Noisy Fluke 6060A/AN signal generator

I own a Fluke 6060A/AM (10 kHz to 520 MHz) synthesized signal generator that appears to be a source of noise. For example, if I set it to 1 Mhz and tune an AM radio to this frequency, I hear a light crashing noise and an occasional pop/click. If I put a scope across the radio's speaker, I can see the pops/clicks. Some other facts:

(1) I've poked around the power supply with my scope and find nothing unusual. (2) The magnitude of this noise is independent of the generator's RF output level. (3) When I switch-off the generator's output, but keep it powered and running, the noise and clicks goes away. (4) The RF output of the generator is clean. No noise or pops/clicks are "floating" on the signal. (5) The noise and pops/clicks only appears after, say, a 10-minute warm-up.

Other than this, the signal generator work fine. I still suspect the power supply--bad capacitors? Or perhaps some other noise source in the house is getting a free ride at the generator's expense.

Any suggestions?

-Dave

Reply to
david
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The AM broadcast band is noisy. Have you tried running the amplitude of the generator all the way up and loop coupling to the AM receiver? This would tend to swamp out external sources of noise.

If it's still noisy, then the generator is indeed acting up. A phase-locked loop is not the easiest thing to troubleshoot.

Reply to
Charles Schuler

I would start monitoring the AC line it's plugged into. it's noise that is just getting a free ride.. also, you should check the PS on the AC side of the Diodes/rectifiers. Bridges/Rectifilers have a happen of causing intermitting noise like this one they warm up and are getting ready to go bad. Also, even though you may not see any signal pops on the PS cap leads, that does not mean the cap is good. I've seen caps arc internally due to bad dielectric but not short. They would at some time how ever. Bad contact switches will also do this. What i do to find problems like this is to use a R.F. sniffer with a small loop so that you can tightly close in the area of detection. I have a grid meter i use for things like this. it works like a charm.. I have found a good many bad components that exhibit this behavior.

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Reply to
Jamie

Yipes. The Fluke 60xx generators are very complex and touchy. There are very complex delay-line phase-noise-reduction loops, with about a dozen delicate adjustments, plus a handful that are "factory adjust only". Plus a few adjustments that are burned into a EPROM.

I'd take it to the top of a tall building and play "how loud is the crash".

Reply to
Ancient_Hacker

I'm fairly sure phase noise is not significantly contributing to what I'm hearing.

Using my amateur radio transceiver interfaced to my home computer running a spectral analysis program, I've looked at the unmodulated signals from the Fluke signal generator and found them to be narrower than the carriers of the local AM broadcast stations. These AM stations provide "full quieting," having no noticeable noise similar in character to what I hear on the signal generator.

Even if the phase-noise reduction loop is the source of the problem, it's no going to matter much because I principally use the generator for receiver alignment. It's the pops and click that I'm looking to eliminate.

-Dave

Reply to
david

Dave ... since the noise starts after 10 minutes, it might be temperature related. Have you tried cold spray?

Reply to
Charles Schuler

Yes, I did.

I sprayed the circuit boards containing the output amp/modulator, controller, attenuator and power supply--no change. I made sure I sprayed the master oscillator--no change. I also made sure I sprayed the power supply capacitors--also no change. However, they are large and covered in plastic, so I doubt the spray had much internal effect on these insulated thermal masses. The only thing I don't recall spraying is the display circuit board.

In any case, I plan to replace the power supply capacitors anyway. They're probably 25 years old and one of them is "bulging."

-Dave

Reply to
david

Ah bulging! Yes, change those for sure.

Good luck. By the way, I have worked on several RF generators and have been able to fix them. It is worth the effort, since the good ones are costly to replace.

Reply to
Charles Schuler

Yeah, that sounds like a robust, field proof design.

Reply to
Homer J Simpson

I finally fixed it, taking more time to find the problem than I'll ever care to admit.

The problem was a surface mount capacitor that's in parallel with the varactor diodes in the voltage controlled oscillator (VCO). Specifically, it was capacitor C3, a 1.5 picofarad (pF) capacitor that's in series with R1, a 180 Ohm resistor. I removed this capacitor and in its place I've temporarily soldered in a 1 pF ceramic disc, zero-temperature coefficient capacitor.

Incidentally, I took Charles Schuler's advice and replaced the power supply capacitors, both electrolytic and tantalum. Although none of these proved to be the problem, the supply voltages are now cleaner.

-Dave, K3WQ

Reply to
david

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