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Technical review

How to select a solid-state HF amplifier ? (II)

ALC and linearity

An 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.

An 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 voltage-controlled resistance.

External and internal view of the Tokyo Hy-Power HL-1KFX amplifier. It develops 600W PEP output in SSB for 1.2kW DC input.

In 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.

In 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.

As 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).

If 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 amplifier.

How to prevent overdrive ? Depending 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.

Low-pass filters

Chebyshev and Butterworth approximations

Located 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.

LPF 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).

Networks obtained by this approximation are the most easily realized. They offer a high attenuation and a steep roll-off near the cutoff frequency.

In 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.

Automatic antenna tuner

Some 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.

The auto-tuner is able to match load impedances in the range 16-150W resistive, 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.

To 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.

At 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.

Band-switcher

Some, 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.

Input interfaces

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.

Don't 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.

Security issues

Controller board

The 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.

The controller board fitted on the side of the Yaesu VL-1000 Quadra amplifier.

Many 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. 

In 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.

In 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).

There 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.

Shielding

All 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.

In 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 entry points 

These protecting measures will maintain RF integrity, minimise internal RF feedback, and ensure regulatory electromagnetic compatibility (EMC) compliance.

Packaging and wiring

Everybody 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 inside !

Some well-known solid-state HF amplifiers

Tokyo Hy-Power HL-2KFX

Icom IC-PW1

Yaesu VL-1000 Quadra

Alpha 87A

Henry Radio SS1200HF°

Ameritron ALS-500M°

°Respectively powered on 28V DC and 13.8V DC (portable).

A 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.

Excepting 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

The 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.

To prevent current surge it should incorporate inrush current limiting (e.g. Ametherm) constituted of thermistors which curves are displayed at right.

For 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.

To 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.

To 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.

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