks inserted into a
robust case is more than recommended when one work with high powers.
So prior to purchase an amplifier, ask to open the cabinet and look
well-known solid-state HF amplifiers
powered on 28V DC and 13.8V DC (portable).
quality solid-state amplifier should use printed-circuit boards
made of FR4/G10 fibre-glass, the sole top-quality boards able to
carry with the high RF and DC currents encountered in the PA stage. PA board
stock should be at least 3 mm thick and solder-plated tracks of sufficient
width and thickness to carry high powers.
Except for on-board
inductors, always select low-inductance layouts. At last, as in any
electronic circuit, lengths of RF component lead should be as short as
practicable. If longer wiring is required, a wire loom is appreciated.
Power supply unit
last but not the least module of your amplifier is the power supply unit
(PSU). If most are external to reduce the weight and bulk of the
amplifier, the PSU may be internal too, linear or switching.
Taking for example the Yaesu VL-1000 Quadra amplifier, it is sold with an
external 2.5 kVA linear PSU developping 48V at 50A output.
prevent current surge it should incorporate inrush current limiting
(e.g. Ametherm) constituted of thermistors which curves are
displayed at right.
safety purposes, a two-pole circuit breaker in the primary
mains circuit is also recommended as well as an interlock fast relay
to disable the power supply if the cabinet is opened while the
unit is switched on.
minimise the effects of mains-voltage drop (it is not exceptional to have a 5% output
voltage drop at full load), the power supply should operate on 220-240V
AC mains (or 110-115V AC) rather than on 13.8V DC (or 28 or 50V DC), this
latter being more suited to power a portable installation. Due to the
high currents flowing in a 220V/48V at 50A transformer at least 5% of
the input energy is converted in heat. Its dissipation requires an adequate
forced-air cooling system.
ensure optimum linearity of power devices, the efficiency of the DC
supply is essential. First the collector/drain must be well-regulated to
prevent voltage drop. Its function is to optimise the collector-to-collector (or
drain-to-drain) load resistance in providing a good efficiency, and
controlling current swing without reaching saturation. Then
the base- or gate-bias regulator must display a flat curve, absolutely stable and free
from modulation (no RF or envelope modulation). In fact a well-designed PSU should include
extensive RF decoupling. These few specs must be carefully monitored in order to provide
the best linearity to power devices.