GBPPR PHOTOANGLO 10 Watt 2.4 GHz RF Power Amplifier

Overview

This is an update to the "10 Watt RF Power Amplifier for 2.4 GHz" project from GBPPR 'Zine Issue #78.  I've tweaked the overall design a bit for use as a final RF power amplifier for the GBPPR PHOTOANGLO radar unit experiments.

The amplifier is based around a Comwave SB010 10 watt, 2.5 GHz Multichannel Multipoint Distribution Service (MMDS) amplifier.  As MMDS sites are being decommission across the U.S., their components are starting to show up at various ham radio swapfests at very decent prices.

The amplifier shown here is from a Comwave SB010 transmitter and served as the RF amplifier for the audio channel.  They put out around 10 watts (+40 dBm) with a 100 milliwatt (+20 dBm) RF input at 2.4 GHz.  These amplifiers were designed for the 2.5-2.7 GHz MMDS frequency range and will drop to around 4 watts output (+36 dBm) at 2.3 GHz.  They quickly roll-off below that.  Internally, the amplifier uses a Fujitsu FLL171 driving a Fujitsu FLL100 high-power GaAsFET.  The amplifier needs +15 VDC at around 2.7A and a -15 VDC gate bias at 12 mA.  The amplifier even has an internal directional coupler with a diode detector pick-off for a rudimentary RF power detector.  The higher the voltage output of the detector, the higher the amplifier's RF output power.  5 watts of RF output gives a reading of around +8.5 VDC.

To generate the -15 VDC bias voltage, we'll be using a Murata NTE0515M isolated single-output DC-to-DC converter.  It's designed to convert +5V to +15V, but since the output ground is isolated we can "reverse" the connections and generate a -15V voltage.  A simple series-inductor / shunt-capacitor filter cleans up the switching noise.  The Murata NTE0515M requires a minimum load for proper operation, so a 1k ohm resistor is on the output.

A simple negative voltage sequencer will also need to be constructed to ensure that the -15 VDC gate bias voltage is applied before the +15 VDC drain voltage.  This sequencer circuit will be very simple, and consists of just about any P-channel MOSFET which can handle the 3 amp drain current, a 1N4743 13V Zener diode, a 2N2222A transistor, and some 10 kohm resistors.

On the RF amplifer's output, we'll add a protective ferrite circulator (Teledyne C-2S43A-1 or HP0960-0046, 2-4 GHz, 20 dB iso.) and a Meca 715-20-2.5 20 dB directional coupler.  An EMC Technology 5307 50 ohm load is on port-3 of the circulator to handle any reflected power.  The circulator and directional coupler are optional, but will make testing easier and will also help to increase the lifespan of the amplifier

Comwave Amplifier's RF Specifications

Frequency (GHz)      Input Power (dBm)     Output Power (dBm)
2.00                 +20.0                 +30.3
2.08                 +20.0                 +31.0
2.16                 +20.0                 +34.1
2.24                 +20.0                 +35.2
2.32                 +20.0                 +35.3
2.40                 +20.0                 +39.1
2.48                 +20.0                 +39.5
2.56                 +20.0                 +40.6
2.64                 +20.0                 +39.0
2.72                 +20.0                 +35.6
2.80                 +20.0                 +28.0

Pictures & Construction Notes

For this amplifier project, we'll being using an external Astec ACV-15N3 +15 VDC, 3A power supply.

The blue disk on the primary is a 150 VAC Metal-Oxide Varistor (MOV) to protect against any voltage transients on the incoming AC mains.

The blue rectangle device on the primary is an optional "snubber."  This device consists of a series 120 ohm / 0.033 µF AC-rated capacitor to prevent the generation of a large voltage spike when power is turned off.

The Astec ACV-15N3 power supply has four output terminals: +15 VDC Output, +15 VDC Return (Ground), Sense +, and Sense -.

The power supply will be built into an old ammo can.

Overview of the incoming AC mains components.

The 120 VAC mains input is via a standard filtered IEC connector.

The black "hot/live" lead then passes through a panel-mounted 15 amp circuit breaker.

It then goes to a SPST switch for power control.

The white wire is the AC mains "neutral."  The green wire is the AC mains Earth ground.

Three optional ferrite beads are on each of the AC mains wires right after the IEC connector to help knock down any incoming EMI on the power line.

Internal overview of the completed +15 VDC, 3A power supply.

There is a DPST switch to select either internal or external sense for the power supply.  This is optional, but highly recommended if your power supply supports it.

There is a green neon lamp for a "power on" indicator.

Front-panel overview of the completed +15 VDC, 3A power supply.

A 4-pin microphone jack is used for the main +15 VDC output and the sense connections.

Pin      Description                Wire Color
1        +15 VDC Output             Red
2        +15 VDC Return / Ground    Black
3        Sense -                    Green
4        Sense +                    White

The banana jacks are for an extra +15 VDC output.  This is optional, but could be useful for powering other projects.

Overview of the -15 VDC negative voltage generator and sequencer circuit board.

The main +15 VDC input is via the solder terminal on the lower-right.  It is regulated down to +5 VDC to power the Murata NTE0515M DC-to-DC converter, which is the black box on the upper-left.

The -15 VDC output is tapped to control a sequencer circuit consisting of a 1N4743 13V Zener diode, 2N2222A transistor, and Vishay SUD45P03 P-channel MOSFET.

This circuit holds off applying the +15 VDC main line to the amplifier before the -15 VDC bias is applied.  This is required to prevent damaging the RF power amplifier.

Alternate view.

The three solder terminals along the bottom connect to the Comwave SB010 are the RF diode detector output, -15 VDC bias, and +15 VDC.

Internal view of the Comwave SB010 10 watt MMDS amplifier.

The RF input (+20 dBm / 100 mW) is on the right-side, RF output (+40 dBm / 10W) is on the left-side.

+/- 15 VDC power, ground, and the RF detector output are via the feed-through capacitors on the lower-left.

Terminal A is a common ground.

Terminal B is +15 VDC at around 2.7 amps.

Terminal C is -15 VDC at around 12 mA and must be applied first.

Terminal D is the DC output of the diode RF detector.

Mounting the Teledyne C-2S43A-1 ferrite circulator, EMC 50 ohm load, and Meca 715-20-2.5 directional coupler.

The optional ferrite circulator will protect the amplifier in case of a SWR mismatch and the optional directional coupler is for sampling the output RF power or for driving the local oscillator port on an external mixer.

The is a large 4,700 µF / 50V capacitor on the +15 VDC input (red) to the case.

The negative terminal of this capacitor is used as a single-point ground.

The Sense + (white) and Sense - (green) connections are also to the terminals on this capacitor.

The stock finned heatsink on the Comwave SB010 was removed and holes where drilled in the case so the body of amplifier can be mounted internally and the heatsink externally.

The amplifier does get fairly warm, but it's not excessive and shouldn't require a fan.

Brass drawer handles prevent the heatsink from being damaged.

Make sure there is a large - but thin - layer of heatsink grease between the amplifier's body, the case, and the heatsink.

Completed amplifier case overview.

RF input to the amplifier is via the N female connector on the upper-left.

The final RF output from the Meca directional coupler goes to a panel-mount N connector.

The 20 dB tap from the directional coupler goes to a panel-mount SMA connector.

All the RF connections should be made using semi-rigid or conformable coax for maximum RF isolation.

Completed amplifier front-panel overview.

The input from the external +15 VDC power supply is on the lower-right via the 4-pin microphone jack.

There is also a 5A fuse and SPST power switch on the +15 VDC input.

10 watt RF output is via the bottom N connector.

The +20 dBm (100 mA) RF input is via the upper-right N connector.

20 dB tap from the directional coupler is via the SMA connector.  There should be a 50 ohm load installed when not in use.

There is a green LED for a "power on" indicator.

Schematic

• Negative Voltage Generator / Sequencer Schematic

Datasheets & Notes

1. Higher resolution pictures and the original project article are available in GBPPR 'Zine Issue #118
2. Murata NTE0515M Isolated DC/DC Converters  (193k PDF)
3. Astec AVC Series Linear Power Supplies  (676k PDF)
4. Vishay SUD45P03 P-Channel MOSFET  (161k PDF)
5. Comwave 10 Watt MMDS Amplifier Schematic  Partial schematic, but is fairly complete.  RF input is on the left (J1), into a Fujitsu FLL171 which drives a Fujitsu FLL100.  The diode and 51 ohm resistor on the output directional coupler should be reversed.