Parameters -

The waveform that the DSM outputs is defined by the following parameters:

Before going into detail about the parameters, a brief explanation of how the board works will help you understand the parameters better. All frequency data is stored on a QDR-SRAM chip on the board. The RAM is capable of storing 458,752 frequency words each of size 32 bits. Each frequency in a chirp is represented as a 32-bit frequency word. Since there are 32 bits available, you have a maximum frequency resolution of about 4.3 billion (232).

Memory Addresses

The actual chip creating the waveforms is the Euvis DS856 direct digital synthesizer (DDS). In order to create a waveform, the DDS must have a clock input. The maximum frequency waveform that the DDS can output is half of the input clock due to sampling restrictions. So if you have a 2.0 GHz input clock the maximum output waveform frequency that you would be able to get is 1.0 GHz. The DSM creates chirping by storing a sequence of frequency words onto the RAM (maximum of 458,752) and then “feeds” these frequency words into the DDS chip sequentially at a maximum rate of 1/8th the input clock. Therefore, every 8 clocks a new frequency word gets sent to the DDS and a new frequency will be output. Each frequency will also be output for 8 clocks. You can change the default time per frequency by changing the Oversampling in the Module Parameter Window. The DDS is phase continuous so the output waveform will be a smooth transition from one frequency to the next without any phase jumps.


Start, Stop, Step Frequency

Now that you know how the DSM operates, explaining the parameters should be easier. The Start Frequency is simply the frequency at which you want to start chirping or the lowest frequency in the chirp. The Stop Frequency is the maximum frequency which you want to chrip. The Step frequency is how much you want the frequency to go up by each time. Of course, you cannot put anything you want for these parameters. The limits are bound by the memory available and your input clock.

The Stop frequency cannot exceed half of the input clock as mentioned before:

Eq 1: Stop Frequency Limit Equation

The total number of steps cannot exceed the limit of 458,752 memory addresses available:

Eq 2: Steps Limit Equation


Data Length

Data Length specifies the number of frequency points that will be output. For a more detailed discussion of Data Length, please see the Data Length page in this section.

Delay

Delay is how many memory addresses to keep at the starting frequency before chirping starts. For example if your Delay was “5” then the first 5 memory addresses will be at the Start frequency then at the 6th memory address, chirping will begin. Please note that the delay is counted toward the data length. So if you had data length set at 32 and you had 30 frequency steps but you also had delay set to 5, then the last 3 highest frequencies will not be output since the delay and the frequency steps together are more than the data length.

Therefore the new Data Length equation becomes:

Eq 6: Final Data Length Equation

where DL is the Data Length and MD is the Memory Depth. This is the general equation to use for determining proper Data Length and Memory Depth.

Ramping Method

The ramping method which is specified in the program as Waveform Code determines the chirping characteristic. The output frequency can either ramp up and then when it reaches maximum frequency return to the starting frequency (like a saw tooth shape) or it can ramp up to maximum frequency and then ramp down to the starting frequency (like a triangle shape). In addition, there is also a reverse chirping available so that the frequency gets smaller and smaller.

Ramping Methdods

Frequency Hold Time

The frequency hold time which is specified in the program as the Oversampling Factor in the Module Parameters window determines how long to output each frequency. The default is to output each frequency for 8 input clocks but you can change this to 16 clocks or 32 clocks by changing Oversampling to 2 or 4 respectively.

Phase Reset

Normally, when the DSM is chirping a waveform in a loop, each iteration of the waveform is not guaranteed to start at the same phase. In order to assure that the DSM will start each waveform at 0 phase, you will need to turn on "phase reset by memory" which is an option in the Phase Reset portion of the Waveform window. In addition, you will also have to specify the three phase reset time parameters: RESET_T1, RESET_T2, and RESET_T3. The three time parameters are specified in data points.

RESET_T1 can be thought of as the phase reset delay. This is how many points you want to wait before the phase reset begins. You would set this to "0" if you want the phase reset to start at the beginning of a waveform.

RESET_T2 is the length of the reset. This is how many points the DSM should hold the phase at 0 before chirping starts again. We recommend that RESET_T2 be no shorter than 5 points.

RESET_T3 is the length of the chirp restart delay. When the DSM resumes chirping, usually it needs a bit of time to get up to the desired frequency that you want to start at. We recommend that RESET_T3 be at least 3 data points.

Below is a graphic description of the three reset time parameters:

Reset time parameter graph