Driving more than one poweramplifier can be done by using a simple passive power splitter arrangement. Drawback of this approach is that you do not have fine grain control on the driving power for each power amplifier. In most cases you need different driving power because the poweramplifiers are not equal. One PA might be a tetrode tube amplifier, needing only 20W of input power to get full output level and the other might be a triode of bipolar PA needing substantial more driving power. One way to overcome this is to use different sets of attenuators (a.o. fixed resistive attenuators, different lengths of coax cable, etc). End result is a time consuming and not satisfactory and optimal operation.
This active splitter design preclude all those drawbacks !. A description is given for a 4 port 144MHz design, but also a 2 port and a 3 port 432 MHz design have been constructed.
- Input power: 5 to 15Watts
- Output power: max 30W on each port
- Output power setting range: 30dB (1000x)
- No interaction between output ports
- Power supply: +12VDC
Blockdiagram of the active power spliter is given below
1. Input attenuator
Input attenuator is a fixed attenuator and the main function is to provide a well defined input impedance, normally 50 Ohms, decoupled from the rest of the circuitry. Input impedance should be constant and irrespective of the setting of the variable attenuators. This attenuator can be constructed as T- or PI- resistor network or a 50 Ohm power (SMD) resistor with a small capacitive pick up.
Splitter will split the attenuated input RF signal into N equal signals. As there is no need for low loss splitter behaviour this splitter can be constructed via a simple resistive arrangement. Table below will give the required resistor values and the corresponding power loss. Interaction between the different ports can decreased by adding some additional attenuation (preferable T-structure) at the output of each port.
3. Variable attenuator
Controlling the output power is done by the variable attenuator.
This can be anything between a low valued (cermet)trimmer resistor or a special coaxial potentiometer as used in this design and shown in the picture on the left.
Care should be taken to make sure that the maximum power handling of this attenuator is not exceeded.
4. Power amplifier
The power amplifier will determine the maximum output power. Beside this the selection criteria should be the needed input power. A too low gain i.e. high input power needed will set a limit on the used variable attenuator. Good results have been achieved by using standard poweramplifier modules from a.o.Toshiba or Mitsubishi. A number of those need about 50mW to obtain full (e.g. 30W) output power. In order to achieve full output and good linearity substantial quiescent current is needed, for the S-AV 33 up to 6A !
Special care should be taken mounting those powermodules, a major fault mode are cracks in the ceramic substrate due to too much mechanical stress. This stress will occur when the mounting surface is not completely flat. Standard (big) heathsinks are not flat enough and do need additional work with a milling machine to get to the required 50um flatness.
RF connection is another concern using those poweramplifier modules.
RF ground is the metal flange and the bottom of the module. There is no "RF ground" pin on those modules.
A special small PCB is needed to achieve good ground connection between the module and the input/output (coaxial) connection.
In some circumstance a special formed copper strap can be used above the modules as well.
Below the details of this particular construction are shown.
Input attenuator is constructed using a high power SMD dummyload with capacitive tap, 4 port resistive splitter and coaxial variable attenuators.
Here the resistive splitter is designed by parallel construction of 2 times a 2 port divider, reason: some resistor values I had at hand....
Power modules used are Toshiba S-AV 33, capable of delivering approx. 30W. On the picture is shown the complete auxiliary circuitry needed to set (and disable) the quiescent current of the module (to achieve low distortion operation for linear modes as SSB). This quiescent current can be quit substancial (> 6 Amp per module).
Front of the active splitter. On the right the input connector and relative power meter to set nominal input power. Output power of the 4 outputs can be independantly set or completely switched off. Also the PTT input is shown, as needed to cut power dissipation during receive.
In the picture above the circuit logic for 2 of the 4 modules. Complete circuit diagram is here as downloadable .pdf
Inside look, the modules are wired up on the input using thin teflon coaxial cable. Cabinet is shown up side down as the heatsink is normally at the top.