Signal generator with distorted wave shapes

[IW2GNB]

Hi Massimo,

Anyways, I have a friend who worked for a long time for the R/S
service department. He said me that almost all the APN 62 he repaired
had the output power stage faulty.
Try to troubleshoot that stage.

Thanks for the info! Will try to take a look to the last stages just before
the ouput connectors. It may be the culprit is some degraded capacitors...
or some much more complicated issue.


Marco

 

[Max65]

Hi Marco,
I experienced a similar issue with my old HP3314A bought on Ebay. It
had the PNP output driver collector-emitter junction shorten with a
25ohm stray resistance (it was blown).
It was a 20MHz generator, and the output PNP driver had a transiton
frequency above 1GHz. Since I can't find on the market a substitute, I
had to rebuild the output stage using and XDSL integrated driver from
Analog Device.
But may be your issue is different from mine, because my FG shown the
distortion at very low frequency (below 10Hz) and with negative offset
greter than -5V, while it looked absolutely perfect used with other
setups.

Anyways, I have a friend who worked for a long time for the R/S
service department. He said me that almost all the APN 62 he repaired
had the output power stage faulty.
Try to troubleshoot that stage.
About the power supply, he say that it's an analog regulator, so my
supposition about the superimposed modulation was wrong.
Have a great day.

Massimo

 

[IW2GNB]

For the wave distortions, just try to put an adequate passthrough load
at the end of the coaxial line before connecting the line to the
scope.

I've connected the output to a load (a dummy load, i.e. purely resistive)
matched to the output impedance (50 Ohm) but nothing changes.

Changing the output impedance from 10 to 640 Ohm does not affect the wave
shape.


Marco

 

[IW2GNB]

I have a R&S APN 62 signal generator (1 Hz - 260 kHz) displaying a strange
behavior-- it generates distorted wave shapes at "high" frequencies.
At low frequencies all the wave shapes are ok but starting from a few
decades of kHz sawtooth and triangular signals are more and more distorted.
Less so the sine wave.
Take a look to the oscilloscope display: www.xenologos.net/1.jpg (from
1.jpg to 8.jpg)

Any idea on the possible causes?
As it is a frequency-dependent malfunction it may be due to some degraded
component (condensers?).



Marco

 

[Max65]

Hi Marco,
Fig. 3.jpg looks like a FM modulation superimposed to the 255kHz
carrier, while Fig. 2 looks like an AM modulation, what doy you canged
between the two figures?
About the other distorted signals they looks like unmatched load
impedance (except for the superimposed AM modulation) do you placed
the correct load at the generator output?
If you like it, you can send me a private e-mail, so we can write
Italian.
Massimo ( IK1IZA )
mporzio@tele2.it

 

[IW2GNB]

Ciao Massimo,
thanks for your reply.

Fig. 3.jpg looks like a FM modulation superimposed to the 255kHz
carrier, while Fig. 2 looks like an AM modulation, what doy you canged
between the two figures?

Good question! let me check as I don't recall. Probably the difference is in
the selected wave shape (all wave shapes seem to tend to a distorted sine
wave as the frequency rises)

About the other distorted signals they looks like unmatched load
impedance (except for the superimposed AM modulation) do you placed
the correct load at the generator output?

No load at all... I have connected the BNC ouput connector of the generator
directly to the oscilloscope input with a RG58 cable. By switching from bal
to unbal does not change anything in the wave shape. I have tested the
generator using a probe but the distortions are always there.

Marco

 

[Max65]

Hi Marco,
I don't know almost anything about the APN 62, the superimposed
signals could be due to a damaged capacitor into the power supply
stage (supposing it's an high frequency switched mode PSU).
For the wave distortions, just try to put an adequate passthrough load
at the end of the coaxial line before connecting the line to the
scope.
Have a great day,
Massimo