RF Amplifier Foldback

[Obsidianxenon]

Hi everyone, I have a project that involves a 30W RF amplifier module at its final stage, the Mitsubishi RA30H4452M1 (using it because a friend has a spare).

Now according to its datasheet, under very high VSWR it suffers "no degradation or destroy". Naturally I want to be safe (and keep the module relatively cool) and start to limit the output power once it hits about 3:1. I can't find much information about foldback solutions right off the bat, so does anyone have any direction they can point me in?

I assume it involves some kind of directional coupler and a way of measuring the reflected power, then attenuating the input proportionally to it. My project will be feeding the PA with a Gilbert Cell configured as a voltage-controlled amplifier so I can manually adjust drive, so I could just introduce a foldback voltage at a summing node (kind of like a PLL no?).

Any help on where to start on this?

[dobsonr741]

I would start by building a microstrip directional coupler/SWR detector for 70 cm. Build and characterize that independently first, testing forward and reflected readings at several power levels and with known mismatches. At 30 W and 440 MHz, layout, coupling, directivity and detector behavior matter — this is no longer multimeter-probing territory.

Once the forward/reflected measurements are trustworthy, the foldback itself is relatively straightforward: an MCU with two ADC inputs for forward and reflected power, and an output controlling the PA drive or bias. That could be a DAC/PWM-derived analog control voltage, or even a simple 3-level scheme: full power → reduced power → shutdown.

I’d also make the hardware capable of very fast reflected-power protection independent of the MCU. Let an analog comparator immediately reduce/kill drive on a severe mismatch, while the MCU handles slower SWR calculation, foldback, hysteresis, timing and recovery.

[Obsidianxenon]

Right, thank you for the clarification.

I'll do my best to come up with an analogue solution for the drive reduction, fitting the theme of my project, but if that turns out to be infeasible, then an MCU solution is not off the table. I've had to use one anyway in order to get fractional TCXO frequency references for PLLs, but an analogue feedback setup would be pretty satisfying to make (not to mention pretty fast if I do it right).

I'll get onto researching directional couplers. Seems like half of this PCB is going to be microstrip filtering and stuff because of the power output. That or expensive RF components for sure. To be fair it's just the tail end of the project but still. 

[BigBoss]

If you use a ferrite circulator at that frequency, the PA will be safe.
There are many drop-in circulators as well as connectorized ones.

[Obsidianxenon]

And thank you very much for a much simpler solution! Of course, whether I'll go the coupler or circulator route depends on final cost, but thank you for letting me know that these things exist!

[RFDx]

I assume it involves some kind of directional coupler and a way of measuring the reflected power, then attenuating the input proportionally to it. My project will be feeding the PA with a Gilbert Cell configured as a voltage-controlled amplifier so I can manually adjust drive, so I could just introduce a foldback voltage at a summing node (kind of like a PLL no?).

Any help on where to start on this?

Take some inspiration from the power amplifiers and their automatic power control (APC) circuit to be found in LMR or ham radio VHF-UHF FM transceivers from Kenwood, Icom, Yaesu, Vertex, Alinco etc.

A few examples:

1. Kenwood TK-8102 uses the same hybrid module as yours. The voltage at the input and output voltage of a lowpass filter is detected, processed and the DC ouput voltage is used to set the gate voltage and therefore the output power of the module.

The similar APC technique can be found with the Icom IC-E2820, Kenwood TK-8102H (discrete high power amps), Yaesu FT-2900R(E) or Alinco DJ-V57 handheld.

2. In the Kenwood TK-8302(H)(U), TK-8360(H)(U)  and the Vertex Standard VX-2100/2200 use printed directional couplers to detect forward and reverse power. Output power is controlled with a variable gate voltage applied to the respective control input of the hybrid amplifier.

3. Another technique found mainly in handheld VHF/UHF FM transceivers like the Kenwood TK-373G or TK-2102G is monitoring the supply current of the final amplifier stage and adjusting the gate voltage of the hybrid amp or the discrete power MOSFETs to keep output power constant or fold it back if necessary.

[RFDx]

Missing attachments.