Wes (W7ZOI) on Oscilloscopes and Test Instruments

In a recent mail from Wes (W7ZOI), he discussed oscilloscopes and test instruments. I thought it was worth sharing his comments with the readers.

Wes wrote -

I used to argue that there were three classes of oscilloscope:

1. No scope at all
2. A scope that would provide an indication of what was happening in a circuit
   
3. A scope that was capable of measurement.

There can be real danger when using a scope though. A major flaw is the "perfect sine wave" syndrome. Folks will look at the output of their 14 MHz transmitter on a 20 MHz scope and see a sinewave. Of course it's a sinewave, for all of the harmonics are well into the attenuation part of the scope bandwidth. That 20 MHz scope is not really useful to see all of the details of a waveform beyond perhaps something for the 160 or 80 meter bands. You need that 465 for anything in the HF spectrum. Still, you can do really a lot with a very modest box.

For years my home 'scope was a Tek T-922, which is a dual trace 15 MHz box. It has a plastic box, so it is a problem for transmitter testing. Still, I could do a lot with it. The key is to understand something of the limitations.

I agree with the digital versus analog meter. An analog meter is a really useful thing. But the DVM is also handy and they are so cheap that one can have both. A frequency counter is, as Farhan noted, extremely useful. If you are going to do work with crystal filters, or oscillator stability, you want it to go down to 1 Hz resolution. That is so easy these days that you may as well build it that way. Don't bother with a grid dipper (or anything else that might dip.) That was for the beginning of the last century.

One of the blog readers commented that a high Z volt meter was a good thing. Na, not really. Yes it is good, but you can get there with an op-amp and a 1000 Ohm per volt meter. So I would opt for a rugged, dependable meter that you could use to measure lots of current or normal voltages. Then I would build a high Z input section for the sensitive measurements, and would also buy a cheap DVM. Actually, I have a Fluke 73 DVM that I have really enjoyed. In an earlier time, a VTVM was the center of my lab, but that ended when I got my hands on a proper 'scope.

DC 40 - A Direct Conversion Receiver

I have just posted a new project on my website:
A simple direct conversion receiver

• You can assemble it in one evening. Just six transistors and any audio amp IC.
  
• It does not use any special components. All out of your junk box.
• Quite a performer. It will probably beat your general coverage, synthesized all mode transceiver with its reception.

The article also traces our journey from idea to finishing the project.

Contributions Needed

Why blog?
Quite a few people have asked me this question. There are quite a few pretty good websites, a great podcast and the news group at rec.radio.amateur.homebrew is pretty active too. Then why should one start a blog? As someone commented, a news group should be enough to discuss most things.

I think that there is a place for everything. The BITX20 required a group of its own due to the sort of discussions it generates (Hans created it to take the discussions offline from the homebrewing news group). The Yahoo group also allowed us to post circuits, swap videos and document the modifications each one of us has been making.

Similarly, Bill's podcast is an armchair QSO. Almost like listening to a ragchew on a quite evening with your favourite caffeine in a steaming mug.

A blog is an easy to handle website that you can update incrementally. It requires little involvement in terms of layout and content management. I will just post if I find something useful to share. It helps to be able to start a topic, post a circuit, discuss and archive it and move onto the next topic.

Looking for contributions
I am looking forward to receiving suggestions, circuits and ideas from all the readers. I will take it upon myself to clean them up and put them on the site. I will, of course, give credits to you. My only requirement is that you draw your circuits in your favourite painting program using the using these symbols (These symbol images are from Harry Lythall's excellent website):



Send me your contributions to my gmail account. My gmail ID is afarhan. (Figure out the email yourself. This is to confuse the email harvesting programs).

Measuring Inductors using a frequency counter

Winding coils and finding their exact inductance is a problem for most beginners. Capacitors and resistors come marked but you have to build your own inductors using whatever comes handy.

The test oscillator shown in here is a very handy thing that can be used to easily measure the inductances and a little bit of work on a calculator.

The Circuit
None of the capacitor values are critical. If you don't have 680pf handy, try 560pf or even 100pf if those are the only ones you have. Almost NPN transistor will work. The output is attenuated down to a 50 ohms impedance through a series drop formed by the 470 ohms and a 50 ohms (actually two 100 ohms resistors tied in parallel). Althought I have shown it to work with a 9volt supply (using a PP3 battery), you can easily power it by the 12v lab suppy.

Operation
Solder the ends of the coil between the 'hot' end of the 330pf capacitor and the base of the oscillator transistor. Power up the oscillator, connect it's output to your frequency counter. Read the frequency.

Calculating the Inductance

1. First, figure out the total capacitance in the circuit. For instance in my circuit, there are two 680pfs in series with each other, their effective capacitance will be 340pf. Then the 330pf between the ground and the inductor will also appear to be in series with the 340pf, hence the effective capacitance will be 165pf. Remember this value for all your calculations in the future.
   
2. Now, divide 1000,0000 by 6.28, then divide that by the frequency read out. square it result and then divide it by the capacitance value from step 1. You have the inductance in uH (micro Henries).

There is a bit of calculation involved. But it is pretty accurate and easy way of doing it. This is essentially what all the LC meters do. Except that you have to manually calculate in our method and the LC meter does it for you.

Thoughts
This method also shows how you can make your own measurements that are as good as those possible with professional equipment if you spend just a little more effort (like using a calculator) to arrive the readings.
This is one of the many applications we will see for this interesting test circuit in combination with the frequency counter.

The Homebrewer's Lab

We are fascinated by the pictures of famous homebrewer's labs. How did they assemble it all? How does one create a junk box? I will try answering those questions in my first post.

The first question usually is about What do I require in my lab? The answer an unhelpful 'depends'.

A Soldering iron and a nail clipper - The impossibly minimal homebrewer
Minimum requirement is a soldering iron. You also require a way to strip and cut wires (forget about using your teeth). I (and my cousin) have used nail clippers to do that. A few 9 volt batteries can power quite a few projects unless you are planning on building power amplifiers.

The Minimal homebrew lab is really really minimal. Unfortunately, to use it you have to be an expert. So it is not really an option for the beginners. Lets see what are the requirements for a decent homebrewer's lab.

A better budget - $200
Setting up a small lab will take some money. You cannot build everything yourself. Also, investing in some good tools is always worthwhile. So here is my list of things:

• Soldering iron: Buy a decent 25 watt iron that has an earth pin too. The chinese irons lining up at your local DIY store are good enough. Check that it has an earth pin too.
• Analog VOM: As a kid, I tried getting by without a VOM for a long time! Don't do that. Buy an analog VOM. It will work without batteries too. The digital VOMs are not a good idea (though cheaper). The analog meter allows you to monitor fluctuations easily. It also works without using the battery (except to measure resistances).
• A small wire cutter. Buy a nice one. Check that the jaws close without leaving a gap and they are made of hardened steel.
  
• A small plier. Same as above. Make sure that both have insulating sleeves on them!
• A screw driver set. The one with interchangeable tips is fine to start with. Don't buy the precision set. Most of them are too small to be used for homebrewers. Buy the sort that has bigger tips that you can use to open PC cabinets etc.
• A good roll of solder. Bad irons and bad solder have dissuaded too many builders.
• A small hacksaw. You will require this for everything from sawing stalks off volume controls to cutting copper clad boards. Buy a few spare blades while you are the shop. Don't buy the big standard hacksaw - it is too unwieldy for most radio work. If you have to buy just one, buy the small hacksaw or buy both.
• Drill. Even a hand powered drill is good enough. Buy a set of drill bits with it too. Electric drill is an overkill. It is too big for the QRP projects. Unless you aspire to be the governer of California, stick to a small drill.
• A flat file and a round file. Buy a small round file that is about 0.2 inch thick and with a fine score. You will need this to enlarge holes in boards. Buy a flat file with a similar score that is about half inch wide. You will need this to straighten the edges of your boards.
• Car battery charger (12V). Ask your mechanic to suggest a cheap one. If you add a 78H12 or 7812 to it, you have a ready and robust power source available.
• A Small Vice. Buy a vice that you can clamp onto your work table to hold a board while you file or saw it. You can usually get by with keeping the board at the edge of a wooden chair or bench and clamping it down with your foot. One hand holds the board and the second hand can work the saw/file.

A second hand Oscilloscope - the big buy

I am not being ridiculous. After living without a scope for years, I finally realized that without an oscilloscope, you might as well try homebrewing blindfolded. As they say, a picture is worth a thousand words. So, to be really successful at homebrewing, you should have a decent scope. This is a the big ticket investment. Make it. I don't think that any serious homebrewing is possible without an oscilloscope. This is a dramatic statement and I am sticking to it.

Get onto eBay and look for a good buy. What should you look for? here are my 5 considerations (I am not an expert, I have bought just one oscilloscope in my life):

1. At least 20MHz bandwidth. More is fine. But 20 Mhz scope will easily work to 30 Mhz. If you can afford better, buy it.
2. Buy an aligned scope. Scopes go out of alignment routinely. Buy it from someone who deals in scopes and can offer you an aligned scope.
   
3. Get the manual including alignment procedure. It is a fairly elaborate drill to align an oscilloscope. Most oscilloscope dealer will align it for you before selling it. This is an important consideration.
4. At least two 10x probes. The 10x probes will attentuate the signal a factor of 10. But they will load the circuit very lightly. Check that the probes are also aligned for you.
5. Don't buy the cheapest. An oscilloscope is a prized possession of a builder. It should last your lifetime. Buy a Tektronix if possible.

I have an Tektronix 265-B. It bought it for $200 with probes about 3 years ago. It is a very fine thing that is rated for 100MHz but allows me to see upto 200Mhz. That way I can observe any VHF parasitics and also occasionally indulge in VHF experiments.

The Frequency Counter
A frequency counter is really a must. You can't survive without it. You can use it to do a lot of things apart from measuring frequency. You can measure capacitors, inductors, transformers, even align crystal filters.

These days it is so easy to build a counter. Just google around for a PIC frequency counter or if you want something really simple, try Han's amazingly simple counter.
Many hams that have told me that they are put off by having to program the PIC. Programming is a PIC is really simple. You don't have to know anything about programming it! All you need to do is solder a simple circuit, connect it to your PC, plug in the PIC, then just run the software to all it to do its magic ... abracadabra! you have a frequency counter. Make this your first homebrew project. If you are a chicken, then you can buy one from any of the ham radio kit suppliers. Anything that goes up to 40Mhz is fine.

Some counters are noisier that others. My own counter uses LEDs instead of LCDs and I find that the noise in a circuit goes up when I connect it. I am OK with that. I never use the counter on radios in service. I would guess that the LED counters are inherently noisier that LCD counters.

Which ever counter is used, have as many digits as you can (mine has 10Hz resolution). House it in an all metal enclosure and make it have it's own power supply. (Rip out electronics from a wall wart 9 volts battery charger and add a 7805 regulator to it).

Ready to build
You will realise that except for the oscilloscope and the frequency counter, the rest of the tools required are no more that those required to build a simple kit. While our instrumentation is minimal, you will realise that you are now able to build the rest of the instruments using these are tools. For instance, even building a spectrum analyzer is possible with just these tools. More on that later. Now to populate your junk box ....

Resistors, Capacitors, Inductors, Transistors and ICs
These should be available as assorted packs from several places. I am told that RadioShack stores as well as www.kitsnspares.com has several assorted packs. If you are assembling a new junk box, here is my recommendations. Buy all the resistors and capacitors in packs of hundred. They are very cheap that way (at least in India).

• Resistors (all quarter Watts): 220K, 100K, 22K, 10K, 2.2K, 1K, 220 ohms, 100 ohms, 22 ohms
• Capacitors: 0.1uF, 0.001uF, 1uF, 10uF, 50uF, 560pf, 220pf, 100pf, 47pf, 22pf, 2.2pf.
• Transistors: npn garden variety, low power transistors like 2N2222/BC148/BC149 that can be used for audio as well as RF work with Ft above 200MHz. Buy 100.
  
• FETs: low power versions like MPF102 or BFW10s (buy a dozen) and a couple of high power HexFets IRF510 or IRF511.
• ICs: not too many, a couple of LM386s and some good quality Op-Amp ICs like the LM3352.
• Odds and Ends: 22pf standard trimmers, 365pF broadcast radio variable capacitors, 10K Log volume control with on/off switch, 100K linear pots, DPDT toggle switches, RCA audio jacks and sockets, 'Banana' jacks and sockets, shielded audio grade cable, 10Mhz, 4MHz and 14.318 MHz crystals.

I buy the plastic lunch boxes available at supermarkets with small partitions inside them to store the components. Two such boxes store most of my components.