P6021 Noise Problem?


Chappy
 

I recently acquired a P6201 active 1X FET probe which appears to have never been used. The inspection tag was still attached, the accessory pack was still sealed, and there was not a single mark on the probe's ground sleeve. All functions of the probe are working but it has a 2mVpp high frequency noise component riding on any signals that I measure.

The manual states a 'Noise Tangental' of 300uV (150uV RMS), a figure well below the noise signal that I am seeing.

I have tried the probe on both my 7403 through a 7A18, and a new Keysight DSOX1102G with the same results. I have tried powering the device with a Tektronix bench power supply, and a home made unit with the same results. Interesting thing is that the noise level riding on the signal is almost non existent when I feed it a supply of +/- 5V. As I increase the supply voltage above this the noise becomes more apparent. Based on this observation I believe that i can rule out noise picked up by the cable between the amplifier and the probe. I have also scoped the power supply rails with negligible abberations on the supply traces on a 2mv/div scale. With the Keysight scope (BW = 70Mhz) a screen capture shows that the lowest component of the noise starts at approx 100MHz.

I am starting to think that there may be components inside either the amp or probe that have degraded over time. Could a degraded or failed electrolytic capacitor contribute to this noise? Or could it be due to a damaged FET or BJT?

If anyone here has one of these I would welcome your input before I go chasing a problem which perhaps is completely normal for this probe. Once again, its physical appearance indicated that it has never (or hardly) been used.

Sincerely,
Steve


Tom Lee
 

Do you live relatively near to an FM radio station?

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/20/2021 13:08, Chappy wrote:
I recently acquired a P6201 active 1X FET probe which appears to have never been used. The inspection tag was still attached, the accessory pack was still sealed, and there was not a single mark on the probe's ground sleeve. All functions of the probe are working but it has a 2mVpp high frequency noise component riding on any signals that I measure.

The manual states a 'Noise Tangental' of 300uV (150uV RMS), a figure well below the noise signal that I am seeing.

I have tried the probe on both my 7403 through a 7A18, and a new Keysight DSOX1102G with the same results. I have tried powering the device with a Tektronix bench power supply, and a home made unit with the same results. Interesting thing is that the noise level riding on the signal is almost non existent when I feed it a supply of +/- 5V. As I increase the supply voltage above this the noise becomes more apparent. Based on this observation I believe that i can rule out noise picked up by the cable between the amplifier and the probe. I have also scoped the power supply rails with negligible abberations on the supply traces on a 2mv/div scale. With the Keysight scope (BW = 70Mhz) a screen capture shows that the lowest component of the noise starts at approx 100MHz.

I am starting to think that there may be components inside either the amp or probe that have degraded over time. Could a degraded or failed electrolytic capacitor contribute to this noise? Or could it be due to a damaged FET or BJT?

If anyone here has one of these I would welcome your input before I go chasing a problem which perhaps is completely normal for this probe. Once again, its physical appearance indicated that it has never (or hardly) been used.

Sincerely,
Steve








Jean-Paul
 

Send scope shots of noise with scale factprs and sketch of entirely setup eg 50 ohm term at scope or1 m?

probe tip open or shorted to grd ring etc.

Check service manual for noise test conditions

Could be bad FET or failed bypass caps.

Jon


Chappy
 

No but I do live next to a military base, and I am about 4000ft away from an air traffic control surveillance radar. You may be on to something there. Is it possible that the high frequency FET and transistors are picking this up? It has not been giving me a problem with my lower frequency probes.

I will try to get some screen shots posted later.

Thanks for the help so far.

Sincerely,
Steve


Michael A. Terrell
 

Chappy wrote:
No but I do live next to a military base, and I am about 400ft away from an air traffic control surveillance radar. You may be on to something there. Is it possible that the high frequency FET and transistors are picking this up? It has not been giving me a problem with my lower frequency probes.
It's quite possible. A microwave diode and a foot of wire for an antenna can let you se when the signal is the strongest, on another channel. Years ago, an EE in Orlando mentioned a consistent tick in his stereo that had recently started. I told him to turn his TV n, when the weather report was on a channel that had just installed a weather RADAR to see if it was pointed at his house during the ticks. It was. A Military RADAR runs a lot more power than a TV RADAR, and they can easily overload sensitive circuits in its path.

The main RADAR at Carin Airfield was 2MW pulsed, in the early '70s. I don't know what they use these days. That was a pair of Westinghouse systems, back then.

The shielding in a probe cable was never intended to deal with this. A simple shield along the wall of aluminum foil will block it, if you can use it. Sheets of foam insulation cann be bought with a foil backing, but you can't beat a properly built and maintained RF screened room.


 

You confused me there with the probe type in the thread title not matching the content (P6021 - Current Probe, P6201 900MHz FET Probe).

Anyhow - if you are matching the test conditions exactly, then I'd be suspicious of:

C170, C180 (both 1uF, 35V) if the serial number is > B079000 (earlier ones used axial tantalum so less likely to be a problem)

C240 (22uF, 15V), and C350 (if it's an electrolytic, schematic disagrees with parts list).

It's unlikely that C300 (5.6uF, 6V) is a problem - it's in the signal path for the AC amp but it is is misbehaving I'd expect it to impact LF AC.


David

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Chappy
Sent: 20 February 2021 21:09
To: TekScopes@groups.io
Subject: [TekScopes] P6021 Noise Problem?

I recently acquired a P6201 active 1X FET probe which appears to have never been used. The inspection tag was still attached, the accessory pack was still sealed, and there was not a single mark on the probe's ground sleeve. All functions of the probe are working but it has a 2mVpp high frequency noise component riding on any signals that I measure.

The manual states a 'Noise Tangental' of 300uV (150uV RMS), a figure well below the noise signal that I am seeing.

I have tried the probe on both my 7403 through a 7A18, and a new Keysight DSOX1102G with the same results. I have tried powering the device with a Tektronix bench power supply, and a home made unit with the same results. Interesting thing is that the noise level riding on the signal is almost non existent when I feed it a supply of +/- 5V. As I increase the supply voltage above this the noise becomes more apparent. Based on this observation I believe that i can rule out noise picked up by the cable between the amplifier and the probe. I have also scoped the power supply rails with negligible abberations on the supply traces on a 2mv/div scale. With the Keysight scope (BW = 70Mhz) a screen capture shows that the lowest component of the noise starts at approx 100MHz.

I am starting to think that there may be components inside either the amp or probe that have degraded over time. Could a degraded or failed electrolytic capacitor contribute to this noise? Or could it be due to a damaged FET or BJT?

If anyone here has one of these I would welcome your input before I go chasing a problem which perhaps is completely normal for this probe. Once again, its physical appearance indicated that it has never (or hardly) been used.

Sincerely,
Steve


Craig
 

I was having irritating low level noise problems at my bench until I replaced the LED bulbs with good old fashioned incandescent. Some LED bulbs are quite noisy.
Craig


Chappy
 

David,

Thank you for picking up on the error in my title, my probe is the 6201 900MHz FET probe. After working on the issue some more I think I have narrowed down the problem. Shielding all of the parts with tinfoil grounded to the chassis did not change anything. However, holding onto the probe lead did.

With a 1Mhz, 5mVpp square wave signal I am able to see a roughly 100Mhz oscillation riding on the square wave. As I carefully move the probe lead around while grasping the probe lead with my entire hand I am able to reduce the oscillation amplitude from 2mVpp to approx 1mVpp. So my main problem is not random noise but a distinct separate signal.

I had another look at the +/-15v supply rails on my power supply and the scope trace for the negative rail is slightly thicker than the trace for the +15V rail on a 5mv/div setting. I am going to investigate this further with my 7A22 plugin with it's greater sensitivity. Once I get this issue resolved I will then look at those capacitors that you have mentioned if the problem still persists. Perhaps I have a small high frequency oscillation in my power supply that is working its way up into the probe head and back, further compounded by degraded electrolytics in the probe system.

Thanks to all who have replied with advice and insight so far.

Steve


Leon Robinson
 

The clue, moving your hand.  A number of years ago I was working with some fast pulses and depending on the placement of the 50 ohm probe cable the scope display changed a lot.  I got some of the clipon ferrite beads and put them on the probe cable.  Problem solved.



Leon Robinson  K5JLR

-------- Original message --------
From: Chappy <sb.chadwick@hotmail.com>
Date: 02/21/2021 11:03 AM (GMT-06:00)
To: TekScopes@groups.io
Subject: Re: [TekScopes] P6021 Noise Problem?

David,

Thank you for picking up on the error in my title, my probe is the 6201 900MHz FET probe.  After working on the issue some more I think I have narrowed down the problem.  Shielding all of the parts with tinfoil grounded to the chassis did not change anything.  However, holding onto the probe lead did.

With a 1Mhz, 5mVpp square wave signal I am able to see a roughly 100Mhz oscillation riding on the square wave.  As I carefully move the probe lead around while grasping the probe lead with my entire hand I am able to reduce the oscillation amplitude from 2mVpp to approx 1mVpp. So my main problem is not random noise but a distinct separate signal.

I had another look at the +/-15v supply rails on my power supply and the scope trace for the negative rail is slightly thicker than the trace for the +15V rail on a 5mv/div setting. I am going to investigate this further with my 7A22 plugin with it's greater sensitivity.  Once I get this issue resolved I will then look at those capacitors that you have mentioned if the problem still persists. Perhaps I have a small high frequency oscillation in my power supply that is working its way up into the probe head and back, further compounded by degraded electrolytics in the probe system.

Thanks to all who have replied with advice and insight so far.

Steve


cmjones01
 

This. It does sound like external interference may be the problem. For a
while I had an office from whose window the tower of the infamous "Palace
of Culture and Science" building in Warsaw was visible about a mile away.
It housed a whole load of FM radio transmitters. They got in to
*everything* analogue. I had to use bandwidth limiting on the scope most of
the time, and just get used to the awful fuzz on the traces if I was doing
any higher-speed work.

Chris

On Sun, 21 Feb 2021, 18:41 Leon Robinson, <leon-robinson@sbcglobal.net>
wrote:

The clue, moving your hand. A number of years ago I was working with some
fast pulses and depending on the placement of the 50 ohm probe cable the
scope display changed a lot. I got some of the clipon ferrite beads and
put them on the probe cable. Problem solved.



Leon Robinson K5JLR

-------- Original message --------
From: Chappy <sb.chadwick@hotmail.com>
Date: 02/21/2021 11:03 AM (GMT-06:00)
To: TekScopes@groups.io
Subject: Re: [TekScopes] P6021 Noise Problem?

David,

Thank you for picking up on the error in my title, my probe is the 6201
900MHz FET probe. After working on the issue some more I think I have
narrowed down the problem. Shielding all of the parts with tinfoil
grounded to the chassis did not change anything. However, holding onto the
probe lead did.

With a 1Mhz, 5mVpp square wave signal I am able to see a roughly 100Mhz
oscillation riding on the square wave. As I carefully move the probe lead
around while grasping the probe lead with my entire hand I am able to
reduce the oscillation amplitude from 2mVpp to approx 1mVpp. So my main
problem is not random noise but a distinct separate signal.

I had another look at the +/-15v supply rails on my power supply and the
scope trace for the negative rail is slightly thicker than the trace for
the +15V rail on a 5mv/div setting. I am going to investigate this further
with my 7A22 plugin with it's greater sensitivity. Once I get this issue
resolved I will then look at those capacitors that you have mentioned if
the problem still persists. Perhaps I have a small high frequency
oscillation in my power supply that is working its way up into the probe
head and back, further compounded by degraded electrolytics in the probe
system.

Thanks to all who have replied with advice and insight so far.

Steve











Jean-Paul
 

The 100 MHz noise can be parasitic oscillation, rather than power rail lack of bypass

The Ft of the transistors can shift over the decades.

One tip-off is if the 100 MHz changes amplitude or disappears as the DC level is changed.

Another test is apply a low frequency small amplitude Signal and see if the noise is just adding to the signal or affects the 100 MHz.

Jon


 

Hi Steve,
I suspect the noise is coming from the circuit you are measuring with the FET probe.
A simple test should be able to determine if the probe or the circuit you connect the probe to is the noise source. Connect the tip of the probe to the gold ground ring adjacent to the tip with a short piece of bare wire. The wire has to be as short as possible to avoid RF pickup. Avoid touching the wire with your finger because your body might be picking up RF noise. Ideally you should touch the bare wire or the gold ground ring of the shorted probe to the ground lug of the scope itself.

If there is still any visible noise it is probably coming from the probe.
If the noise level is now within spec try connecting a few different value 1/4W resistors between the tip and the gold ground ring. Again keep the length as short as possible to avoid RF pickup. Start with 51ohms. Then 1K, then 20K. As the resistance increases the thermal noise from the resistor will increase.
The noise voltage of a perfect resistor depends on its resistance, its temperature, and the bandwidth being measured. The formula for the RMS noise voltage is
Vn = square root (4 x Boltzmann's Constant x Kelvin Temperature x resistance x bandwidth)
If you make your measurement at room temperature (300K) and the bandwidth of the probe is 900MHz then if you connect a 51ohm resistor across the probe the resistor will generate
Vn = square root ((4*1.38E-23*300*51*9E+8) = 2.76E-05 = 27.6uV RMS. That will be too small to see.
A 1,500ohm resistor will generate 150uV RMS of noise if your measured bandwidth is from a 900MHz probe. That will also be too small to see because your scope's bandwidth is not 900MHz even though the probe's bandwidth is that high.

Since the 7403 is a 60MHz scope and the Keysight scope is 70MHz we can redo the calculation assuming a 70MHz bandwidth. The noise formula says a 20Kohm resistor will generate 150uV RMS of noise with a 70MHz bandwidth. So any circuit you attach your probe to with 20Kohms or greater of resistance may be generating the noise you are seeing.

There are two ways to reduce the noise:
1) Reduce the impedance. This is one reason high frequency plugins like the 7A24, 7A19 and 7A29 have 50ohm inputs and not 1Megohm inputs), or
2) Reduce the bandwidth. The 7A22 plugin is able to measure 10uV / Div. because it only has a 1MHz bandwidth.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Chappy
Sent: Saturday, February 20, 2021 1:09 PM
To: TekScopes@groups.io
Subject: [TekScopes] P6021 Noise Problem?

I recently acquired a P6201 active 1X FET probe which appears to have never been used. The inspection tag was still attached, the accessory pack was still sealed, and there was not a single mark on the probe's ground sleeve. All functions of the probe are working but it has a 2mVpp high frequency noise component riding on any signals that I measure.

The manual states a 'Noise Tangental' of 300uV (150uV RMS), a figure well below the noise signal that I am seeing.

I have tried the probe on both my 7403 through a 7A18, and a new Keysight DSOX1102G with the same results. I have tried powering the device with a Tektronix bench power supply, and a home made unit with the same results. Interesting thing is that the noise level riding on the signal is almost non existent when I feed it a supply of +/- 5V. As I increase the supply voltage above this the noise becomes more apparent. Based on this observation I believe that i can rule out noise picked up by the cable between the amplifier and the probe. I have also scoped the power supply rails with negligible abberations on the supply traces on a 2mv/div scale. With the Keysight scope (BW = 70Mhz) a screen capture shows that the lowest component of the noise starts at approx 100MHz.

I am starting to think that there may be components inside either the amp or probe that have degraded over time. Could a degraded or failed electrolytic capacitor contribute to this noise? Or could it be due to a damaged FET or BJT?

If anyone here has one of these I would welcome your input before I go chasing a problem which perhaps is completely normal for this probe. Once again, its physical appearance indicated that it has never (or hardly) been used.

Sincerely,
Steve











--
Dennis Tillman W7pF
TekScopes Moderator


Chappy
 

To all,

Thank you for all of your tips and advice. I may take me a few days to have a look at this again, but I will look at each one of your suggestions in turn and post the results.

Sincerely,
Steve


Chappy
 

I finally had a chance to get back to this yesterday. Thanks again to all who provided suggestions, and especially Dennis with his detailed reply. I have been a longtime lurker on this site and I have learned a lot about Tek equipment and troubleshooting in the process.

Tests performed:

1. I attached a normal Tek 100Mhz probe to two separate power supplies that were powered OFF. On a 2mv/div scale I was able to see a few cycles of a decaying signal that appeared to be approx 100Mhz. Visible on both my Tek and Keysight scopes.

2. I attached a 10ft loop of wire to my scope. I did not have a microwave diode but I think the wire alone is providing a clue. The wire picked up a 93Mhz signal with varying amplitudes from 4 to 6mVpp!

3. I hooked up the 6201 probe to my Keysight 1102G scope, injected a 1Mhz 5mVpp square wave. The square wave had both noise and an additional signal riding on the square wave. Using the single shot capture I was able to break out a 93Mhz signal riding on the square wave from the noise.

4. I tested my 6201 probe following the procedure that Dennis suggested by shorting the tip to the ground sleeve and then shorting that to the scope. I am still able to see the 93Mhz signal under the noise. Both the noise and the signal had an amplitude of less than 1mVpp.

I suspect that the 93Mhz signal is getting into the circuitry of the 6201 probe either through the the power cable, or the probe cable.

For now I am going to leave the probe as is. Most of the hobby work that I do right now is below 1Mhz, and if I need to use the probe, I will run it through my 7A13 plugin with the 5Mhz BW limit enabled. I will check this issue again when I move to a new location. Reference the 93Mhz signal, could this be a sub harmonic of a surveillance radar working in the GHz range? Could it also be a sub harmonic of wireless routers that can be as low as 900Mhz? I turned off all of the other potential noise sources in my house while performing these tests, to include fluorescent lights, and all SMPS wallwarts.

Sincerely,
Steve


Jim Ford
 

Nearby FM radio station, maybe?                   Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Chappy <sb.chadwick@hotmail.com> Date: 2/27/21 8:07 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] P6021 Noise Problem? I finally had a chance to get back to this yesterday. Thanks again to all who provided suggestions, and especially Dennis with his detailed reply.  I have been a longtime lurker on this site and I have learned a lot about Tek equipment and troubleshooting in the process. Tests performed:1. I attached a normal Tek 100Mhz probe to two separate power supplies that were powered OFF. On a 2mv/div scale I was able to see a few cycles of a decaying signal that appeared to be approx 100Mhz.  Visible on both my Tek and Keysight scopes.2. I attached a 10ft loop of wire to my scope. I did not have a microwave diode but I think the wire alone is providing a clue. The wire picked up a 93Mhz signal with varying amplitudes from 4 to 6mVpp!3.  I hooked up the 6201 probe to my Keysight 1102G scope, injected a 1Mhz 5mVpp square wave.  The square wave had both noise and an additional signal riding on the square wave.  Using the single shot capture I was able to break out a 93Mhz signal riding on the square wave from the noise.4. I tested my 6201 probe following the procedure that Dennis suggested by shorting the tip to the ground sleeve and then shorting that to the scope. I am still able to see the 93Mhz signal under the noise.  Both the noise and the signal had an amplitude of less than 1mVpp. I suspect that the 93Mhz signal is getting into the circuitry of the 6201 probe either through the the power cable, or the probe cable. For now I am going to leave the probe as is. Most of the hobby work that I do right now is below 1Mhz, and if I need to use the probe, I will run it through my 7A13 plugin with the 5Mhz BW limit enabled. I will check this issue again when I move to a new location. Reference the 93Mhz signal, could this be a sub harmonic of a surveillance radar working in the GHz range?  Could it also be a sub harmonic of wireless routers that can be as low as 900Mhz?  I turned off all of the other potential noise sources in my house while performing these tests, to include fluorescent lights, and all SMPS wallwarts.Sincerely,Steve


Ozan
 

2. I attached a 10ft loop of wire to my scope. I did not have a microwave
diode but I think the wire alone is providing a clue. The wire picked up a
93Mhz signal with varying amplitudes from 4 to 6mVpp!
This reminds me a time when I was chasing a ~ 88MHz coupling. Finally I connected a wire antenna to spectrum analyzer and turned on FM demod, it turned out to be a local station (KQED 88.5MHz). Antenna is not very close, over 40 miles away. If you have a spectrum analyzer with FM demod you can try it on the signal.
Ozan


Chappy
 

JIm, Ozan,

I didn't think that the nearest station would be an issue at over 10 miles away. I just checked, and they are broadcasting on 94.5Mhz. I narrowed down my problem frequency to between 93 and 95Mhz. Looks like we have found the culprit.

Steve


Tom Lee
 

That's why I asked the question that I originally did. When I was a teaching assistant back in the days when disco was king, students regularly complained about noise or "low-level oscillations" in their circuits "around 100MHz". They were seeing the same thing you were, and are, seeing.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/27/2021 12:19, Chappy wrote:
JIm, Ozan,

I didn't think that the nearest station would be an issue at over 10 miles away. I just checked, and they are broadcasting on 94.5Mhz. I narrowed down my problem frequency to between 93 and 95Mhz. Looks like we have found the culprit.

Steve




Chappy
 

Tom,

When I first read your post I discarded the idea thinking that the local radio station was too far away to be a problem and I hadn't yet discovered the signal buried under the noise. Once I had determined that there was a repeating signal buried under the noise, I jumped to the conclusion that it was somehow coming from the ATC radar across the street from my house.

Apparently I still have a lot to learn! Thanks again to everyone who helped.

Sincerely,
Steve


Jim Ford
 

Yep 88 to 108 MHz in the US is automatically suspected to be FM radio.  I understand FM broadcasts in Japan go down to 65 MHz.            JimSent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Tom Lee <tomlee@ee.stanford.edu> Date: 2/27/21 1:07 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] P6021 Noise Problem? That's why I asked the question that I originally did. When I was a teaching assistant back in the days when disco was king, students regularly complained about noise or "low-level oscillations" in their circuits "around 100MHz". They were seeing the same thing you were, and are, seeing.--Tom-- Prof. Thomas H. LeeAllen Ctr., Rm. 205350 Jane Stanford WayStanford UniversityStanford, CA 94305-4070http://www-smirc.stanford.eduOn 2/27/2021 12:19, Chappy wrote:> JIm, Ozan,>> I didn't think that the nearest station would be an issue at over 10 miles away. I just checked, and they are broadcasting on 94.5Mhz.  I narrowed down my problem frequency to between 93 and 95Mhz.  Looks like we have found the culprit.>> Steve>>> >>