¿ªÔÆÌåÓý

ctrl + shift + ? for shortcuts
© 2025 Groups.io
  1. TekScopes
  2. Topics

Scope bandwidth

Steve
 

Hi all,

I have a Tek 2235. Its -3dB bandwidth specification is "DC to at least 100MHz" when using the 5mV/division to 5V/division vertical scales.

For no particular reason, I decided to verify that specification. I connected an HP8657B signal source to the CH1 input with a 36in RG59 coaxial test lead and used a BNC-tee to terminate the scope input with 50 ohms. Then I made some measurements using 0dBm at 1MHz as the reference. Using this method, the -3dB point is about 20MHz on both CH1 and CH2. It's down about 24dB at 100MHz.

I was expecting the -3dB point to be at least 100MHz. Is my methodology flawed? Do I have a bad scope?

Thanks.

Steve
 

Hi Steve,

The reference frequency used typically is 50 kHz. However there shouldn't be much difference in scope response from 50 kHz to 1 MHz.

It sounds like there is a problem either with your scope, or the 8657B, or the 3 ft. coax lead. To narrow things down you would need a known good scope...

RG59 coax which is 75 ohm impedance probably has 75 ohm BNC connectors installed that do not perfectly fit the 50 ohm BNC connectors on your equipment. That might cause a problem at higher RF frequencies. So an inexpensive thing to try would be to buy a correct RG58 cable with 50 ohm BNC connectors.

I have usually found that the Tek bandwidth spec is significantly conservative. Actual bandwidth is around 20% greater than spec on my 2465A and 2467 while the actual bandwidth is around 60% greater than spec on my TDS 3032. I am using a Tek UHF leveled sine wave generator for these measurements.

Good luck.

Patrick Wong AK6C

--- In TekScopes@..., Steve wrote:

Hi all,

I have a Tek 2235. Its -3dB bandwidth specification is "DC to at least
100MHz" when using the 5mV/division to 5V/division vertical scales.

For no particular reason, I decided to verify that specification. I
connected an HP8657B signal source to the CH1 input with a 36in RG59
coaxial test lead and used a BNC-tee to terminate the scope input with
50 ohms. Then I made some measurements using 0dBm at 1MHz as the
reference. Using this method, the -3dB point is about 20MHz on both CH1
and CH2. It's down about 24dB at 100MHz.

I was expecting the -3dB point to be at least 100MHz. Is my methodology
flawed? Do I have a bad scope?
 

¿ªÔÆÌåÓý

RG-59 is 75 ohms. You want to use some 50 ohm?cable (RG-58)?for your jumper. Your BNC T with a 50 ohm termination will introduce some VSWR to the measurement. Use a 50 ohm through adapter for the best result. Or, you could place a 20 dB attenuator in place of the through adapter.
?
Tom
?
?

----- Original Message -----
From: Steve
Sent: Friday, January 25, 2013 10:41 AM
Subject: [TekScopes] Scope bandwidth

?

Hi all,

I have a Tek 2235. Its -3dB bandwidth specification is "DC to at least
100MHz" when using the 5mV/division to 5V/division vertical scales.

For no particular reason, I decided to verify that specification. I
connected an HP8657B signal source to the CH1 input with a 36in RG59
coaxial test lead and used a BNC-tee to terminate the scope input with
50 ohms. Then I made some measurements using 0dBm at 1MHz as the
reference. Using this method, the -3dB point is about 20MHz on both CH1
and CH2. It's down about 24dB at 100MHz.

I was expecting the -3dB point to be at least 100MHz. Is my methodology
flawed? Do I have a bad scope?

Thanks.

Steve
 

Hi,

RG59 is 75ohms. That may be your problem. Use RG58.


Jerry Massengale



-----Original Message-----
From: Steve
To: TekScopes
Sent: Fri, Jan 25, 2013 9:41 am
Subject: [TekScopes] Scope bandwidth

?
Hi all,

I have a Tek 2235. Its -3dB bandwidth specification is "DC to at least
100MHz" when using the 5mV/division to 5V/division vertical scales.

For no particular reason, I decided to verify that specification. I
connected an HP8657B signal source to the CH1 input with a 36in RG59
coaxial test lead and used a BNC-tee to terminate the scope input with
50 ohms. Then I made some measurements using 0dBm at 1MHz as the
reference. Using this method, the -3dB point is about 20MHz on both CH1
and CH2. It's down about 24dB at 100MHz.

I was expecting the -3dB point to be at least 100MHz. Is my methodology
flawed? Do I have a bad scope?

Thanks.

Steve
Ray
 

¿ªÔÆÌåÓý

Steve,
I don't have a 2235 but my 465 has a "20 MHz bandwidth" filter on the input.? If the 2235 has something similar, make sure it is not engaged.? If not:

The standing wave ratio for your test setup would be about 1:5 to 1.? Although that would cause some inaccuracies in you test indications, I doubt seriously that it would amount to response of? -24 db down at 100 MHz.? I would then guess you have another possibly more major problem.? Go ahead and use a piece of 50 ohm cable like RG-58 for your test.? You will likely still see a major loss at that frequency.

Ray, W4BYG

On 1/25/2013 11:21 AM, jerry massengale wrote:

Hi,

RG59 is 75ohms. That may be your problem. Use RG58.


Jerry Massengale
-----Original Message-----
From: Steve
To: TekScopes
Sent: Fri, Jan 25, 2013 9:41 am
Subject: [TekScopes] Scope bandwidth
?
Hi all,

I have a Tek 2235. Its -3dB bandwidth specification is "DC to at least
100MHz" when using the 5mV/division to 5V/division vertical scales.

For no particular reason, I decided to verify that specification. I
connected an HP8657B signal source to the CH1 input with a 36in RG59
coaxial test lead and used a BNC-tee to terminate the scope input with
50 ohms. Then I made some measurements using 0dBm at 1MHz as the
reference. Using this method, the -3dB point is about 20MHz on both CH1
and CH2. It's down about 24dB at 100MHz.

I was expecting the -3dB point to be at least 100MHz. Is my methodology
flawed? Do I have a bad scope?

Thanks.

Steve

 

Using 50 ohm cable or adding an attenuator like Tom suggests would
give better results but I do not think that is the problem in this
case.

Having a -3dB point at 20 MHz is awfully suspicious. Is the 20 MHz
bandwidth limit of the oscilloscope activated? It is controlled by a
front panel button just to the left of the ADD ALT CHOP switch labeled
BW LIMIT. When tested, all of my 22xx series oscilloscopes come very
close to the 100 MHz bandwidth specification and also very close to
the 20 MHz bandwidth specification when the bandwidth limit switch is
pressed.

Note that you can also test bandwidth if you have a pulse generator
with a fast transition time. The rise and fall time of a 100 MHz
oscilloscope should be 3.5 nanoseconds. When the 20 MHz bandwidth
limit is selected, the rise and fall time should be 17.5 nanoseconds.

Rise or Fall Time in nanoseconds = 350 / Frequency in MHz

On Fri, 25 Jan 2013 10:41:33 -0500, Steve <steve65@...>
wrote:

Hi all,

I have a Tek 2235. Its -3dB bandwidth specification is "DC to at least
100MHz" when using the 5mV/division to 5V/division vertical scales.

For no particular reason, I decided to verify that specification. I
connected an HP8657B signal source to the CH1 input with a 36in RG59
coaxial test lead and used a BNC-tee to terminate the scope input with
50 ohms. Then I made some measurements using 0dBm at 1MHz as the
reference. Using this method, the -3dB point is about 20MHz on both CH1
and CH2. It's down about 24dB at 100MHz.

I was expecting the -3dB point to be at least 100MHz. Is my methodology
flawed? Do I have a bad scope?

Thanks.

Steve
 

As others have stated, be sure to use a 50ohm coaxial jumper, a thru termination if you have one, and be sure that the 20MHz BW limit switch isn't turned on.

Beyond that, there have been some scopes that have a lower BW rating for the highest sensitivity settings. I don't recall if the 2235 was one of them. Try measuring at something greater than 5mv/div.

I did a video a few weeks ago on this topic, and showed my basic measurement setup. Not terribly calibrated, but works well enough as a check:


Alan

--- In TekScopes@..., Steve wrote:

Hi all,

I have a Tek 2235. Its -3dB bandwidth specification is "DC to at least
100MHz" when using the 5mV/division to 5V/division vertical scales.

For no particular reason, I decided to verify that specification. I
connected an HP8657B signal source to the CH1 input with a 36in RG59
coaxial test lead and used a BNC-tee to terminate the scope input with
50 ohms. Then I made some measurements using 0dBm at 1MHz as the
reference. Using this method, the -3dB point is about 20MHz on both CH1
and CH2. It's down about 24dB at 100MHz.

I was expecting the -3dB point to be at least 100MHz. Is my methodology
flawed? Do I have a bad scope?

Thanks.

Steve
Steve
 

Thank you Ray and David and Alan. I was unknowingly testing the response of the 20MHz filter and looks like it meets spec :-)

Disengaging the 20MHz filter yields more reasonable results. At 100MHz, CH1 is down 3.5dB while CH2 is down 2.9dB -- numbers that I was expecting to see first time around.

My apologies to all for introducing an element of confusion regarding the test setup. I am using RG58 coax and 50 ohm BNC connectors, misstated it in the initial post. I have from time to time need a 50 ohm thru termination and will endeavor to acquire same.

Appreciate the guidance from all.

Steve

On 1/25/2013 12:43 PM, w2aew wrote:
As others have stated, be sure to use a 50ohm coaxial jumper, a thru termination if you have one, and be sure that the 20MHz BW limit switch isn't turned on.

Beyond that, there have been some scopes that have a lower BW rating for the highest sensitivity settings. I don't recall if the 2235 was one of them. Try measuring at something greater than 5mv/div.

I did a video a few weeks ago on this topic, and showed my basic measurement setup. Not terribly calibrated, but works well enough as a check:


Alan


--- In TekScopes@..., Steve wrote:
Hi all,

I have a Tek 2235. Its -3dB bandwidth specification is "DC to at least
100MHz" when using the 5mV/division to 5V/division vertical scales.

For no particular reason, I decided to verify that specification. I
connected an HP8657B signal source to the CH1 input with a 36in RG59
coaxial test lead and used a BNC-tee to terminate the scope input with
50 ohms. Then I made some measurements using 0dBm at 1MHz as the
reference. Using this method, the -3dB point is about 20MHz on both CH1
and CH2. It's down about 24dB at 100MHz.

I was expecting the -3dB point to be at least 100MHz. Is my methodology
flawed? Do I have a bad scope?

Thanks.

Steve

------------------------------------

Yahoo! Groups Links
 

How do you know the generator's amplitude is flat ?
It may not be.

Be aware that some scopes have a reduced bw spec at the higher sensitivities.

Use RG-58, not RG-59

HankC, Boston
 

--- In TekScopes@..., "Patrick Wong" wrote:

Hi Steve,

The reference frequency used typically is 50 kHz. However there shouldn't be much difference in scope response from 50 kHz to 1 MHz.

It sounds like there is a problem either with your scope, or the 8657B, or the 3 ft. coax lead. To narrow things down you would need a known good scope...

RG59 coax which is 75 ohm impedance probably has 75 ohm BNC connectors installed that do not perfectly fit the 50 ohm BNC connectors on your equipment. That might cause a problem at higher RF frequencies. So an inexpensive thing to try would be to buy a correct RG58 cable with 50 ohm BNC connectors.

I have usually found that the Tek bandwidth spec is significantly conservative. Actual bandwidth is around 20% greater than spec on my 2465A and 2467 while the actual bandwidth is around 60% greater than spec on my TDS 3032. I am using a Tek UHF leveled sine wave generator for these measurements.

Good luck.

Patrick Wong AK6C
Good point Patrick,
Tiny impedance mismatches can really throw off a BW test. High speed scopes usually place a pad right at the input to reduce the reflected energy back into the cable. If you are testing a 100 MHz scope with a 50 generator and a 75 ohm cable, the mismatch is huge and could easily account for bad measurement results.

- Steve
 

On Fri, 25 Jan 2013 23:21:16 -0000, "Steve" <ditter2@...> wrote:

--- In TekScopes@..., "Patrick Wong" wrote:

Hi Steve,

The reference frequency used typically is 50 kHz. However there shouldn't be much difference in scope response from 50 kHz to 1 MHz.

It sounds like there is a problem either with your scope, or the 8657B, or the 3 ft. coax lead. To narrow things down you would need a known good scope...

RG59 coax which is 75 ohm impedance probably has 75 ohm BNC connectors installed that do not perfectly fit the 50 ohm BNC connectors on your equipment. That might cause a problem at higher RF frequencies. So an inexpensive thing to try would be to buy a correct RG58 cable with 50 ohm BNC connectors.

I have usually found that the Tek bandwidth spec is significantly conservative. Actual bandwidth is around 20% greater than spec on my 2465A and 2467 while the actual bandwidth is around 60% greater than spec on my TDS 3032. I am using a Tek UHF leveled sine wave generator for these measurements.

Good luck.

Patrick Wong AK6C
Good point Patrick,
Tiny impedance mismatches can really throw off a BW test. High speed scopes usually place a pad right at the input to reduce the reflected energy back into the cable. If you are testing a 100 MHz scope with a 50 generator and a 75 ohm cable, the mismatch is huge and could easily account for bad measurement results.

- Steve
Just for the heck of it I ran this test using a 50 ohm cable and a 32
inch RG-179 video cable with my SG503, 2232, and a 50 ohm feedthrough
terminator. I set the SG503 to x 0.1 mode so the internal attenuator
would be inline and the oscilloscope vertical sensitivity was
50mv/div.

With the 50 ohm cable, the bandwidth was right at 100 MHz like it
should be. With the 75 ohm cable, I measured it at about 78 MHz.

1 - 11 of 11