September 12, 2018

Test Equipment You Can Build ...

~ Crystal Measurement using the SNA Test Gadget ~

Siggen + D.U.T. + Power Detector ==> Crystal Parametric Measurement


A simple Test Gadget daughtercard contains both a Siggen and and an RF Detector, that plugs into either the Nano- or ESP32-based Test Gadget motherboard.


Our topic this month considers the combination of two test gadgets developed in previous installments into a single measurement system called the Scalar Network Analyzer.  By connecting the Siggen test gadget to the input of a Device Under Test (like a crystal filter), and the output of the D.U.T. to our RF Power Meter test gadget, we have a complete loop that can be used to determine the response characteristic of that crystal filter.  In other words, we squirt some RF into a filter and measure how much of that signal comes out, thus giving us a good feel for how that filter will perform in a given circuit implementation.

Additionally ... We have some good progress to show for our two "standardized" Test Gadget platforms:  (1) The Nano Test Gadget, for quick & simple Gadget experimentation; and (2) The ESP32 Test Gadget, for powerful simultaneous control and graphical display for all Gadgets we develop.  Prototype PCB motherboards are now in-hand for each of these Test Gadget platforms, and a PCB is in-progress for the SNA Gadget that we're talking about here this month!

 73, George N2APB, Joe N2CX, Dave AD7JT and DuWayne KV4QB


CWTD Episode #90:  Crystal Parametric Measurement using the SNA Test Gadget!



First ... Scalar Network Analyzers






Crystal Filter Measurement

WHY measure a Crystal?

  • To determine if crystal still works!  (i.e., mechanically intact)

  • To determine how well a crystal works (e.g., its Quality Factor)

  • To determine or verify the frequency of a crystal (e.g., if frequency not noted on part)

  • To sort them for individual, exact-frequency use (e.g., crystal in LO to match the corresponding IF filtering being used ... in SW30+)

  • T sort them for combined exact frequency use (e.g., multiple crystals for use in IF filters, as in the SW30+)

  • To determine circuit parameters (Cm, Lm, Rs) ... for equivalent network design and analysis)

What Is able to be measured?

  • Center Frequency (Fc)

  • Bandwidth (3dB, in Hz)

  • Passband Ripple

  • Insertion Loss (in dB)

  • Series Resistance (Rs, in ohms)

  • Motional Capacitance (Cm, in pF)

  • Motional Inductance (Lm, in mH)

  • Quality Factor (Q)


HOW we will Measure Crystal(s)

Referring to the Block Diagram below ...

  • Arduino commands Siggen to generate a "range of frequencies" at-and-around the crystals' given frequency ...which in our case is 7.68 MHz, just as in the SW30+ project!

  • The crystals filter that incoming square wave to produce a sinewave-like output from "a little before" to "a little after" the crystals' given frequency.  This range is the crystals' bandwidth (BW). 

  • No other frequencies get through the crystal network, and thus the crystal filter is able to effectively do its job of only allowing the signals of interest to get through in the IF stage of a superhet. 

  • The RF Detector samples that filtered output of the crystal network to produce an analog DC voltage that is proportional to the logarithm of the input RF energy.  Thus, this block is also called a Log Detector.

  • Why a logarithmic relationship? ... A much-improved Dynamic Range is able to be measured!

  • The Arduino controller inputs this this log signal using its built-in Analog-to-Digital Converter (ADC) and computes "Many Things" ... which is the function of an SNA.


Detailed Block Diagram

Referring to the Detailed diagram below ...


SNA Test Gadget

The Test Gadget pcb being designed and laid out right now has an Si5351 chip as a siggen, and an AD8307 log amp as the RF detector.  These two signal paths are accessible to a D.U.T on one's workbench by means of some BNC connectors.

However as a convenience, we are also placing a "prototyping grid" of 0.1"-spaced pads that one can put specific circuitry onto without needing to go off-board, thus saving some connectors and cabling.

Further to assist, especially in relation to this episode's topic of Measuring Crystals, we're placing dedicated circuit traces to accommodate the crystal sockets and interface networks for specifically making these measurements, without the need for external cabling, muss or fuss.  All one needs to do in this case would be to add jumpers at the input and output of the crystal circuits to pump the output of the Siggen into the crystal circuits, and another jumper to take the outpu of the crystal network and sent it to the AD8307 log detector.

Of course, the SNA Test Gadget plugs into either the Nano Test Gadget motherboard, or the ESP32 Test Gadget motherboard for actual operation.



AD8307 RF Detector

The circuit for the AD8307 RF Detector is lifted from the straightforward implementation of this log amp on the Midnight SNA instrument. 


AD8307 RF Detector ... using circuit from the Midnight SNA


SNA Test Gadget, v0.5 ... by Dave AD7JT

This is hat we have so far in the design and layout process.  The orientation of the board and BNC connectors may change as we finalize things, but these are the basic components.  (The sharper eye will spot a 5-pole elliptical filter just to the right of the Si5351 siggen chip (U2) in the lower-left corner of the board rendition.  We are considering adding such a filter to optionally remove much of the odd harmonic energy and thus provide a much more sinusoidal signal for use in the testing process.  There may be some instances whereby such a purer signal is helpful in accurately representing the signals coming from the Device Under Test.  (For example, a classic "reflectometer" for measuring reflected energy from an antenna and feedline would very desirably only want to consider the fundamental frequency, and not the harmonics that would also be reflected and thus skew the SWR.)


SNA Test Gadget, v0.5 ... by DuWayne KV4QB

DuWay is taking a parallel path ith his implementation of the SNA Test Gadget, but currently without an onboard prototyping area or dedicated circuit traces for specific applications.  Which approach (AD7JT or KV4BQ) will "win out"?  Maybe both, as the beaty of Test Gadgets is that many different incarnations may be used within the common pinout structures used on the Nano and ESP32 Test Gadget motherboard!



The Nano Test Gadget ... by DuWayne KV4QB

This is DuWayne's Nano Test Gadget fixture pcb installed in a custom-made 3D-printed enclosure.  Pretty nice!


The ESP32 Test Gadget ... by DuWayne KV4QB


PCB and Partial Kit Planning

1) PCBs and Parts -- We’ll certainly be getting a feel for interest levels in the two Test Gadget motherboards (Nano and ESP32 versions), and the expected stream of the dedicated Test Gadget plug-ins themselves.  We’ll certainly offer the pcbs, and perhaps “partial kits” containing the PCB and most of the board-mounted components, such as voltage regulator, Rs & Cs, sockets, and joystick and rotary encoders.  Homebrewers would only need to get the displays and processors on their own via links provided for each vendor.

 2) Enclosures – DuWayne’s 3D printed case is really beautiful and would be highly desirable by all, but it is clearly not able to be mass produced. We intended to have users obtain their own preferred commercially-available enclosures, based on their specific implementation plans.  For example:

  1. My Nano Test Gadget and the 2x26 LCD are mounted on a small piece of blank copper-clad board as a base.  The base also contains room on both sides to accept the individual Test Gadget plug-ins to facilitate open probing and experimenting, which is the primary intent of this smaller “individual & interchangeable” platform.  I'll be using some standard signal connectors on this copper-clad base board, thus enabling easy connection to the interchangeable Test Gadget plug-ins.  I have a sizable amount of copper-clad material, so perhaps such a blank baseboard would also be of interest to Chatters (i.e., to the CWTD audience).
  2. My ESP32 Test Gadget, on the other hand, will be mounted inside a moderately-sizable commercial enclosure, maybe about the size of an Elecraft K3. The user interface (display and joystick) will be brought out to the front panel, and room will be available inside for numerous Test Gadget plug-ins to be connected in a stacked/parallel fashion inside the enclosure on both sides of the ESP32 motherboard.  In this way, this powerful “multi-gadget” platform can better survive in regular, extended use on the bench as a hub for ham/RF-based test and measurement needs in the shack.  And of course the various connectors would be on the rear panel, appropriately labeled for the intended signal and purpose, as you describe.

TUTORIAL ... Measuring Crystals  <== download this pdf!

Crystal Measuring ...

Crystal Matching ...



Crystal Filter Design for the Ambitious Designer!

Several references:

To design filters, you need to match the crystals to within a small fraction of the desired bandwidth ... and then determine the crystals' parameters. 

There are methods described in :

How to actually perform the Crystal Measurements?

  • A very practical approach is outlined in Simplified Tools and Methods for Measuring Crystals by K8IQY

  • An automated method using the Midnight Design Solutions SNA can be found in a thorough tutorial written by Dave AD7JT called Crystal Characterization and MatchingThe crystal test fixture (CTF) described in this document is based on the K8IQY method. 

  • SNA Jr. II ... by DuWayne KV4BQ

  • A configuration using our Siggen Test Gadget and our RF Power Meter Test Gadget (or another RF detector circuit like that used in the N2CX Accuprobe) is another good way to manually determine the crystal parameters needed for the design program