P6302 / AM503B 330MHz Oscillation Where is this from?


 

I have been testing a P6302 50MHz Current Probe just given to me by one of
our members. It is connected to an AM503B I inherited from someone who got
it cheap because it had a number of intermittent "issues".

Fortunately for me the AM503B has been working lately and I have had no
trouble getting around the few issues it has reported intermittently lately.

One of the tests I performed on the P6302 was to measure the noise floor.
For this test it is impossible to separate the RMS noise of the AM503B,
which is 250uA typical, from the 300uA noise of the P6302. All you can do is
sum them together (square root of the sum of the squares), which gives 390uA
of combined noise, to see if the result meets the combined noise spec.

There is some subjectivity to estimating the noise amplitude visually but
what I saw on the CRT was enough that I think its fine. However what I
noticed and I cannot explain is the 1mA p-p 330MHz signal on top of the
noise. It is very obvious in the photo I took. Neither the AM503B nor the
P6302 have that kind of bandwidth. So where is it coming from?

My test configuration consists of a late generation Tek TM500 High Amplitude
Test Generator (AKA Braatz Generator) outputting 500mV into a 50ohm power
splitter. One leg of the power splitter goes to a 7A29, which displays 175mV
on the 7844 scope. The other leg goes to a Tek 067-0559-00 50ohm current
measurement fixture. 175mV across the 50 ohm fixture will generate 3.5mA of
current. The P6302 is degaussed and then hooked onto the 067-0559-00. The
AM503B is in a TM504 mainframe. It is set to 1mA/Div. (its most sensitive
range). Its output goes to another 7A29. The pulse from the AM503B measures
3mA on the CRT of the 7844. That is pretty darn close.

My two questions are:
Where is the repeatable (it isn't changing at all) 1mA 330MHz signal coming
from?
Why isn't it present right before the rising edge of the pulse?

I uploaded two photos to TekScopes. The first one clearly shows the 330MHz
signal is present on the current pulse and afterwards. The 2nd photo clearly
shows the pulse is not present prior to the rising edge of the pulse. The
photos are at
https://groups.io/g/TekScopes/album?id=264402&p=Created,,,100,2,0,0
The album is called
"Mystery 330MHz signal on top of a 3mA pulse from a P6302 - AM503B
Combination"

Dennis Tillman W7pF


Albert Otten
 

Dennis, in your pictures I see about 3 periods of oscillation in 20 ns. Isn't that corresponding to 150 MHz in stead of about 300 MHz? (I hesitate to bring this up...).
I tried this with similar setup, HAPG, AM503/A6302, 7854/7A29s. I used 5 mA/div to avoid noise. And guess: I also saw oscillations about 3 periods per 20 ns! These oscillations (excited by the rising and falling edges) fade away long before the next pulse occurs. Hence there is nothing to see prior to the rising edges.
You normally won't check these current probes with sine waves far above 50 MHz. I tried this with the 220 MHz - up generator because of the mentioned 330 MHz. There was a slight increase in AM503 output about 380 MHz, not very convincing. Have to repeat this with the SG503. Did you ever try this?

Albert


Albert Otten
 

(continued) No pronounced resonance with SG503 either. Albert

You normally won't check these current probes with sine waves far above 50
MHz. I tried this with the 220 MHz - up generator because of the mentioned 330
MHz. There was a slight increase in AM503 output about 380 MHz, not very
convincing. Have to repeat this with the SG503. Did you ever try this?

Albert


 

Hi Albert,
First: what is HAPG?
Second: that was an excellent suggestion to try feeding it a signal from a SG503 to see how it behaves with higher frequency signals. The SG503 only goes up to 250MHz. The SG504 goes higher which would let me test the AM503/P6302 at up to 1.05GHz

Dennis

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: Monday, May 24, 2021 7:00 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] P6302 / AM503B 330MHz Oscillation Where is this from?

Dennis, in your pictures I see about 3 periods of oscillation in 20 ns. Isn't that corresponding to 150 MHz in stead of about 300 MHz? (I hesitate to bring this up...).
I tried this with similar setup, HAPG, AM503/A6302, 7854/7A29s. I used 5 mA/div to avoid noise. And guess: I also saw oscillations about 3 periods per 20 ns! These oscillations (excited by the rising and falling edges) fade away long before the next pulse occurs. Hence there is nothing to see prior to the rising edges.
You normally won't check these current probes with sine waves far above 50 MHz. I tried this with the 220 MHz - up generator because of the mentioned 330 MHz. There was a slight increase in AM503 output about 380 MHz, not very convincing. Have to repeat this with the SG503. Did you ever try this?

Albert







--
Dennis Tillman W7pF
TekScopes Moderator


Dave Wise
 

High Amplitude Pulse Generator, I bet.

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Dennis Tillman W7pF via groups.io
Sent: Sunday, May 30, 2021 8:14 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] P6302 / AM503B 330MHz Oscillation Where is this from?

Hi Albert,
First: what is HAPG?
Second: that was an excellent suggestion to try feeding it a signal from a SG503 to see how it behaves with higher frequency signals. The SG503 only goes up to 250MHz. The SG504 goes higher which would let me test the AM503/P6302 at up to 1.05GHz

Dennis
-=-=-=-=-=-=-=-=-=-=-=-


Albert Otten
 

First: what is HAPG?
Dennis, I meant the High Amplitude Pulse Generator. Meanwhile I tried a bare P6302 probe with, using merely pins M and N as inputs (signal and GND) to a 7A19. This converts 500 mV at the current fixture to 10 mV 7A19 nput. I saw no sign of that "ringle" or whatever it should be called.
Albert