Saturday, March 21, 2015

Level Controlled RF CW Synthesizer

Using an amplifier, attenuator, splitter and power detector (Prj135) along with an older project with an ADF4351 synthesizer (Prj131 - a variant of Prj133 without the mixer), a Beagle Bone Black, and a SPI interface (Prj130), a controlled level RF CW synthesizer configuration was created.  Without some kind of feedback a standalone signal source is susceptible to level variance, particularly with load reactance and interaction with cable length - an example is here.  By measuring and attenuating you get an AGC loop to set the level.  A block diagram of the setup is below.
Block Diagram of Constant Level Synthesizer Setup using a Beaglebone Black.
A picture of the physical setup is below.
Level Controlled Synthesizer Setup.  Beaglebone Black at left with interface board, synthesizer in upper center, and level control board on right.
The synthesizer has a coarse level control (3 steps of approximately 3dBm).  The digital step attenuator provides 30dBm of attenuation while the amplifier +20dB of gain up to +8dBm through 2700MHz. Software on the Beagle Bone Black sets the synthesizer at its lowest output level at the desired frequency along with maximum attenuation at the digital attenuator.  The power at the detector is measured (multiple samples averaged over 5mS) and compared to the desired level.  The attenuation is reduced by half the delta until the detector reading is within 0.25dBm of the target or until the attenuation setting is 0dB.  If the level cannot be achieved, the synthesizer output level is increased one step and the entire process is repeated.  The following diagram captures measured outputs at various levels and with different measurement devices.
Level Controlled Synthesizer 0.040 - 2 GHz Results
All devices show are +/-2dB. The 0dBm output at 1900MHz shows a sag where the synthesizer output has dropped enough with frequency that the amplifier cannot bring it quite to 0dBm.  You can see the SA0314 has a couple of small spikes in response around 500MHz and 1100MHz. You can also see the 7L12 agrees quite well until about 1500MHz (the traces stop at 1800MHz as this is its limit).

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