dds-sweeper

Raspberry Pi Pico interface for the AD9959 DDS.

If you use this project, please cite as:

E. Huegler, J. C. Hill, and D. H. Meyer, An agile radio-frequency source using internal sweeps of a direct digital synthesizer, Review of Scientific Instruments, 94, 094705 (2023) https://doi.org/10.1063/5.0163342

Specs

• The timing capabilities of the DDS-Sweeper are tied to the number of clock cycles the pico takes to send the next instruciton.

• At the default Pico Clock frequency of 125 MHz those clock cycle counts correspond to the following times:

• The number of instructions you can store in the table depends no the type of sweep being performed and the number of channels being used.

How to flash the firmware

Download the latest dds-sweeper.uf2 file. On your Raspberry Pi Pico, hold down the "bootsel" button while plugging the Pico into USB port on a PC (that must already be turned on). The Pico should mount as a mass storage device (if it doesn't, try again or consult the Pico documentation). Drag and drop the .uf2 file into the mounted mass storage device. The mass storage device should unmount after the copy completes. Your Pico is now running the DDS Sweeper firmware!

Notes

• Output Amplitude is dependent on frequency (some people on Analog Devices forum mentioned amplitude and frequency are related by a sinc function)

• The frequency resolution of the AD9959 is $= \frac{f_{sys clk}}{2^{32}}$. At the default system clock of 500 MHz, the frequency resolution is $\approx 0.1164$ Hz. Any frequency input to the dds-sweeper will be rounded to an integer multiple of the frequency resolution.

• The phase resolution of the AD9959 is $= \frac{360^\circ}{2^{14}} \approx 0.02197^\circ$. Phase offsets will be rounded to a multiple of this resolution

• The amplitude resolution of the AD9959 is $= \frac{1}{2^{10}} \approx 0.09767\%$. Amplitude scale factors will be rounded to a multiple of this resolution

Sweeps

• Setting up a sweep:
  
  Sweeps are defined by two parameters, sweep delta and ramp rate.
• Sweep Delta defines the change in output amplitude/frequency/phase on each sweep step
• Ramp rate defines how often a sweep step is taken. It is based off of the AD9959's sync clock signal which will be one quarter of the AD9959's system clock. The ramp rate parameter specifies the number of sync clock cycles per sweep step. A ramp rate of 1 will cause the sweep delta to be applied every 1 sync clock cycle. For upward sweeps, the ramp rate parameter can have a value of 1-255. For downward sweeps the ramp rate can only be 1.

The time between sweep steps can be calculated with: $$ t = \frac{\textrm{Ramp Rate}}{\textrm{Sync Clock}} $$ Using the Pico's 125 Mhz with a 4 times PLL Multiplier gives the AD9959 a system clock of 500 MHz and therefore a sync clock of 125 MHz. For upward sweeps the time between sweeps can range from $\frac{1}{125 MHz} = 8 ns$ to $\frac{255}{125 MHz} = 2.04 \mu s$. Downward sweeps will apply the sweep delta every $\frac{1}{125 MHz} = 8 ns$.
Given the frequency resolution of $\frac{f_{sys clk}}{2^{32}}$, the smallest sweep delta is $= \frac{1}{2^{32}} = 0.1164$ Hz. With the maximum ramp rates, the DDS-sweeper has a minimum sweeping rate of $\approx 47871$ Hz/sec when sweeping upwards or $\approx 12207031.25$ Hz/sec when sweeping downward.

Downward Sweeps:

Downward sweeps are not well supported by the AD9959, but they can still be done. The best method I have found for doing a downward sweep is to send the instructions via serial first, with the sweep autoclear bit set to active and the rising sweep tuning word set to the maximum. Then issue the IO_UPDATE signal while keeping the profile pin for that channel high. I belive this clears the sweep accumulator then quickly refills it with a max rate sweep before beginning the downward sweep. Other combinations of autoclear bit active or not and timing of the profile pins seem to cause even more issues with downward sweeps. The biggest downside of this method is that you cannot slow down the downward sweep ramp rate as much as the upward sweep ramp rate.

• Autoclear Accumulator Active, drop the pin after the update:
  Seems to work, you just cannot slow down downward sweeps.
  

• Autoclear Accumulator Active, drop the pin before the update:
  Downward sweeps just dont work at all.
  
  

• no autoclear, drop pin before update:
  You cannot do consecutive down sweeps - every down sweep must be preceeded by an up sweep
  
  

• no autoclear, drop pin after update:
  A down sweep after an up sweep cannot cover a greater distance than the upward sweep
  
  

• You can also generate a downward sweep by operating the AS9959 above the nyquist frequency. This method does work but causes discontinuity when switching which band the AD9959 is working in.
  

Serial API

Note: Commands must be terminated with \n.

• version:
  Responds with a string containing the firmware version.

• abort:
  Stops buffered execution immediately.

• status:
  Returns the opperating status of the DDS-Sweeper. 0 status indicates manual mode. 1 status indicates buffered execution.

• getfreqs:
  Responds with a multi-line string containing the current operating frequencies of various clocks (you will be most interested in pll_sys and clk_sys). Multiline string ends with ok\n.

• debug <state:str>:
  Turns on extra debug messages printed over serial. state should be on or off (no string quotes required).

• mode <sweep-type:int> <trigger-source:int>:
  Configures what mode the DDS-Sweeper is operating in

  • 0: single stepping / manual mode
  • 1: amplitude sweep
  • 2: frequency sweep
  • 3: phase sweep

  The operating mode must be set before buffered execution instructions can be programmed into the DDS-Sweeper.
  A trigger-source of 0 means the Sweeper is expecting external triggers. A trigger-source of 1 means the Sweeper will send its own triggers and set commands will require an aditional time argument. The Sweeper must be in manual mode in order for the setfreq, setphase, and setamp commands to work.

• setfreq <channel:int> <frequency:float>:
  Manually set the output frequency of a specified channel. Channels are 0-3 and frequencies are in Hz. If debug is set to on, it will respond with the actual frequency set. The Sweeper must be in manual mode.

• setphase <channel:int> <phase_offset:float>:
  Manually set the phase offset of a specified channel. Channels are 0-3 and offsets are in degrees. If debug is set to on, it will respond with the actual degree offset set. The Sweeper must be in manual mode.

• setamp <channel:int> <amplituce_scale_factor:float>:
  Manually set the amplitude scale factor of a specified channel. Channels are 0-3 and amplitude scale factors are a precentage of the maximum output voltage. If debug is set to on, it will respond with the actual frequency set. The Sweeper must be in manual mode.

• set:
  Sets the value of instruction number addr for channel channel (zero indexed). addr starts at 0. It looks different depending on what mode the sweeper is in. If Debug is set to on it will respond with the actual values set for that instruction.

  • Single Stepping (mode 0): set <channel:int> <addr:int> <frequency:float> <amplitude:float> <phase:float> (<time:int>)
  • Sweep Mode (modes 1-3): set <channel:int> <addr:int> <start_point:float> <end_point:float> <delta:float> <ramp-rate:int> (<time:int>)

  start_point is the value the sweep should start from, and end_point is where it will stop. delta is the amount that the output should change by every cycle of the sweep clock. In the AD9959, the sweep clock runs at one quarter the system clock. ramp-rate is an additional divider that can applied to slow down the sweep clock further, must be in the range 1-255. The types of values expected for start_point, end_point, and delta different depending on the type of sweep

  • Amplitude Sweeps (mode 1)
    start_point and end_point should be decimals between 0 and 1 that represent the desired proprtion of the maximum output amplitude. delta is the desired change in that proprtion. For all three of those values there is a resolution of $\frac{1}{1024} \approx 0.09766\$
  • Frequency Sweeps (mode 2)
    start_point, end_point, and delta are frequencies in Hz. They can have decimal values, but they will be rounded to the nearest multiple of the frequency resolution.
  • Phase Sweeps (mode 3) start_point, end_point, and delta are in degrees. They can have decimal values, but they will be rounded to the nearest multiple of the phase resolution (always $= 360^\circ / 2^{14} \approx 0.02197^\circ$).
  • Sweep and Single Stepping Mode (modes 4-6): set <channel:int> <addr:int> <start_point:float> <end_point:float> <delta:float> <ramp-rate:int> <secondary1:double> <secondary2:double> (<time:int>)

  These modes perform a linear sweep on one of the parameters, while simulaneously single stepping on the other two parameters.

  • Amplitude Sweeps (mode 4)
    secondary1 is the frequency, and secondary2 is the phase offset.
  • Frequency Sweeps (mode 5)
    secondary1 is the amplitude scale factor, and secondary2 is the phase offset.
  • Phase Sweeps (mode 6)
    secondary1 is the amplitude scale factor, and secondary2 is the frequency.

• numtriggers:
  Responds with the nmumber of triggers processed since the last call of start

• setclock <mode:int> <freq:int>:
  Reconfigures the source/reference clock.

  • Mode 0: Use pico system clock as reference to the AD9959
  • Mode 1: Sets the AD9959 to recieve a reference clock not from the pico

• setmult <pll_mult:int>:
  Sets the AD9959's pll multiplier on the Reference Clock input. The default value is 4, giving the AD9959 a system clock of 500 MHz with the pico's 125 MHz reference. Valid values are 1 or 4-20.
  If changing the reference clock and PLL multiplier, you should set the reference clock frequency first.
  The AD9959's PLL has an output range of 100-160 MHz or 255-500 MHz with VCO gain enabled. The pico will automatically enable the VCO gain bit if the requested frequency is in the upper range. If trying to use the PLL multiplier to generate a frequency between 160 and 255 MHz, there is no guarantee of operation.

• setchannels <num:int>:
  Sets how many channels being used by the table mode. Uses the lowest channels first, starting with channel 0. If number of channels is set to 1, buffered execution instructions will be written to all 4 channels simultaneously.

• save:
  Saves the buffered execution table to nonvolatile memory so that it can be recovered later (after a power cycle).

• load:
  Retrieves the buffered execution table stored in nonvolatile memory and restores it to system memory so that it can be run. A saved table must be loaded before it can be run.

Compiling the firmware

If you want to make changes to the firmware, or want to compile it yourself (because you don't trust binary blobs from the internet), we provide a docker configuration to help you do that.

1. Install docker desktop and make sure it is running (if you are on Windows, you may have to mess around a bit to get virtualisation working at an operating system level)
2. Clone this repository
3. Open a terminal with the current working directory set to the repository root (the `docker-compose.yaml`` file should be there)
4. Run docker compose build --pull to build the docker container
5. Run docker compose up to build the PrawnBlaster firmware.

Step 4 will take a while as it has to build the docker container. If it is slow to download packages from the Ubuntu package repositories, consider providing an explicit apt mirror that is fast for you: docker compose build --pull --build-arg APT_MIRROR="http://azure.archive.ubuntu.com/ubuntu/".

If you want to change which version of the pico SDK it builds against, this is set in the build/docker/Dockerfile file. Just change the git tag of the pico SDK that gets cloned out by git, then rebuild the docker container (see step 4).

Note once the docker container is built, you can run step 5 as many times as you like. You do not need to rebuild the container, even if you make changes to the PrawnBlaster source code. You only need to rebuild the docker container if you modify the build/docker/Dockerfile file.