Signal generator with distorted wave shapes

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:

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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

Reply to
IW2GNB
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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 ) snipped-for-privacy@tele2.it

Reply to
Max65

Ciao Massimo, thanks for your reply.

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)

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

Reply to
IW2GNB

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

Reply to
Max65

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

Reply to
IW2GNB

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

Reply to
Max65

Hi Massimo,

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

Reply to
IW2GNB

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