Golborne Vintage Radio

One or two forum mebers have mentioned scope calibration, and I've referred to it in my posts about crystal calibrators. Given that most of us first add a scope to our range of test gear, then when we've got it, try to learn how to use it, maybe a few words of explanation about scope calibrators might be of interest, though I'm not setting myself up as an expert. First, I might be stating the obvious as to what a scope does, but basically it draws a graph of voltage - the vertical ('y') axis, over time - the horizontal ('x') axis. If you get x and y mixed up, think 'y' as in sky, which is up, and 'x' as in 'a cross'. These two axis are calibrated in precise units of Voltage and time per centimetre of deflection, with ther graticule marked off in 1 cm squares.

When it comes to checking the calibration, the 'x' timebase can be calibrated by displaying a precise frequency from the output of a crystal calibrator or other frequency standard. (Many scopes have a built in 50Hz square wave derived from the mains frequency). When it comes to the calibration of the 'y' amplifier & attenuator, it requires a square wave of a known amplitude over the range of 1mV to 5V peak-to-peak to match the usual 1-2-5 sequence of the oscilloscope attenuator range.

Of course, the calibration test gear doesn't actually calibrate the scope - it simply checks the calibration, and if it's out, then it's necessary to delve into the scope handbook to see how the claibration may then be adjusted.

That said, my eyes glaze over when people on certain forums bang on about the importance of calibration, or buy or recommend very complex scopes which far exceed their needs or undertstanding. Many such scopes are years old, full of aging, obsolete chips, and the higher the compnnenet count, the greater the probabilty of something failing over time, be it resistors going high, transistors dying, or capacitors developing a high ESR. To the questions 'which is the best scope?' the answer must be 'the simplest cheapest one that meets your likely needs',(unless of course a higher spec one falls into your lap, then all well and good!).

There's nothing that we do as hobbyists which calls for a highly calibrated scope - we're primarily interested in observing waveforms rather than in measuring precise frequencies. We can do that with a frequency counter, which give a more precise and unambiguous display. UIndeed, unless we work with scopes every day, it's unlikely that we'll be able to do the mental arithmetic which tells us that 1kHz = 1 mSec, 100 kHz = 10uSecs etc. A frequency counter will tell us to one Hz what the frequency is. But if, for example, we wanted to see the shape of the waveform and any distortion, a counter can't do that, but we could set the scope to display ten cycles, then check the output waveform to see if only one cycle is displayed. (On a dual beam scope we could check both waveforms at the same time). We could check the actual frequency with a frequency counter of course, but the counter wouldn't show us what shape the waveform is.

I've attached a pic which show a 10kHz frequency from my homebrew waveform generator connected to my homebrew frequency counter, from which the frequency can be seen. (It's actually showing 10.003 because the frequency is set by hand on a single turn pot and to set it precisely to a few Hz is a bit ticklish!). I've also attached pics showing that frequency displayed on my scope with the scope set to 0.1 secs, for three waveforms - sine, square and triangular. The scope is set to 2Volts/cm, and two vertical divisions will be seen, which equates to 4 Volts peak-to-peak, to which I've set the adjustable output, as can be seen on the far RH knob of the waveform genrator.

I mentioned earlier that I intend to design a cheap and simple crystal calibrator to give square outputs at 5 MHz, 1 Mhz, 100 kHz, 10 kHz and 1 kHz, as a homebrew 'forum project'for interested parties. But meanwhile, I thought it wouldn't go amiss to say that if you've got a scope, the likelihood is that its calibration will be as close as it needs to be for any conceivable purpose to which any of us are likely to put it. Whilst we like to htink our test gear is accurate, we're not putting rockets into space of reparing safety critical life support systems - we're hobbysits, mending old radlios and tellies for fun!

About the only use for a scope on radios that I can think of, is setting up the IF stage, which requires a signal generator, a frequency counter, and a wobbulator. Most people are content with just using a signal generator.

So why, I wonder, do so many radio hobbyists lust after 200mHz digital storage scopes and the like? Ask them the question and they don't know the answer. (Big boys toys?).

Just my own thoughts.

Hope it's of interest.

David

Hi David,
Info like this is so valuable to electronic 'numpties' like myself so please can we have more information like this as I am always will to learn.

Many thanks

Rob

PS - I have a waveform generator and frequency counter to I think that I am well on the way to being able to roughly calibrate my scope - thanks

I routinely push my 400MHz Tek 2465B to the limits of its capability but then I do design work professionally. However most of the time, even when I'm doing design work, it's just idling along doing little that a simple instrument could not. The basic calibration accuracy of this sort of scope isn't much better than any reasonably modern instrument but what makes it much more accurate are the time and voltage cursors which I would find it difficult to live without.

Much of my work is TV related and it's worth noting that under almost any circumstance the 64us line period of a 625 signal is several orders of magnitude more accurate than the scope's calibration. Even when using the cursors. I use it as a routine check on the scope.

Because it's always there on the bench and I'm utterly familair with it I'll use the 2465 for vintage work too. Just have to remember that the lowest sensitivity is 5V/cm, hence 50V/cm at the tip of a 10:1 probe. Occasionally this is only just about OK for vintage work. Maybe I ought to get a 100:1 probe.

Again because it's always there I'll often do faultfinding with the scope which could perfectly well be done with an Avo or DVM. Even DC voltage checks where a quick glance will tell me if things are about right.

(20-04-2011, 08:52 PM)Yorkie Wrote: [ -> ]About the only use for a 'scope on radios that I can think of is setting up the IF stage, which requires a signal generator, a frequency counter, and a wobbulator. Most people are content with just using a signal generator.

So why, I wonder, do so many radio hobbyists lust after 200MHz digital storage scopes and the like? Ask them the question and they don't know the answer. (Big boys toys?).

David

When I'm doing almost any type of electronic design or repair work, after the Fluke DMM (and occasionally the AVO 8), my 'scope is the next essential item of test equipment I have and regularly use; it's simply instinctive to switch it on and have a 'poke around' when tacking any fault, other than the "no d.c. rails" symptom. Properly used - and recognising its limitaions - a 'scope can tell you far more what is 'going on' in any electronic circuit that any other single piece of test equipment: the list is simply too large to detail it!

As for "Why are 'scopes that 'go to' 200 MHz so popular", I can only speak for myself. On account of my radio interests, I frequently have the need to get some idea (see further down the page) of what's going on at freqs. in excess of 100 MHz. Now there are several things that need to be recognised (and which are frequently overlooked or are mis-understood) in the "200 MHz 'scope" animal. First, and I assume that this figure relates to the vertical amplifier & not the timebase, is this question: Is that "200 MHz" the response of the vertical amplifier, from BNC input right through to the Y plates, at - 1 dB? Or -3dB? Or worse? This "200 MHz" figure rarely turns out to mean a perfectly flat response to 200 MHz, and rolling off after that. (And what is the rate of that roll-off, too?) And then secondly there is the frequently overlooked characteristic of the probe: your 'flat to 200 MHz 'scope' will not be 'flat' to that freq. at all with most 'probes - unless they are the very expensive 'compensated - at - the 'scope - end' types.

As for digital work, a 'fast 'scope' - and especially if it has a delayed time-base and is a storage one - is essential for looking for fast glitches and other abberations that didn't 'show up' - or were overlooked - at the design stage. And with any type of pulse train, the ability to accurately measure the rise time can be very important: fast rise times require a fast 'scope - typically a "200 MHz" type - or more. Again, relevant to logic circuits.

And finally, when it comes to aligning IF amplifier strips, this is probably the one area when I use a 'scope the least! That is assuming that the amp. is 'behaving itself' and is not doing something horrible like oscillating, has gone very low gain or distorting, for example, (caused by a defective AGC line).

In terms of 'importance to me', my list of test equipment is as follows:
Fluke DMM.
Tek. (or HP) 200 MHz 'scope (+ quality probes).
HP 8640B sig. gen.
'Home brew' LV d.c PSU (adjustable).
Marconi frequency counter; (1 GHz).
Anritsu (or Advantest) spectrum analyser.
Marconi 2202C synthesized sig. gen.
'Home-brew' AF sig. gen. - sine, square, TTL, pulse and ramp outputs.
Wide freq. range comms. receivers: Racal RA-17 and an Icom R-7000 (goes to >1 GHz).

As you can see, the 'scope is well up on my list! I suppose it all depends on what your interests are & where they take you.

Finally, whilst on my 'soap box', I must stress one point that is often not fully appreciated: a piece of test kit that 'tells you lies' is worse than not having that item of test kit in the first place! In the latter case, the worst that you can do is make an inspired guess; in the former, you trust what you are told - and can waste a lot of time as a consequence - or worse! :@

Al.

A few months ago I purchased a very basic valve RF signal generator. It works but checking it with the 'scope shows the sine wave to be quite distorted. I suppose this might be more of a problem with an AF sig. gen. but how concerned should I be about this distortion? I don't have a frequency counter and I think that is my next item on the test equipment shopping list.

(27-06-2011, 07:17 AM)fido Wrote: [ -> ]A few months ago I purchased a very basic valve RF signal generator. It works but checking it with the 'scope shows the sine wave to be quite distorted. I suppose this might be more of a problem with an AF sig. gen. but how concerned should I be about this distortion? I don't have a frequency counter and I think that is my next item on the test equipment shopping list.

Just a thought, but when you say 'distorted' do you mean that the peaks and troughs are wide rather than a fine line? If so, that may be because the modulation of the sig genny is switched on. If you switch it off, you'll see a sine wave, unless of course the generator really is faulty! To illustrate the effect of modulation on a sine wave, , I've attached below two scope traces. They're actually of a 650kHz VFO in a MW modulator that I made to enable signals from any source (I-pod DAB, AM/FM) to be fed into a radio (legally) tuned to 650 kHz in the MW band. It's an alternative to a 'micro transmitter' which are unlicensable anywhere in Europe, not that anyone worries too much about that! The modulator works by the audio input from a device such as an i-pod, FM radio, CD player or whatever, modulating the 650kHz VFO, then playing this through the antenna socket of a vintage radio tuned to the 650kHz. (460M). The modulator in tunable from 430M - 490M. (It was built on three homebrew PCBs - the rest of the bits mostly came from my spares box).

The first trace shows the unmodulated sine wave, which is what you should see from a signal generator, the second is the 650kHz sine wave modulated with a 1kHz tone. On most signal generators you can adjust the depth of modulation.

I've attached a couple of other pics - one showing how you can check the frequency of an oscillator on a frequency counter simply by placing a loop of insulated wire close to the oscilator, another pic shows my homebrrew 200MHz frequency counter which I built in 1976, when such commercial instruments were far beyond the pocket of constructors, but since I'm a constructor, I constructed one! I later built a 1gHz counter, but theres' not much point these days when you can buy a hand-held one for about £30.

Hope that's of interest, and helps a bit.

David

(27-06-2011, 07:17 AM)fido Wrote: [ -> ]A few months ago I purchased a very basic valve RF signal generator. It works but checking it with the 'scope shows the sine wave to be quite distorted.

I have a collection of Advance Signal Generators. The af ones seem to produce quite a good waveform, but the rf ones, particularly the older ones on the higher ranges, are fairly distorted. They seem to have expected it as the Manual often says that you can use the harmonics for higher frequencies.
Alan

Distortion of the high-freq. wave: "The harmonics can be used for the higher freqs." I had a Heathkit RF-1U years ago that produced a distorted wave (before I re-engineered it to get rid of the distortion) - and that comment was there, in the manual, IIRC.
Seems to me that it is just another dubious marketing ploy: "if you've a problem that you can't resolve . . . turn it into a 'feature'! "

Al.

Do bear in mind to check the ht smoothing on your signal generator, excessive hum will cause Frequency Modulation of the signal.
Really the VFO ought to be fed from a stabilised (neon regulator) supply.
Rob.

I hadn't realised that in my earlier post, I'd repeated almost verbatim an even earlier post from a couple of months ago, with the same pics. Sorry about that - it comes with age - I know my name and address, but if you ask me what time it is, I forget where I live!

I've just quickly lashed up my Heathkit RF1U sig genny to my scope and have squirted 1 MHz unmodulated, and then internally modulated by the RF1U into the scope. I've also taken a pic of a 5 MHz unmodulated trace. The traces appended below. As can be seen, the unmodulated traces are clean sine waves, (given the limitations of a budget priced kit-built sig genny some thirty years old, viewed on a Gould 300 of similar vintage rescued from the skip), but on the modulated 1 MHz trace, the effects of modulation can clearly be seen.

Of course, the problems with your sig genny might be nothing to do with the signal being modulated - it may be due to it being discombobulated - an altogether different thing!


David.