Schematic diagram for Tektronix probe, please ?

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Schematic diagram for Tektronix probe, please ?

Post by Externet » Sun Aug 24, 2014 1:23 pm

Hi.
It is a plain passive Tektronix P6105A; very curious of what is inside.

Also, what probe should I look for turning ~2uV = -101dBm up to ~200uV = -61dBm into a proportional DC signal that I can see on a DVM or DC oscilloscope ? Is it called an active detecting probe ? Any particular preference?
-I need to measure tiny RF levels from ~2uV to ~200uV on 50MHz to 800MHz and the oscilloscopes I have left run only to 20MHz.
I do have a signal generator to 1GHz.
Miguel
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Re: Schematic diagram for Tektronix probe, please ?

Post by Robert Reed » Sun Aug 24, 2014 9:53 pm

I am afraid that you are SOL on what you want to do here. The P6105 is a garden variety passive probe with all their shortcomings. Basically a series 9 meg resistor shunted by a 10 pf capacitor at the tip. The comp box at the other end has a variable capacitor to ground that is adjusted in conjunction with the scope input resistance of 1 meg to give an exact frequency compensated 10:1 division. This would end up as Rscope= 1meg and a parallel capacitance of 90 pf to ground (C total = cable, scope input and compensation capacitor).
Each manufacturer may or may not add a low value series resistor and a very small series inductance in the signal path for parasitics. This particular probe has a sensor prong on the BNC assy. which tells the scope that 10:1 attenuation is taking place and adjusts the display accordingly. Active probes are just that being that they have an active device inside them and also require external power. These are generally FET devices and the probe as a whole will have a gain ranging from 0.1 to 1.0, rarely having any positive gain. They are usually limited to 15 volts DC at there input. Their crowning advantage is the extremely low input capacitance of 1pf or less. The 10 meg spec on scopes is meaning less beyond 50 MHz- capacitance is the sole contributor to circuit loading and distorted wave forms.
Detector probes use a low voltage turn on diode and 200 millivolts is low. below that , square law takes over and there performance dwindles and they craps out quickly.
There are some dedicated chips out there that can boost wide band performance performance but they don't even begin to pick up the signal until it reaches a neg. 50 db or higher. As to spectrum analyzers, some may reach down to 2uV, but you do not even want to hear the price. I wish you good luck on your search, but I think you may be beating a dead horse.

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Re: Schematic diagram for Tektronix probe, please ?

Post by jwax » Mon Aug 25, 2014 3:59 am

Sounds like you need a wideband preamplifier, perhaps an antenna preamp?

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Re: Schematic diagram for Tektronix probe, please ?

Post by Externet » Mon Aug 25, 2014 8:54 am

Hi.
Robert:..."As to spectrum analyzers, some may reach down to 2uV, but you do not even want to hear the price.

Yes, I have one. -130dBm to 0 dBm; up to 1000Mhz. Of the big price sticker type. But never used to full of its capabilities. The intention is on the paper:
---->http://s588.photobucket.com/user/Innern ... ort=6&o=77
(with text below picture)

John: I have several TV antenna broadband preamplifiers. How would you do it? What would be their typical output level ?

Or can it be done with a tracking generator feature ?

Miguel
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Re: Schematic diagram for Tektronix probe, please ?

Post by Dean Huster » Fri Oct 10, 2014 8:50 pm

You'll find that the center conductor of the P6105A is resistance wire of fairly low value. This is a modular probe and both the compensation box and the probe head snap off easily. Other than the replacement of the three main components (head, cable, comp. box), the probe is not repairable.

Active FET probes can handle high frequencies to 900 MHz or more with low resistice and capacitive loading.

RF detector probes, if properly biased, can get a reading in the millivolt range. Using an RF millivoltmeter is the option. Models made by -hp- or Millivac are excellent for this .
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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Re: Schematic diagram for Tektronix probe, please ?

Post by Robert Reed » Sat Oct 11, 2014 3:41 pm

Externet
Can't down load your Photo Bucket. Says it does not exist.

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Re: Schematic diagram for Tektronix probe, please ?

Post by jwax » Sat Oct 11, 2014 5:00 pm


Robert Reed
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Re: Schematic diagram for Tektronix probe, please ?

Post by Robert Reed » Sun Oct 12, 2014 4:27 pm

Are you trying to measure the input impedance of the tuner which we would assume to be 75 ohm. I think you had a posting a while back about trying to do this, but if I remember right you did not have top of the line equipment as you show in the photos. If I am reading this right - you show a 1.5 db loss in the cable and connectors with just a direct connection of SAs output and input ports. Probably within reason if you are using any BNC connectors in that setup (rather than type N) and especially at that frequency. Now adding the tuner load in parallel to the SAs 50 ohm input drops that figure by 3.3 DB.This is exactly what a double terminated 50 ohm load would do, so my assumption is that the tuner input is 50 ohm. However, along with the mismatch introduced, VSWR comes into play and coax line lengths (especially stubs) will alter the expected results. At 650 MHz even a 3" section of RG-58 is 1/4 wavelength so the setup at these frequencies become very critical and BNC connectors will affect absolute accuracy. Unfortunately when trying to get laboratory test results with a generalized test set up at these frequencies leaves a lot of room for minor errors that can add up and leave one unsure about the results.

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Re: Schematic diagram for Tektronix probe, please ?

Post by Externet » Sun Oct 12, 2014 5:50 pm

Robert Reed wrote:Are you trying to measure the input impedance of the tuner which we would assume to be 75 ohm.
Hi Robert.
I did not and cannot believe a modern TV tuner is 75 ohm input impedance. In the best case, it could be 75 ohm but only at a few points in its receiving spectrum.
Impedance, by definition, is at a given frequency only. Not on a 600+MHz wide band
So what I did is connect the SA signal generator output to the SA analyzer input and noted the signal drop (loading) when the TV tuner was paralleled in the path. Doing it every several MHz, gave me the readings for the TV tuner impedance varying around 150 ohms at most of the spectrum if I remember well. I would try to find the notes and post.

The whole story was about trying to prove that "75 ohm TV antennas and RG59 cable" are a nominal wrong match to a modern TV tuner.

I receive TV over the air. Had permanent marginal reception at my location deep in the bunnies. After the evaluation, changed (made) the antenna to a 300 ohm and now I have good reception permanently.

----> http://s588.photobucket.com/user/Innern ... ort=6&o=80
(Some text under the picture)
----> http://s588.photobucket.com/user/Innern ... ort=6&o=79
Miguel

This would really need to be in another subject titled thread, it just drifted into this discussion.
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Re: Schematic diagram for Tektronix probe, please ?

Post by Robert Reed » Sun Oct 12, 2014 7:11 pm

Hello Externet

Robert Reed wrote:

Are you trying to measure the input impedance of the tuner which we would assume to be 75 ohm.

I probably should have phrased this as THEY WOULD ASSUME and not WE WOULD ASSUME.

I agree with you that stated impedances probably vary quite a bit especially in wide band Equipment. In a closed system its probably accurate, but off the air and through one antenna it has to stray. This must be why the term Nominal is so popular as in Nominal Impedance.
How did you calculate 150 ohms in your photobucket test setup, or was this done in a different test setup?

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Re: Schematic diagram for Tektronix probe, please ?

Post by Externet » Mon Oct 13, 2014 7:35 am

Hi.
I tried ohm's law in lack of better knowledge.

Example:

Code: Select all

Gen 71uV--------/\/\/\/--------------P-------------/\/\/\/-------------GND
               50 source             |             50 load
                                     |-------------\/\/\/\-------------GND
                                                   ? load
With the tuner (? load) disconnected, noted the level at point P which had only the 50 ohm analyzer load and cable loss.
Paralleling the tuner as (? load) at point P, noted the new level at point P.
Calculated what (? load) paralleled to 50 ohm load would produce the new level. Did that at many frequencies.

Please let me know a better way.
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Re: Schematic diagram for Tektronix probe, please ?

Post by Robert Reed » Mon Oct 13, 2014 10:51 am

Hi Externet

I might be missing some thing here, but as stated before - the level difference when adding the unknown load (tuner) had a 3db drop which would exactly indicate double terminating of the 50 ohm source Z , hence unknown Z would be 50 ohm. Again, will only be reliable in a perfect test set up.

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