25-11-2013, 10:30 AM
Remove C4 and look at the Output.
I think that will cheer you up.
Alan
I think that will cheer you up.
Alan
RB RF Probe puzzler - any thoughts please?
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25-11-2013, 10:40 AM
I fully agree that it is working correctly.
If it is designed to also work as an AM detector you should be able to get audio out of it when probing an IF signal.
Here's the first of a two part video about the Radio Bygones wobbulator, it rambles on about the Jasonkit one for a bit then switches to the Radio Bygones one, part two is around somewhere. It explains what's what. The bloke has quite a few radio related videos
http://www.youtube.com/watch?v=tx4xx4umG4I Lawrence. [EDIT: Part two....] http://www.youtube.com/watch?v=z86w3CwLMXo
25-11-2013, 12:57 PM
Interesting videos. I'll have to try the marker idea as my sig gen/wobbulator is a little non linear with its sweep. (I know the marker idea works with the spectrum analyser.)
The wobbulator output in the video looks to be a little distorted. Ideally it should be a sine wave so perhaps the oscillator needs checking. I was using my wobbulator last week to peak the IF of a Roberts radio that had been subject to the "phantom twiddler", ended up breaking one of the cores but managed to find a replacement and peaked the IF. It's now working but I suspect it needs a little more attention as it's not as sensitive as I'd hoped. Keith
Yes, I noticed the wobbulator waveform distortion too, anyone point me to an online diagram for that wobbulator?
Lawrence. [EDIT: Couldn't see for looking, it's on this forum so ignore above message]
25-11-2013, 02:06 PM
Thanks everyone for your advice and comments, which is much appreciated.
I can understand on the one hand that the probe will rectify the RF waveform and just display a DC voltage, but on the other hand, that doesn't square with the comments and the pics of traces as shown in the clips of the article which I've appended below. The wobbulator article states: "Signal levels can be too low to produce a display on the 'scope. A suitable RF probe to amplify and detect the signal will be described in the next issue of Bygones". Again, that suggests that the output of the probe should produce a display (amplified) on the scope, of an RF waveform. Yet with regard to C4, Alan, it's role seems to be to ensure that there isn't a waveform at the output of the probe - just a Voltage, because the articles states: "Residual RF is bypassed by C4, and R4 is included to provide a DC current path for the diode network". In other words, any trace of RF still existing at the cathode after rectification by D2 is shunted to chassis by C4, and only the DC voltage passes to the output of the probe. (Which I suppose is a bit like the smoothing & reservoir caps on an HT line from a rectifier smoothing out the 50/100 Hz ripple). One of these days, enthused by Keith's talk and demo at Golborne, I'll have to use my wobbulator in anger - if need be, minus the probe. Meanwhile, I'll watch the youtube videos to see if that moves me along the path of enlightenment. If not, I think I'd better book a place in a home for the terminally bewildered!
Regards, David.
BVWS Member. G-QRP Club Member 1339. 'I'm in my own little world, but I'm happy, and they know me here' (25-11-2013, 02:06 PM)Yorkie Wrote: I can understand on the one hand that the probe will rectify the RF waveform and just display a DC voltage, but on the other hand, that doesn't square with the comments and the pics of traces as shown in the clips of the article which I've appended below. The wobbulator article states: "Signal levels can be too low to produce a display on the 'scope. A suitable RF probe to amplify and detect the signal will be described in the next issue of Bygones". Again, that suggests that the output of the probe should produce a display (amplified) on the scope, of an RF waveform. The article is written correct so far as I read it David, "A suitable RF probe to amplify and detect the signal will be described" That is what that circuit does, it's no different from a standard envelope detector that you would encounter in an AM radio apart from the fact that the detector is configured as a voltage doubler. If you modulated the 1Mhz input that you have fed into it with a sine wave then that sine wave would be shown on the output and thus on the scope, thus the diode has demodulated/detected it. If the 1Mhz input is CW then there's no modulation, if there's no modulation then there is no modulation to detect, the detector will thus produce a constant DC output proportional to the amplitude of the CW signal fed into the detector. The waveforms shown in the diode probe article are the typical waveforms you would expect at the output of the probe when the input frequency to the probe is swept between two frequencies (the sweep width) and has passed through a typical IF stage, they show the DC voltage output of the detector across the sweep width. I don't know if you have used the wobbulator yet? If not then that would be the next stage, you will need a sawtooth input to it to obtain the sweep, this is normally taken from the scopes timebase output, normally a socket on the back of the scope, however some scopes do not have that facility, if not then you can tap into the scopes timebase at a suitable point, the other alternative might be to build a seperate sawtooth generator to trigger the scope and sweep the wobbulator. Lawrence.
25-11-2013, 03:49 PM
(25-11-2013, 02:06 PM)Yorkie Wrote: One of these days, enthused by Keith's talk and demo at Golborne, I'll have to use my wobbulator in anger - if need be, minus the probe. Go on David, you know you want to use it. You can always call on Wobbulators Anonymous for support Keith
25-11-2013, 10:05 PM
Thanks very much for the continued advice and encouragement - greatly appreciated.
As to the sawtooth Lawrence, I built a sawtooth generator into the wobby as an addition because another constructor said that was the way to go and kindly provided me with a circuit and PCB layout. Really, I have to admit that having built the wobby and probe, I've endowed myself with delusions of adequacy and must now make some attempts to put it into use and familiarise myself with it. I need to be more focused and concentrate on one thing at a time! Admitted Yorkie, (Duty numpty).
Regards, David.
BVWS Member. G-QRP Club Member 1339. 'I'm in my own little world, but I'm happy, and they know me here'
26-11-2013, 12:25 AM
I have just given this a little thought.
The demodulator is no more than an AM detector and the RF produces DC as expected with a blank carrier. The project was designed in the days when a X10 scope probe was beyond the means of the home constructor. X1 is the setting and the fact that the demodulator is a voltage doubling type is another clue as affordable scopes only had a little bit of gain and the doubler was needed in those days. With the probe set to X10 first. So if you modulate the carrier with a signal generator and sweep the audio up and down the carrier will produce a constant DC level with a very high modulation frequency a little more DC will appear and as you sweep down the DC will slowly get what looks like ripple on rectified mains and at a much lower frequency it will become a sine wave. Set the audio so that it looks like rectified mains again. Set the sensitivity of the scope down by 10 (0.01V/cm to 0.1V/cm for example) and then set the probe to X1 and see what happens. If what happens makes you very confused. Congratulations you have just got the scope set to the correct input impedance for your probe! The capacitor across the output of the probe is set for a 1M ohm scope input impedance and there is not enough loading to discharge it, you will just get DC. It looks like they tested it with audio filters connected and this means the capacitor value may be a little high unless the sawtooth rate is quite low and the scope they used had an old radar tube in it. Now it just needs a little bit of tinkering with the capacitor value and sawtooth frequency to suit a modern scope tube. |
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