Making a headache by musing on electronics measurement

I have been experimenting in a field well outside my comfort area! I have started to play with RF construction and measurement. I have a reasonable set of test gear from my Radio Ham days but very little of it is suitable for serious "Research". Well I will never have the best gear but I either have or can build some measurement tools that let me start to get a better feeling for the fundamentals of RF.

Currently I have an RF Explorer Spectrum Analyzer (which is very good for it's price) and an RF Explorer Signal Generator (also a bargain). These together cover about 25Mhz to 2600Mhz (actually a bit more as the generator go's up to 6Ghz and the analyzer go's down to 10Mhz)  but I have started building some home brew kit to cover from about a Megahertz up to about 50 Mhz.

Signal generator / recording voltmeter

Front Crystal test jig / Back 3db splitter

Overview of setup

The first picture shows a PSOC5LP development board and an AD9851 module. This board takes the output from the AD9851 and converts from the 200ohm filter output to a 50ohm SMA socket using a bifiliar wound impedance matching transformer.  The PSOC5LP also supports a high resolution ADC which is used to measure the output voltage from the Log Amp that is in the diecast box on the left.
The code on the PSOC5 is partially mine and partially stolen from other open source projects. This code lets the PSOC configure and drive the AD9851. It also reads the high res ADC fed from the Analog Devices logamp.

The second picture shows a crystal test jig that is basically two SMA sockets with a pair of 4:1 impedance transformer to better max the crystal characteristic impedance which is around 111ohms.

You can also see the Mini-Circuits splitter (ZFSC-2-2500-S+) that I used in my tests.

I firstly ran the test with a wire jumper in place of the crystal, With nothing between the generator and the Crystal test rig, I then inserted a 32Mhz crystal and took another set of readings. I then inserted a Mini circuits splitter setup with a 50ohm dummy load on the second output. Next was a home brew resistive splitter also terminated on the unused output .



The blue line is the reference run with the short circuit instead of the Crystal.
The orange line is the setup with just the crystal.
The yellow line is with the 6db attenuation of the resistive splitter.
The grey line is replacing the resistive splitter with a high quality (transformer) splitter rated for 10-2500Mhz usage.


As you can see the grey line is the closest to the expected classical shape. at the extreme right the lines are spaced as expected in 3db intervals (The sensor has a slope of 25mv per db of power detected).

The left hand side is not as expected. The splitter insertion loss showing as near zero!
Also at the anti-resonant point the shape is much closer to what I would expect.

However as you see I have a lot to learn and understand. My head hurts!

By the way all of this is because I want to build a few crystal filters and to do that I have to characterize the crystals (I have a bag of 100 32Mhz crystal purchased very cheaply on ebay).

G4DCP - Peter D Hull