Strange amplitude problem with Hewlett Packard 3325A function generator

[Glubb]

Hello,

First I wich you a happy new year, second please apologize for my english : I'm french.


I have a very strange problem with a 3325A HP function generator :
all amplitudes with all signals (sine, square, triangle) are multiplicated by two at the output :
2,5 mV on the display give me 5 mw on the BNC output
it starts from 1 mV to 10 mV peak to peak

I've checked the following tensions :

a) Test on TP4 (with ACD grounded) :
all tension (with and without DC offset are good (correct : identical as the values given by the service manual)

b) Pulling out W23 : then TP4 reach to 6,64 V (correct)

c) Test on AMPin : correct

d) Test on AMPout : correct

e) Pressing AMPTD CAL : no CAL FAIL

f) Pressing AMPTD CAL with jumper AMPin disconnected : CAL FAIL

g) Attenuator relay checked : OK

h) Attenuator resistor checked : OK


Sommething very interesting : all the relays of the attenuator pad out at TWICE the normal tension :
K3 switches on "pad out" at 3 mV (expected : 1,5 mV)
K2 switches on "pad out" at 10 mV (expected : 5 mV)
K1 switches on "pad out" at 100 mV (expected : 50 mV)


For me the problem is not on the sine amplitude path, but before U29 : on the controller board (you can see U29 on the page 347 of PDF (noticed : Data Lach)

Where is the failure ?

Do you think it would be possible to make an atenuator 2/1 ?


For all your answers and help : thanks !

Regards.

GLubb

 

[Andy Watson]

Have to eliminate the obvious question - are you using a 50\$\Omega\$ load ? The output voltage is only correct when loaded.

[free_electron]

Have to eliminate the obvious question - are you using a 50\$\Omega\$ load ? The output voltage is only correct when loaded.

jup.... my thoughts exactly

[Glubb]

Hello,

Thanks for your answers.

50 ohms load changes a bit the value but it's not OK.

I've tried with 162 ohms resistor in series with 200 ohms multiturns trimmer... the good value is 300,00 ohms (exactly).

OK I have a solution, but I don't understand where is the failure, and why with 300,00 ohms load the amplitude is good (not absolutely perfect, but good ).

Glubb

[Tony_G]

Hey Glubb,

How are you measuring the voltage? Like Andy & Free_electron, it sounds to me like a 50 ohm termination issue.

If not that then maybe the issue is that you're entering an amplitude value in Vpp and trying to read it as a Vrms or Vp value.

Can you provide some more detail on what your setting on the 3325A (e.g. Press AMPTD key, then 1, then VOLT key, measured using a scope and see Vpp to b X volts) so we can help.

TonyG

[ogden]

Your "problem" is explained in every FAQ of signal generators:

https://www.keysight.com/main/editorial.jspx?ckey=1948055&id=1948055&nid=-11143.0.00&lc=eng&cc=MY

Even here on this forum it was discussed before:

https://www.eevblog.com/forum/beginners/signal-generator-output-impedance-question/

[Lorenzo_1]

Here's a couple of pictures that might help you with setup if you're unsure.  HP3325A with 1V sine wave run into scope through 1MOhm probe, 50Ohm lead and 1MProbe with 50Ohm terminator T-ed in.  The scope vertical and horizontal are the same in all pics.  Note the 1MOhm probe gives ~2V and is corrected to ~1V by 50Ohm termination.

[free_electron]

that hing has a 600 ohm mode too i believe. .. check the output impedance settings of the machine.

[Lorenzo_1]

I think the high impedance mode is only in units with the HV Option 002 fitted. If that's missing, as per mine, then  it's 50 ohm output only/

[Glubb]

Hello,

Thanks a lot to everybody for your answers 

I know how to use a scope and the difference between VRMS and Vpp.

How did I test the HP 3325A ?
- I connect it to a scope : Metrix OX8042 (with a BNC cable)
- F selected : 1000 Hz
- Amplitude : push "AMPTD" then push key "1" than key "MHz Volts"
- Reading amplitude on the screen of the scope : 2 Vpp

I've done this also with a Picoscope : same result (and for many amplitude values and many frequencies).
I've done also the test with the scope probe... same results...

But if I put a 300 ohms resistor à the middle of the cable (between ground and signal of course), I can read the good values for every amplitudes and every frequencies... Well it's not perfect : for very small amplitudes it doesn't fit exactly the value that I have programmed on the 3325A, but since approximately 5 mVpp to 10 Vpp it's OK.

The only way for me is to put a 300 ohms resistor load... and not a 50 ohms... that's strange...

Glubb

[Tony_G]

What is the measured value of R252 - It should be 50 ohms.

TonyG

[Lorenzo_1]

Can you confirm whether your 3325A has the high voltage option fitted - allows up to 40v output using button on lower RH corner of front panel. Mine doesn't have this option so front panel button switches output front to rear. As I read the manual the hv option has to output into >=500 ohm load. Page 3-12 of user manual sets out test procedure. page 4-22 sets out comprehensive amplitude test procedure which involves using 50 ohm terminator below 100kHz and 500ohm terminator to test hv amplitude. I'd suggest run through this completely before concluding there's a fault.

[Tony_G]

As I read the manual the hv option has to output into >=500 ohm load.

Interesting - My 3325B doesn't have Option 002 either, I hadn't looked at it - Great suggestion.

TonyG

[Glubb]

Hello,


Everything is now OK with an auto-term 50 ohm T BNC... (see picture 1) 

I still don't understand why a 300 ohm pure resistor also works (see picture 2).

In fact I don't know with what the auto-term 50 ohm is done (the pure resistance of it is : 69,8 ohm (with a multimeter, so mesured with a DC tension))...


Thank a lot for all answers !

Glubb

[Andy Watson]

Looks as if you are using oscilloscope probes to connect the signal (?) Measure the resistance of the probe tip to BNC plug - I bet it's not 0\$\Omega\$ !

[Glubb]

Hello,

For Andy : yes you are absolutely right. It's about 270 ohm ! So in fact I've done an attenuator 2:1 with three resistors... (see picture)

It's true that we often forget the cable resistance (impedance) for mesure, because current is so low that usualy it doesn't impact the result.

Thanks for all.

Glubb

[Armadillo]

I think this old HP is kind of silly in this modern world context.
When you set 1V, its going to output 2V and demand you to form a divide by 2 network at the output.
Imagine you build a circuit having a high input impedance of 100Kohms, hence its kind of troublesome to use it.
The reason is its output impedance cannot be varied.

But as long as you know the context and requirements, using it has never been a problem to many.   

[ogden]

I think this old HP is kind of silly in this modern world context.

Modern world of Arduino?   

It does not matter - old generator or new. When you use RF tools like signal generators, spectrum analyzers and so on, you shall know basics about source/cable/load impedance. Live with that.

[Armadillo]

I think this old HP is kind of silly in this modern world context.

Modern world of Arduino?   

It does not matter - old generator or new. When you use RF tools like signal generators, spectrum analyzers and so on, you shall know basics about source/cable/load impedance. Live with that.

The reason is its output impedance cannot be varied.

But as long as you know the context and requirements, using it has never been a problem to many.   

It's also kind of silly why people don't read and understand.
No, we don't need to live with that. Use a modern gen where the output impedance can be selected.

[TheUnnamedNewbie]

I think this old HP is kind of silly in this modern world context.

Modern world of Arduino?   

It does not matter - old generator or new. When you use RF tools like signal generators, spectrum analyzers and so on, you shall know basics about source/cable/load impedance. Live with that.

The reason is its output impedance cannot be varied.

But as long as you know the context and requirements, using it has never been a problem to many.   

It's also kind of silly why people don't read and understand.
No, we don't need to live with that. Use a modern gen where the output impedance can be selected.

Please, feel free to give us a list of those modern synth/generators that have that option? Because I've worked with some very modern ones and they were all 50 Ohm. Sure, some AFGs allow you to pretend it's not 50 ohm and display signal voltage into a high-Z load, but the output level is than not guaranteed to be within specifications, nor is the distortion performance.