Oscilloscope question

[Sparky Williams]

I am a novice, and am learning to use an oscilloscope that I have access to in order to troubleshoot a wireless remote control project (transmitter & receiver)I've built from an article featured in Nuts & Volts Magazine (April 2002).The remote control unit can be used to turn on, say a livingroom lamp, by pressing a button on the remote control transmitter. <p>I can't get it to work, and I'm trying to figure out whether the problem lies in the transmitter, the receiver, or both. My question is this: <p>In testing the wireless transmitter (it operates at 418 MHz) can I confirm whether it is putting out a signal by attaching the black lead from the scope to the "ground" and the red lead to the bare wire antenna and then pushing the transmit button? If so, how will such confirmation show up on the scope? Will the scope show a frequency reading of 418MHz?<p>I'm learning on two scopes: One is a Tektronix TDS 340A (Digital), and the other is an analog Tektronix TAS 465. I've downloaded the manuals for both and am trying to learn on my own with the help of a signal generator. <p>Any help will be greatly appreciated.<p>Zeek

[Edd]

I thought that vaguely sounded familiar, seems like its so far back that it has drifted off current topics, here is a refresh off archives:<p>Those are quite stable units, looks like they should be foolproof. As far as stability, looks like the xmitter is using a trimmed SWIF filter in its osc design to assure stability. Also the receiver is a quantum leap in its design in using a Swif in its filtering as well as a full blown superehet in that small package profile. Thanks to technology trickle down from the cell phone industry.
Your units data:
http://www.rfdigital.com/pdf/rfd24005.pdf
RXM-418-LC-S…the Receiver module
http://www.rfdigital.com/pdf/rfd24002.pdf
TXM-418-LC…….the Xmit module
Since you are crimped on your test equipment, looks like the very simplest test might just have to do . Start with an initial monitoring of the transmitter and confirm that Pin 2 data input does not have a logic high on it from any attached circuitry, it needs to be low.. Then move over to the receiver and check its Pin 8 and confirm its also at its logic low state. I am unsure if your xmit Vcc supply is operated at its lower limit spec of 3vdc or at the 5vdc level. Whichever , meter pin 8 of the receiver on 5vdc meter scale range via a mini clip and then go over to the xmitter and connect a 1k resistor between its Vcc ( Pin 7) and its data in (Pin 2)
That high level should emit an RF carrier from your xmit module and you should see a single voltage level shift on your metering on the rec end.
Now if you only had a scope and a pulse generator….oh well…..
If the above checks out , looks like your RF link of the project is working ok. <p>Addenda: DTG 6/9/2004/9:19:29 PM
Now you do have a scope …and two fine ones at that!.. so just probe the points referenced above as mentioned and use in the scope vert amps DC coupling mode.
Individual RF sine wave nodes viewing at that hi of a freq is more in the order of utilization of a sampling o-scope. However, on the transmitter unit , you should be able to see the RF carriers bursts of serial modulation,close proximity capacitively coupling at a low MV sensitivity vert input settting, using the hoz time bases in its auto triggering mode.<p>Also a refresh on your queries:
[ How do I confirm that pins 3 and 8 are at the low position? ][ Correction -- I meant pin 2, not pin 3.]…………..
Using your Scoping/ Metering in the DC mode:
Less than (decimal).4 Vdc= Logic low.
Greater than 2.5 VDC = Logic high, in the logic family that you are working with.<p>So on the transmit RF module you you should be able to view the Holtek decoders pulse train input at the xmit modules pin 2.
Likewise, if a successful RF handshake has been made between the units, there should be a scope viewable pulse train output at pin 8 of the receiver RF module.

73's de Edd
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<p>[ June 09, 2004: Message edited by: Edd Whatley ]</p>

[Sparky Williams]

Dear Edd:<p>You have a good memory! It's taken me months to gain access to an oscilloscope. I finally found a technical school in the area that has been kind enough to let me use the scopes in their lab to do the tests on my project. Thus far, I've only been able to learn the bare essentials - and probably not even those - such as how to measure voltage and frequency. Thanks to you, I've learned that a voltage reading of less than .4 Vdc = logic low and greater than 2.5 Vdc = logic high.<p>If I understand you correctly, hooking up the probes in the way stated in my question would not tell me anything useful. <p>Could you please clarify for me your statement, "... close proximity capacitively coupling at a low MV sensitivity vert input setting, using the hoz time bases in its auto triggering mode"? I do not understand how to set this up on the scope, nor do I understand how to set the "auto triggering mode." Also, although I've been reading about "triggering" I must confess that I do not understand it.<p>Thanks for your help.<p>Zeek

[ajc-28]

In order to use an oscilloscope to view the transmitter pulses, you need to make a detector which will consist of a single turn of wire about 1 inch in diameter. One side of the coil is "ground" which you connect the scope ground wire to. The other side of the coil goes to the anode of a germaniam RF diode and the cathode of the diode is the positive output pulses which go to the scope probe. You would want to connect a 10 pf capacitor from the cathode of the diode to ground. Use very short leads for the connections. Hold the coil near the antenna of the transmitter and you should be able to pick up enough amplitude of the pulses to verify that the transmitter is operating.

[wd5gnr]

Hi Zeek,<p>In an indirect answer to your question, let me point out that learning to use a scope will be one of the most important things you ever do related to electronics since the scope is the principle way we "look" at electronics.<p>The screen of the scope is a graph. The X coordinate (horizontal, left to right) is time and the Y coordinate (vertical, up and down) is voltage.<p>So when you set the horizontal sweep time, you are setting how much time each little square on the screen represents. So setting it to 1mS means each square is worth 1mS and the total time is 10mS (most scopes have a 10x10 grid on their screen).<p>The vertical sensitivity sets the voltage scale, so 1V/div means each square (vertically) is worth 1V. However, you often use a X10 probe which means 1V/div is really .1V/div because the probe cuts everything by 10. This has some advantages. Many probes that do this have a little pin near the BNC connector that the scope can sense and figure out that the probe is a X10 and it adjusts its readings accordingly. If your probe doesn't have this pin or the scope doesn't do that, then you have to do it your head. If it doubt, read a DC voltage (like the scope's calibrator or a 9V battery) and see what happens.<p>Now, that sounds simple, right? It is! But here's the trick. Sure, each horizontal division is worth 1mS (or whatever time we set it to equal). But what is the absolute value of the leftmost square? In other words, when does our graph start? That's the purpose of a trigger.<p>When you set the trigger slope and level, you are telling the scope that when the input signal rises (or falls, depending on the slope + or -) to a certain level, start the graph running for one sweep. So say you have a sine wave that goes from -5 to 5V. By moving the trigger level, you can make the left part of the screen show any starting point along that sine wave that you like. Keep in mind that on a 2 channel (or more) scope you have to pick which channel you want to trigger on. Also, most scopes have an external trigger you can select (more on that later).<p>When you are in single trigger mode, the scope will sweep once and that's it. For a digital scope, this is handy because you can watch one event. My analog scope has this too and then it is mostly an alarm. If you set the trigger for, say, 3V, go to lunch and come back, you can see if the signal ever went above 3V (the scope will be triggered if it did) but you can't really see the waveform because it isn't a storage scope. On an analog scope you don't use this much. For a storage (which today means digital) scope you use it a lot to capture one event of interest.<p>Manual trigger mode waits for a trigger event. The scope doesn't sweep until it gets one. This is great when you have the trigger set up right. Otherwise, it makes it hard to see the trace if there is one at all. <p>Auto trigger waits a while for a trigger event and if it doesn't see one, it generates a fake event. This is good for reading a DC voltage or when you just want a sense of the activity on a line. But it can also cause a garbled display.<p>For example, suppose you set the sweep to 1mS/division and you have a square wave that happens every 15mS. The scope triggers on the first square wave and then 10mS later hits the end of the screen. Now, before 5mS elapses (and the next pulse arrives), the scope gets tired of waiting and starts sweeping. So your eye will see the square wave kind of "float" around -- it won't always be in the same place. With manual triggering, you'd see the square wave right at the start of the screen and it would stay put which is what you want if you are going to measure it accurately. Some scopes have a trigger hold off that can combat this effect also.<p>There is also something called delayed sweep, but that's another discussion for a later time (or maybe an article). The short version is that it lets you "zoom in" on a part of a waveform.<p>So triggering is very important. Let's say you want to look at signal when another signal hits a level (maybe a data line on an I/O device when a chip select asserts). That's what the external trigger is for. You would trigger with the chip select and watch the data lines to see what was being written. In fact, you can get logic analyzers that will generate a trigger on certain events (for example, when a data bus has a certain number on it).<p>If you want to see a practical use of this, look at my 2 part logic scope article in the last 2 N&V magazine. This logic device uses triggering and a D/A converter to show 4 logic waveforms on one scope channel! Triggering is the key to making this work.<p>There's a lot more to know, but that will get you started and I've gone on long enough anyway. If you want more info check out:
Tektronix XYZs of Scopes and don't forget Virtual Oscilloscope Simulator which is very cool!

[Dean Huster]

Zeek, if you want to look at the signal of the transmitter directly, the TAS465 won't be really useful. It has a 100MHz bandwidth, MIGHT show that signal at 418MHz, might even trigger on it, but the amplitude will be really, really small if you can see it at all. I don't know what the bandwidth of the TDS340A is. Never been that much of a fan of digital scopes over analog models.<p>As mentioned, if your scope bandwidth is to low to see the actual RF, you can make a detector probe. But it will be useful only if the signal is amplitude or pulse modulated. You used to be able to find detector probe construction in the Radio Amateur's Handbook published by the ARRL.<p>Dean

[Edd]

Aaah-so:
That Tech training schools generosity explains TWO primo scopes. Since your last posting I thought [in order to have come up with those two units] that you had won the Lotto or made some dark alley findings in the manner of ”Aah,….. such a deal I have for you, already.”
I just mentioned that scopes auto-triggering mode to initially simplify matters, as typically I see first time operators having initial confrontations in just acquiring a trace on scopes or even finding it, if deflected/displaced off scale.
The coupling to the antenna via 1x probe at the scopes most sensitive vertical setting was on the hope that you might possibly see the ~1-3kc freq of serial burst data as very weak “snivets” on your scope trace. BTW , that TDS 340 dig scope is also a 100 meg b/w unit. It would take its big brother TDA380 at 400meg and more importantly its 2 gig sampling capability for scope analysis use at this freq. Therefore, as far as xmitter output signal ,and assuredly assuming that you have no 400-450 mhz scanner receiver, I now will suggest this:
You will need one common TV receiver that has cable band tuning capability. You place it in the cable band mode tuned to cable channel #56.A couple feet of ~20-22ga plastic insulated hook up wire with 1 bared end inserted within the TV’s antennas F-connector, into its center connection..
With no grounding being used, then interwrap the xmit antenna with that aforementioned insulated wires other end. Power up the whole remote xmitter unit and transmit a signal in the same manner as you would be using it in the future .Simultaneously observe the TV screen and sound for lines or herringbone interference in the raster, confirming a valid RF signal being sent out from your xmit module.
As far as receiver operationality one would only need to monitor its output pin mentioned before and if that signal generator that you mentioned covers 410-415 Mhz, have its output coupled in like manner mentioned above to the receiver antenna and a rocking of the tuning dial across the 418 mhz should produce a level shift output as is tuned past each time.<p>If you happen to go to the Hameg site referenced by Al, you might be able to polish up your knowledge on scope control’s cause and effects . Typically you wouldn’t have that much continual access time on the schools units. Looks like they give you 4 BNC sourced signal inputs to click and drag into your Ch 1 or 2 inputs and scale up and evaluate.
Initially confirm that your two (Ch 1 & 2)vert controls vernier adj's (red arrow at 9:00 pos) and also the hoz time base(red arrow at 9:00 pos) are at the cal position and then you should read:
Green BNC…..1 1/2V p/p sine signal @2200cycles
Brown BNC…..8V p/p.sine signal @4310~
Blue BNC…..4.4V p/p sine signal @1225~
Purple BNC…..3V p/p sine signal @1225~
Seems like the trainer has a shortcoming in the vert positioning control, it having only 20 digital steps, it needs many more to give enough vernier action for vert trace repositioning while taking vert amplitude measurements. It requires interpolation as it is.<p>73's de Edd
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<p>[ June 11, 2004: Message edited by: Edd Whatley ]</p>

[Sparky Williams]

To Everyone:<p>I want to thank everyone for their very informative responses to my question.

[Dean Huster]

Edd, using a 1X probe on a scope to view a 418MHz signal would be like connecting a 2 ohm resistor from the test point to ground because of the huge capacitance the probe would exhibit. 1X probes sound so attractive because of the increased vertical sensitivity you get, but the tradeoff in capacitive reactance usually makes the 1X probe good for little other than audio work. I know you know that, but I thought that I'd mention it for the edification of those learning their way around scopes. Yes, a 1X probe lets you have the full 5mv/div sensitivity the scope offers, but a 10X probe isn't that bad, limiting you to 50mv/div. After all, most of the signals we view are large enough to force us to use even higher attenuator settings that that.<p>Dean