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/optimal operation.
This active splitter design preclude all those drawbacks !. A description is given for a 3 port 432MHz design, but also 2-and a 4 port 144MHz designs have been constructed.
Some more picture's:
- 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, 6A
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 output power obtainable. 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 (e.g. RA13H4047M) or Mitsubishi. A number of those need about 50mW to obtain full (e.g. 20W) output power.
Special care should be taken mounting those powermodules, a major fault reason 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) flat heathsinks are not flat enough and need additional work with a milling machine to get to the 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
On the picture shown also is the auxiliary circuitry needed to set the quiescent current of the module to achieve low distortion operation for linear modes as SSB. This quiescent current can be quit substancial (1-2Amp).
Front of the active splitter. On the left the input power meter to set nominal input power. Output power of the 3 outputs can be independantly set or completely switched off. Switch off by just cutting DC power to the powermodule. On the right the input connector
Quiescent current of the modules is cut during receive (PTT needed)
Inside look, the modules are wired using thin teflon coaxial cable. Cabinet is shown up side down (heatsink is normally at the top)
Input attenuator constructed using a SMD dummyload with capacitive tap, 3 port resistive splitter and the coaxial variable attenuators