Category Archives: Engineering

Experiments with Phase-Frequency Detectors

Over the past few evenings, I have been experimenting with phase-frequency detectors. I have an upcoming project that requires the use of one, and, I figured I’d refresh my memory on them. I ended up using my Lattice MachXO2 breakout board as the development platform.

The first step was to divide a 10 MHz crystal oscillator down to 10 kHz. That was easily done with a counter, counting from 0 to 499, and then resetting to 0. Every reset would simultaneously toggle an output bit, with the net result being a square-wave clock on the output bit, periodic every 1000 cycles of the input. A divide by 1000 counter.

Next was to understand the Phase-Frequency Detector (PFD). This is commonly referred to as a Type 2 detector, since it detects not only phase difference but frequency difference. This means that the PLL will only ever lock to the fundamental frequency, and not harmonics. It also means that when the loop is unlocked, the PFD knows which way to drive the VCO to regain lock. A Type 1 detector only uses phase information, and so drives the oscillator in the direction of the phase difference until the loop locks – as a result, Type 2 detectors lock quicker.

The Type 2 detector has two outputs, up and down, which pulse for the required direction with a duty cycle proportional to the phase difference.

I spent a few days designing and simulating the PFD using the Aldec Active-HDL simulator to confirm that my circuit did indeed perform as expected:

I then added a simple lock detector, which set a locked signal high if the phases were in lock for the past 10 cycles as a proof of concept. In reality, a much longer observation window will be used. It is possible to see the lock signal becomes high after 10 cycles.

The final stage of this project snippet was to test on real hardware. The Verilog code was pushed through synthesis, place and route, and a configuration file for the Lattice FPGA generated. This was then programmed into the board, and the board taken to the lab – you can see all the main parts of the setup in the photo below.

Below, you can see the scope traces from the probes in the lab bench photo. The yellow trace shows the 10 MHz VCO frequency from the Trimble 34310-T2 OCXO. The green trace is a debug from the FPGA output showing the 10 MHz signal divided down 10 kHz. The blue trace is GPS locked 10 kHz reference output. Finally, the purple is phase detector output, here from the ‘down’ output of the detector since we see that the divided VCO output (green) slightly leads the GPS reference (blue). The ‘up’ output is at logic-0 throughout.

The next part of the project was to create the charge pump circuit which converts the ‘up’ and ‘down’ pulsed signals into an analogue control voltage for the VCO.

The parts for the charge pump took a few days to arrive, and while waiting I contrived the following circuit. Since the OCXO generates a 6V reference voltage for use with the VCO input (actually 5.4V in my case), it seemed wise to use that. Some crude experiments had lead me to a tune voltage of around 3.5V. The circuit uses a PNP transistor (Q1) to put pulses of energy into the filter network via R1. Similarly, it uses an NPN transistor (Q2) to remove pulses of energy from the filter via R1. R5 and R6 serve as a current limit in case both Q1 and Q2 are both powered. A further NPN transistor (Q3) acts as a voltage interface between the ~6V on the base of Q1 and the FPGA IO at 3V3 maximum. Only the values of components in the filter section are critical (R1, R7, C1, C2, C3); the others were chosen from what I had laying around.

The loop filter was tested and tweaked in LTspice using the values above. The loop filter has around 62 dB of attenuation at 10 kHz (our reference [and thus up/down pulse] frequency).

A look in the time domain shows we can expect about 1.5mVp-p at 10 kHz from visual estimation. An output of 1.5mVp-p is approximately 0.53 mVrms, which gives us around -66 dBV of attenuation (similar to we saw above). The curve of the waveform is the DC levels settling out at the start of the simulation.

And finally the steady-state ripple; for a 1V square wave (0V-1V) input, a 1.23 mV ripple exists at the output (455.719-454.486).

The penultimate step was to build the circuit and confirm it worked in real life. I made the charge pump on a scrap of strip-board, with pin headers for the main signals. On the left, +6V VCO reference input, the up and down signals from the FPGA, and on the right, ground and the VCO tuning input. The circuit is pretty much laid out as per the schematic, with the addition of an LED.

The final step was to watch the PLL lock on the scope! In the short video clip below, you can see the yellow trace is the 10 kHz reference frequency from the GPS. The green trace is the 10 MHz from the VCO. The blue trace is the 10 MHz divided down to 10 kHz. The purple trace is the VCO tune voltage – the output of the charge pump.

The closing remark is that this project was a learning exercise. The Verilog code & TB is presented here on GitHub and you’re invited to take a look.

Installing Eclipse IDE and PyDev onto Ubuntu 18.04

This page assumes some basic familiarity with Linux. It assumes a clean install of Ubuntu 18.04 and installs Eclipse Photon. I install the CPP version, but you’re free to choose when the option presents itself!

Update the OS

First thing to do is to update the operating system. This is easily achieved by running the following two commands:

sudo apt update

sudo apt upgrade

These commands may take a while to complete, depending on what there is to update and how fast your internet connection is.

Installing Java Development Kit (JDK) 8

Since Eclipse is written in Java, we will need the latest version. I’m not sure if the Java Runtime Environment (JRE) alone is enough, but I have installed the full JDK anyway.

Firstly we add the third party JDK PPA repository and update the package manger index

sudo add-apt-repository ppa:webupd8team/java

sudo apt update

Next we must actually install the JDK:

sudo apt install oracle-java8-installer

This will pull in a few extra packages, such as java-common, oracle-java8-set-default (which makes sure that this installed version of Java is the system default), font packages and so on. You’ll be guided through the Java 8 installation with an ncurses based installer:

You must accept the Oracle Binary Code licence. The download for Java 8-1u171 was 182 MB. To confirm the installer completed correctly, scroll up in the terminal window. You should see something explaining that the installation finished successfully. To confirm this, and check the version installed, you can run the following from the terminal:

javac -version

javac 1.8.0_171 [or similar result expected]

Installing Eclipse

The Eclipse installer can be found on the Eclipse project download page: https://www.eclipse.org/downloads/. At the time of writing, the Eclipse Photon installer was 45.9 MB. I downloaded it using the Mozilla Firefox browser, and saved the installer into my user’s download folder (/home/geosma01/Downloads/).

Once downloaded, switch back to your terminal program, and change directory into the downloads folder and extract the downloaded TAR/GZIP archive and change directory into it:

cd ~/Downloads/

tar xvfz eclipse-inst-linux64.tar.gz

We’re now ready to run the installer. If we change into the newly extracted folder and then run the installer, we should be good to go:

cd eclipse-installer

./eclipse-inst

I ran into trouble installing Eclipse as root, so I install it into my own user space: ~/eclipse/cpp-photon

Accept any licences it prompts for:

Once the installer has finished, you can start Eclipse by pressing Launch. We’ll make a desktop shortcut in a few moments…

And eventually…

Now we have Eclipse running, we should get ourselves an icon to easily start it.

Creating a Menu Icon

Next we will create a menu shortcut. We will create this using a basic terminal-based text editor (nano). Running the following will open the file:

nano ~/.local/share/applications/eclipse.desktop

Then, copy and paste the following, as you see appropriate – you should change my username (geosma01) to your own at the very least:

[Desktop Entry]
Name=Eclipse CPP Photon
Type=Application
Exec=/home/geosma01/eclipse/cpp-photon/eclipse/eclipse
Terminal=false
Icon=/home/geosma01/eclipse/cpp-photon/eclipse/icon.xpm
Comment=Integrated Development Environment
NoDisplay=false
Categories=Development;IDE;
Name[en]=Eclipse

If all has gone well, you’ll see something like the following inside the menu:

Installing PyDev

PyDev can be easily installed through the Eclipse Marketplace. From inside Eclipse, click Help on the menu, and select Eclipse Marketplace. You should be presented with the following window. You can then enter “pydev” into the search, and then click Install on the entry shown below:

Confirm your selections, accept the licence conditions, and you’re good to go! Once installed, click on Restart Now and you’re done!

Once restarted, you can open a PyDev Perspective by selecting Window from the menu, selecting Perspective > Open Perspective > Other and selecting PyDev:

You’re ready to go!