CWTD - Nov 26, 2013

(Now a bi-weekly program)

November 26, 2013

Troubleshooting Techniques ... Live!!
Measuring and diagnosing a 10 MHz oscillator circuit LIVE and in REAL-TIME over the Internet

Overview

We have a groundbreaking, trail-blazing, state-of-the-art program planned this time as we attempt to get a 10 MHz variable crystal oscillator (VXO) working that happens to be sitting here on the bench at Chez N2APB. The circuit is the Crystalizer, a fun and useful RF oscillator that we designed and made available to all attendees of an Atlanticon QRP Convention some 8 years ago.

But the WAY COOL part of our program this time is that we will be finding the fault(s) in this known-working circuit by probing with a DVM, viewing oscilloscope traces, and making spectrum analyzer measurements... interactively, in real-time, and viewable in real-time video streams over the Internet!

So in addition to learning some troubleshooting techniques and approaches from the more experienced homebrewers on the program, and being able to ask questions like "why probe here?" or "what does that signal look like?", you be able to SEE the measurements being made! Imagine having a circuit that produces a "distorted RF waveform that almost looks sinusoidal but the negative half is skewed to the left a bit and there's a squiggle at the positive peak" ... We'll actually be able to SEE the waveform!   Man, talk about a picture being worth a thousand words!

So put on your thinking caps and help us determine what the problem is in our 10 MHz oscillator tonight!

73, George N2APB & Joe N2CX

Audio Recording ... (Click to listen or download)

Discussion Notes:

<20:08:53> "Alan W2AEW": George - I'll forgive you in advance for the A* scope ;-)
<20:23:27> "Eamon - VE2EGN": The circuit is working ... so what needs fixing?
<20:30:01> "Jay K4ZLE": with 4.9 volts from reg, is the p-p output sig higher than when reg volt was low?
<20:30:36> "Alan W2AEW": mis-wired pot?
<20:31:07> "Pete - WB2QLL": Bad ground on U1?
<20:32:17> "Eamon - VE2EGN": I was going to say bad ground on U1 too.
<20:32:37> "Eamon - VE2EGN": That's the only thing that could perhaps cause it go restrict the current.
<20:32:54> "Eamon - VE2EGN": If there were an open on U1 in or out, it would just not work at all.
<20:33:02> "Alan W2AEW": Is the main supply current limited?
<20:34:35> "Nigel VA2NM": why not try using a 9V battery instead of the 78L05?
<20:34:35> "Joe N2CX": But a missing ground could foul up operation of the regulator
<20:35:07> "Eamon - VE2EGN": If we had negative resistance in the circuit there could be a startup / inrush problem with insufficient current limit on the supply, but that's not the case here.
<20:37:10> "Eamon - VE2EGN": Nigel, we have to make it FAIL before we can fix it.
<20:37:21> "Joe N2CX": MV204 - dual MV104...
<20:38:12> "Mike WA8BXN": how much variation in frequency do you expect?
<20:38:15> "Nigel VA2NM": trying to isolate the suspicious component.
<20:38:50> "Alan W2AEW": But, you'll also be adding about 10pf across R5 from the probe
<20:39:22> "Jay K4ZLE": first step in prob solving is define the problem --- what is the problem. This ckt appears to be wkg!
<20:44:02> "Jay K4ZLE": camera definitely adds to presentation
<20:44:35> "Dave W4VU": Video is great - and this is my first time at CWTD
<20:45:24> "Alan W2AEW": You'll never saturate Q2 since there is no collector load
<20:45:33> "Eamon - VE2EGN": maximum swing
<20:45:44> "Eamon - VE2EGN": or close
<20:45:46> "Alan W2AEW": Q2 is a CC amp - no voltage gain
<20:46:55> "Alan W2AEW": Here's my tutorial on common emitter, common collector (emitter follower) and common base transistor amp configurations: https://www.youtube.com/watch?v=zXh5gMc6kyU
<20:48:19> "Alan W2AEW": stick a 450 ohm resistor in series with the output to the SA input. thus, load will be now 500 ohms, and SA has enough dynamic range to account for the 20dB loss
<20:49:03> "Alan W2AEW": what is you CF and Span?
<20:49:09> "Joe N2CX": Yes Allen that gives an impedance that the EF can drive linearly.
<20:49:15> "Eamon - VE2EGN": dc in the middle?
<20:49:42> "Alan W2AEW": yep, you have the DC spur in there. Set CF to 10MHz and span to something <20MHz
<20:49:44> "Joe N2CX": Eamon, yes.
<20:50:11> "Alan W2AEW": or set start to <10MHz, and stop to >50MHz so you can see harmonics easier
<20:51:34> "Alan W2AEW": loading with 50ohm SA input is likely introducing distortion from buffer amp. series resistor will help keep output linear
<20:52:21> "Ray K2ULR": Not here, Joe... whenever George keys up the video, I get it right away.
<20:58:37> "Pete - WB2QLL": What's the purpose of R4?
<20:59:55> "Joe N2CX": R4 Supplies a DC bias return for the top varicap diode
<20:59:56> "Alan W2AEW": that's exactly what I'm doing, Jeff
<21:01:49> "Alan W2AEW": If you still have the scope probe on R5 - that's adding about 10pF to ground at that point
<21:01:55> "Eamon - VE2EGN": series cap
<21:03:42> "Eamon - VE2EGN": It would be real cool to put an audio tone in and listen to FM
<21:04:03> "Joe N2CX": Very narrow band FM!
<21:04:16> "Jeff - W7WWA": modulate via the varicap?
<21:04:34> "Eamon - VE2EGN": that's true, it would be pretty narrow
<21:08:12> "Alan W2AEW": GREAT book!
<21:08:43> "Alan W2AEW": http://www.amazon.com/Troubleshooting-Analog-Circuits-Series-Engineers/dp/0750694998
<21:11:46> "Joe N2CX": We control the horinzontal, we control the vertical - Twilight Zone!
<21:12:35> "Terry WB4JFI": Actually, The Outer Limits!
<21:15:00> "Rick K3IND": qik is owned by Skype
<21:15:15> "Terry WB4JFI": Thanks George and Joe!
<21:15:16> "Rick K3IND": which is owned by MS
<21:15:43> "Terry WB4JFI": Another advancement in CWTD!
<21:16:39> "Jeff - W7WWA": Yeah, delay about 2 seconds here. Not bad at all.

CWTD Episode #65: Troubleshooting Techniques ... LIVE!

Here's the opening segment of the live video feed we had going during showtime ...

For more, see the video segments at the bottom of this page.

How We'll Be Doing Things

Normally here on CWTD we just post a bunch of static images (photos, schematics, diagrams, etc) and point people to different parts of the graphics that we happen to be discussing.

This time, we'll be working with a master schematic (shown below) and we'll be showing a live video stream over the Internet, viewable at www.qik.com/cwtd, where you'll be able to actually see me (N2APB) making the measurements with the DVM, scope and spectrum analyzer.

So the idea is that we will be conducting our CWTD audio program just like normal on Teamspeak, and listeners will just open another browser window to watch the videos at the QIK site. Pretty cool, eh?!

And the way we'll be working the program is that we'll have a fault (or two) on the board (short, open, wrong value component, wrong voltage, etc) and through "group iteration and dialog" we'll trace down the problem and fix the circuit ... all with real-time viewing of the measurements (and foul language(??) that often comes about when trying to get something working ;-)

Viewing the Streaming Video

Some browsers will be able to see the video stream "live", while others will be able to click a link to download and play each short video segment that we make and post. Regardless, even our podcast listeners will be able to see the videos later on by going to our CWTD channel there on the QIK site.

You can test your system in advance to see that you can access the QIK site and view the videos we plan on having there during the show. Just go to http://www.qik.com/cwtd and click on on of the test videos we have posted there right now. You will need to have the Adobe Flash Player loaded on your computer in order to view these videos ... so you should get that common driver loaded if it says so in the window where the video will be playing. And yes, platforms like the iPhone and iPad that do not have the ability to run Flash videos will not be able to participate in the streaming video part of our program. (Macs are okay.)   Afterwards, we'll have the video clips loaded here on this page for you (and the podcasters) to view later.

    This is the QIK website (www.qik.com/cwtd) ... just double-click on the video clip of the Crystalizer above to test the process.

The Instruments We'll be Using

Here are some quick 'n dirty snapshots of the instrucments we'll be using "live" to measure and diagnose the Crystalizer circuit ...



The Circuit

The Crystalizer is a 10 MHz variable crystal oscillator, or VXO, that can be put to use in many ways around the shack. Once calibrated to WWV, this VXO can serve as an accurate frequency standard for receiver alignment, or as a PLL standard, or even as an LO for a transmitter. (Note: Some temperature control/stabilization will probably be necessary to prevent drift of the VXO frequency ... which was the subject of Atlanticon "contest" back then :-)

The Crystalizer produces an approximate 2V p-p signal at RF Out when C3 is in the top position (C3b on the pcb). This signal is quite ragged but very suitable for driving a balanced mixer like an SA612. (Think “producing a beat frequency with the WWV RF signal being placed on the other input of the mixer.”) When C3 is placed in the lower, “C3a” position on the pcb, a 200 mV p-p near-pure sinusoid is delivered at RF Out, suitable for transmission, after amplification, if desired.

It is possible to “bend” the crystal by adding a 10K pot to the control line of a varicap diode across the crystal in order to place the oscillator dead-on at 10 MHz. The Adjust signal on connector P1 pin 1 can be the wiper of a simple potentiometer connected between 5V and ground that presents a nominal 4 volts to varicap D1.



Crystallizer Troubleshooting Tips

-        Verify that all of the component values are correct.

-        Note that there are two 220 pf capacitors and only one 22 pf

-        Some resistor color bands are tough to distinguish

-        Ensure that components are installed in the correct locations.

-        Tabs on Q1 and Q2 and flat on U1 are shown in instructions

-        Other component locations are shown in instruction photo

-        L1 is generally not needed, a short length of component lead takes its place

-        C3A and B are correct on board layout, instructions are backwards.

-        Check carefully for hairline shorts or bad solder joints

-        DC tests

-        Is U1 hot? Baad, it's installed backwards or you have a short

-        There should be 5VDC on one side of C7

-        If not you may have a short

-        Is the input DC above 8V?

-        Q1 voltages

-        Collector ~ 5V

-        Base ~ 2.5V

-        Emitter ~ 1.8V

-        Q1 voltages will likely change if you put your finger on the base lead

-        Q2 voltages

-        Same as Q1

-        Is oscillator errr... oscillating?

-        Make sure you have some DC on the Adjust pin

-        Check for RF at emitter of Q1 3-4V P-P on emitter

-        If not check component locations or shorts

-        Can you hear a signal on a nearby receiver set for CW?

-        With a diode detector and DC voltmeter you should see about 1V or more on Q1E

-        If output from Q1 but not Q2 check to see that C31 or C3b installed.

-        With C3B, output is only about 200 MV p-p

-        Output distorted

-        Should be ok on hi-Z scope

-        WILL be distorted if 50 ohm load or spectrum analyzer or low Z freq counter is used

-        Not designed to drive low impedance

-        Frequency does not change with variable Adjust voltage

-        D1 installed incorrectly

-        Wrong R1 value installed

-        Needs 100k for isolation

-        Xtal goes out of oscillation with some tuning voltages

-        Could be normal with a low activity crystal.

-        Interesting observation!

-        Component leads are “live” on bottom of pc board. If you lay it on a metal bench or even some black SD foam you can cause inadvertent shorts

More Segments from our Live Video Feed during this Show

Segment 2: Overview of the test bench and instrumentation before we get into the troubleshooting activities.

Segment 3: Initial discussion of the circuit operation and connection points.

Segment 4: Why was the 5-volt bus being pulled down?

Segment 5: Measuring the control voltage to the Adjust line ... trying to "bend" the crystal.

Segment 6: Measuring oscillator's better (more sinusoidal) signal output on oscilloscope. And showing "to scale" button on the scope to view better.

Segment 7: Checking out the 10MHz signal on the spectrum analyzer.

Segment 8: Adjusting the spectrum analyzer to see the spurious components.
.

Segment 9: Best view of the spectrum analyzer screen ... and analysis.

Segment 10: Views & description of measurement cable routing and Frequency measurement ... while adjusting the Crystalizer "frequency adjust" potentiometer.

Segment 11: Final measurement ...feeding the oscillator signal into a ham receiver to illustrate how "bending" (varying) the Crystalizer frequency can be detected as a changing audio tone in the radio.

Segment 12: Wrap up & thank you to all "live" attendees and podcasters. Please visit our home page at www.cwtd.org to see list of all past episodes and podcasts.