Author Archives: Chris

RedPitaya Phase Noise Plot

Working for Golledge gives me access to some great specialist kit. One example is the phase noise test set.

I bought one of the first batch of redpitayas by joining in the crowd funding of the development. Since I have not seen phase noise data appear elsewhere hopefully people will find this plot of use.

The above was measured at 50MHz.

The frequency setting of the redpitaya was slightly off 50MHz in order to get it as close to 50MHz as possible, and within the tuning range of the OCXO being used as a reference oscillator for this measurement.

Verifying Simulation

It’s good to check simulations against measurement sometimes to ensure all is well. Here is a nice result 🙂

The blue traces are from direct measurement, and the red from measurement of smaller elements combined in simulation. Read on for more detail.

What I was checking today was two approaches to generating characteristic plots of a 4 pole crystal filter. The filter comes split into 2 separate 3-leg UM1 packages. Its a 15kHz wide (-3dB) filter centred on 55MHz. The termination impedance is 500 ohms in parallel with 2pF. The filter requires the user to provide a 10pF shunt capacitor at the junction of the two parts. This arrangement is a right pain in production, but common in the industry, fortunately now being superseded by advanced packaging techniques that can accommodate the two quartz blanks in a single surface mount package.

The method of measuring crystal filters I have been using recently is to measure the S-Parameters of the unmatched filter using the 450 ohm vector network analyser (VNA) I developed at Golledge. The 450 ohm normalised S-Parameter data can then be analysed in a circuit simulator with the termination impedances set as required for the crystal filter. 450 ohms is used in place of the conventional 50 ohms as it is closer to the working impedances of crystal filters, usually in the range 300 ohms to 3k ohms. Measurements in 50 ohms are not accurate enough.

What I was comparing today was approach 2) measuring the two packages of the crystal filter separately then combining them along with an ideal centre coupling capacitor, with approach 1) measuring the two packages connected with a ceramic centre coupling capacitor as a single filter.

Here is the simulation running both approaches

The separate package approach has benefits when measuring the stopband as test fixture stopband leakage is less of a problem.

Now that’s what I call stopband performance

Some ancient wisdom on coil winding

Occasionally I come across individuals who have been in the industry a very long time, and have become great experts in their field, often ending up working as independent consultants. Bruce Carsten is one of them. He even has a grey beard, at least on the probably quite out of date photo on his website. Ancient RF experts were once referred to as “grey beards”, back in the days when it was more wizardry than the modern CAD driven engineering of today. The gems of knowledge that attracted my attention were Bruce’s work on inductor and transformer design. Some excellent papers of his can be found on the Micrometals website. Happy reading.

RS is 80!

Working in an industry where rapid change is the norm, it comes as a bit of a surprise to learn that RS, one of the dominant 3 broadline distributors has been around for 80 years. The others are Farnell and Digikey.

RS was originally called Radiospares, the company started in 1937 in the UK. They now have operations in 30 countries across Europe, North America and Asia. Although I occasionally use the others, plus Mouser, RS is my preferred choice of component supplier (other than Golledge of course) because their component search engine is a bit more sorted than the other broadlines, their lack of a minimum order value, and free next day delivery. The free delivery must be a loss leader for some small orders. I wish them many more prosperous decades to come.
More details on RS being 80 can be found here.

SamacSys Component Search

This is a great CAD library, search tool, and library part builder. It works with most of the common electronic circuit CAD programmes. The search interface can be found here. I particularly like the quick component builder that lets you create your own library elements and then share them with other users. It has already saved me a great deal of time, as it is much faster creating new library parts for Eagle than using the component editor within Eagle, even when starting from a similar part already in Eagle.
For the uninitiated Eagle is the schematic capture and PCB design tool I use at Gollege.

Ultra Miniture Antenna Using Acoustic Resonators

I have recent ley come across academic papers making big claims in radio antenna miniaturisation. This paper in Nature using SAW resonators and this one in IEEE Transactions on Antennas and Propagation using bulk acoustic wave (BAW) resonators. It may be quite a while before this technology is available as usable devices on the market, but it always good to know what is on the horizon.