While searching through eBay listings I came across a step attenuator that was listed aa 0-50 dB in 10 dB steps attenuator, and good for DC to 18 GHz. It was recently priced so I decided to spring for it. When it arrived I started to research the origin. It turned out to be from an old HP 8555 spectrum analyzer. Most likely a unit no longer working, but the attenuator was advertised as tested. I found a users/service manual that described the operation and the voltages needed to activate the solenoids. It looked to be simple as it was using 12V solenoids. The attenuator has three solenoids that are used to enable/disable 3 different attenuators in series. After putting the attenuator on the test bench I confirmed that all 3 was working and providing 10, 20 and 40 dB attenuation respectively. So the range is 0-70 dB and not as the specs for the HP 8555 analyzer listed (0-50 db). In the spectrum analyzer the attenuator is placed between the N connector on the front panel and the mixer and can handle +33 dBm or 2W maximum. Good for many microwave applications.

With the information about how the attenuator is operated and the knowledge that all 3 stages was working as expected I started designing the control unit. The first step was to decide how to generate the voltage needed for each of the three solenoids. In order to engage the solenoid the two terminals needed a +12V pulse for 150 ms and in order to switch it back the polarization of the pulse should change ro -12V for 150 ms. This can be achieved with an H-bridge. This is basically 4 transistors configured to have 2 inputs and two outputs. This will allow a positive voltage when the input is high and low and a negative output when the inputs are low and high. In addition there would be no voltage across the outputs if both inputs are low. H-bridges are commonly known to drive DC motors, allowing them to turn in both directions depending on the voltage being positive or negative. Amazon is selling a package of 4 H-bridge modules based on the L298N chip for just $10. It would not make sense to try to build one from scratch at those prices. The module provides 5V input logic and 12V output, exactly what's needed to use an Arduino Nano as the controller.

With the parts ordered I designed a 3D printed enclosure that could be glued on to the side of the attenuator, covering the terminals for the solenoids and providing push buttons to change the attenuation up and down and a display to show the current value. Since the solenoids are latching it's possible to connect the power, set the desired attenuation and then disconnect the power. That way there will be no noise from the Arduino board while the attenuator is in use. I also made the code to set the default attenuation to 70 dB when the unit is powered up. This is needed to set the highest attenuation but also because it's not possible to read the current value/state of the each attenuator. Bringing them to a default value makes sense.

The finished attenuator and control box all assembled.

The inside of the control unit showing the two H-bridge boards on the left and the Arduino Nano on the right. I placed the Arduino nano on an breakout board that allow for 12V power input and easy access to the pins needed to control the H-bridges and the two input buttons.

And finally the unit with power applied. The small display is a 128*32 pixel OLED display with a I2C interface. When power is applied it will show my call sign and then set the attenuator to 70 dB.

Here are links to the Arduino sketch and the 3D stl file.

I have a couple of different GPS Locked reference oscillators that are used to lock PLL oscillators in microwave equipment. Some of these have a nice sine wave output at 10 MHz and others have a square wave output causing harmonics of the base frequency to be present in the signal. This is a factor that will increase the phase noise of the PLL oscillator. Getting rid of the harmonics is relatively simple with the help of a low pass filter. W1GHZ, Paul Wade has a nice article about the design of such a filter for a 10 MHz reference oscillator. The article is linked on the small projects page of his web site.

Today I decided to build a version of this low pass filter using SMD components. I did not have the exact values of all components, but was able to get close enough. I started out by creating a NC file for my CNC router and cut out a small PCB as seen on the image below. The PCB is about 8x19mm. This only a small amount wider than the SMA connectors I used.

The at the end I added some heat shrink tube to finish the project.

As you can see I ended up making two filters to check how accurate they were. It turned out that the first one gave the best performance. One of the inductors I used was in a 0402 package and way too small to work with. I'm going to order some in 0603 package and create a few more to see if this can be repeated with similar performance.

The output of the 10 MHz reference without the filter looks like this:

The 3rd harmonic is only about 10dB down from the 10 MHz signal and there is a substantial amount of harmonics. After adding the filter the output looks like this:

Almost all of the harmonics are gone, indicating that the signal now is much closer to a sine wave. Using the spectrum analyzers tracking generator to show the filter response looks like this:

This shows 40-45dB attenuation at the 3rd harmonic and almost no loss at the base frequency.