Great video, I enjoyed seeing a comparison of how the different probes changed the output of the waveform. I'll agree 100% that How to use test probes is a excellent book. I've had my copy for about 12 years now and it goes into great detail about probes for scopes and vtvm's of the era. There are even some tube designs in there that are quite interesting. Probes by Bruno Zucconi Gernsback publishing 1955. Is another one that it a interesting read from around the same era.
Both Sencore probes are voltage doubler detectors. It only takes two diodes to double the voltage, as in the first Sencore probe. The 1N695 is a nothing special germanium diode, like the 1N34 in modern packaging. The 27K load resistor in the first Sencore probe produces more distortion at low signal input level. It could be much higher in resistance. Using an oscilloscope with a 1 megohm input with DC coupling you don't need a load resistor, the scope is the load. RF bypassing is accomplished by the capacitance of the cable. A series resistor of modest value between the 2nd diode and the coax cable will improve the output waveform a bit more. In my probe I use a 100 microhenry RF choke there instead. Using the probe with four feet of RF-58 coax the the typical 25pF input capacitance yields a nice bandwidth well beyond what's needed for IF alignments for FM and TV IF amplifiers.
On the DP213 schematic it shows an output "smoothing" capacitor of 390pF with a resistor of 560K to discharge it. That has a cut-off frequency of 728Hz, so if you're feeding it into a high impedance measuring instrument like a scope, any frequencies above 728Hz will be attenuated by that capacitor. That's why you can't recover a 5KHz modulation. The clue should have been that the distorted waveform is exactly what you see when you rectify ac to dc and draw a little current from the output capacitor. If you want to recover higher frequencies accurately, you need to load the probe much more, for example feeding a 50ohm coax. That would effectively load the capacitor through the 12K resistor is series with the output, reducing the output a lot, but having a cutoff frequency of 34KHz. Probably the best load would be in the region of 10K to 20K, allowing accurate audio demodulation while only reducing the output by a factor of about one half.
Nice vid Tom thanks.. Can i ask whats the ds4000 like to live with any quirks etc? I think they just may have to squeeze a little to much from the 2000 I tried updating the firmware but did not fix the issues shame
If I said the B&K works at 60 Hz I misspoke. I meant to say that the sweep generators of the day (50's&60's) worked at 60 Hz sweep so that was all the demodulator probes of the day needed to pass. I am doing a new video about the probe I called the "Dinosaur Bone" and others used in applications from the past and present. I will try there to clear up any confusion as well about probes, response, etc.
Congratulations on the comparative demonstration between the probes. I have a "dinosaur bone that works great". A hug from the South of Brazil!
Great video, I enjoyed seeing a comparison of how the different probes changed the output of the waveform.
I'll agree 100% that How to use test probes is a excellent book. I've had my copy for about 12 years now and it goes into great detail about probes for scopes and vtvm's of the era. There are even some tube designs in there that are quite interesting.
Probes by Bruno Zucconi
Gernsback publishing 1955. Is another one that it a interesting read from around the same era.
Would love that book on probes, will search! TFS, GB :)
Both Sencore probes are voltage doubler detectors. It only takes two diodes to double the voltage, as in the first Sencore probe. The 1N695 is a nothing special germanium diode, like the 1N34 in modern packaging. The 27K load resistor in the first Sencore probe produces more distortion at low signal input level. It could be much higher in resistance. Using an oscilloscope with a 1 megohm input with DC coupling you don't need a load resistor, the scope is the load. RF bypassing is accomplished by the capacitance of the cable. A series resistor of modest value between the 2nd diode and the coax cable will improve the output waveform a bit more. In my probe I use a 100 microhenry RF choke there instead. Using the probe with four feet of RF-58 coax the the typical 25pF input capacitance yields a nice bandwidth well beyond what's needed for IF alignments for FM and TV IF amplifiers.
Got my copy of the book. Pretty decent shape considering the age. Looks to have a lot of good information.
On the DP213 schematic it shows an output "smoothing" capacitor of 390pF with a resistor of 560K to discharge it. That has a cut-off frequency of 728Hz, so if you're feeding it into a high impedance measuring instrument like a scope, any frequencies above 728Hz will be attenuated by that capacitor. That's why you can't recover a 5KHz modulation. The clue should have been that the distorted waveform is exactly what you see when you rectify ac to dc and draw a little current from the output capacitor.
If you want to recover higher frequencies accurately, you need to load the probe much more, for example feeding a 50ohm coax. That would effectively load the capacitor through the 12K resistor is series with the output, reducing the output a lot, but having a cutoff frequency of 34KHz. Probably the best load would be in the region of 10K to 20K, allowing accurate audio demodulation while only reducing the output by a factor of about one half.
Great video, but what in the world is an "analog discovery"?
Just ordered a used copy from Amazon.
Nice vid Tom thanks.. Can i ask whats the ds4000 like to live with any quirks etc? I think they just may have to squeeze a little to much from the 2000 I tried updating the firmware but did not fix the issues shame
Your saying the B&K works at 60hz but the instruction book says the response is from 100khz to 950 MHz? Can you help understand. Thanks.
If I said the B&K works at 60 Hz I misspoke. I meant to say that the sweep generators of the day (50's&60's) worked at 60 Hz sweep so that was all the demodulator probes of the day needed to pass. I am doing a new video about the probe I called the "Dinosaur Bone" and others used in applications from the past and present. I will try there to clear up any confusion as well about probes, response, etc.