The AM Forum

   I don't understand the waveform of the 400 Hz modulated rf output from my BK 200D signal generator. I expected to see something similar to the output of my URM-25F (attached jpg). Instead I saw the second waveform(attached jpg).

   What am I seeing from the BK instrument?

   First time I've tried to attach pics to a message. Hope it works.

Ed

Check the ripple on the power supply of that generator.

Thanks BIL,
   I'll check that out. But don't be misled by the apparent difference in frequency of the two waveforms. The BK output is shown on a different timescale. The modulation? is, in fact, 400 Hz as best as I can estimate.

Ed

Hey Ed,

I've seen a waveform pattern just like your second one! Only it was on my URM-25D. Went through that old beast and did the Dallas Lankford capacitor replacement stuff. (his URM-25 Notes 11-88 rev 1-94) Good pattern on the O scope now.

As a reminder to the choir, a good tech in a quality workplace sends his test/calibration equipment to a metrology lab on some sort of a schedule. Results should be kept in a data base of the test equipment and what stuff it was used to check or calibrate.

Always had a sigh of relief when the lab reports came back and the equipment was up to snuff. Bad day at the salt mines if it didn't.

Can't afford that on my own dime any more, but always do my darnest to test the test equipment somehow first. Got to wonder how many hams get bad results working on their radios because their test equipment was an auction place special in perfect working condition. ???

Compare apples to apples. Different sig-gens at the same freq and 400 Hz, use same scope and same scope settings. What do you see?

The first waveform shows one signal modulated by another.

The second one shows the two signals with little or no intermodulation.  Just the sum of two signals at widely different frequencies.

Yup!  That's what it is

Al

I think Buddly nailed it. Hay Bud!

   Thanks to all who replied. I seem to have fixed the problem by doing what the manual says and terminating the output with 50 ohms. Doh! Previously I was going direct to the scope thru a 10 meg probe.
   I don't, however, understand why the difference in impedance would change a modulated waveform to an unmixed superposition.
   In any event, I can use the generator.

Ed

With the high impedance, you are likely picking up 60 Hz from various fields in the vicinity of your probe (test equipment power supplies, lights, AC wiring).

I have a B&K E-200D signal generator and found that the high impedance mircrophone connector on the front panel (lower right) was susceptible to interference.

I replaced it with high quality BNC panel mount connector and problems dissappeared.

While I had the covers off, I cleaned all switches with Deoxit which improved frequency and output level stability.

Phil - AC0OB

The B&K may have a crappy modulator design that needs the output circuit to be terminated to work right. Well as long as it is working, and you know about the termination requirement, you are good to go.

The only issue is that the item you attach it to could disturb the termination and therefore the wave. So just always double check the waveform.

Or, use a 50 Ohm 10dB attenuator on the B&K, if possible. That will firstly terminate the output well, and secondly give some isolation between the generator and the circuit to be injected.