Noise on VCO voltage ramp

I've now uploaded 3 photos of what I'm seeing. The most obvious noise (apart from being obvious) is the 'comb' across the screen. I'm getting the most spurii at the lowest 'fundamental' (around 5Mhz) but as I sweep up in frequency the number of spurs declines markedly so by

The other form of noise I mentioned in an earlier post is the more attenuated and spread out one visible just to the right of the centre-frequency line. This noise, unlike the 'comb' is constantly flickering and random.

Not to mention ingominous failure! Well, the traces I'm seeing (and have now posted images of to the group) do bear a certain distinct similarity to what you described, Jeff, so if that be the case I'm wondering if one of the oscillators is being over-driven. I'm guessing that might give rise to 3rd harmonic spurii, don't you think?

Sorry for the late reply, but I had to waste the last few days dealing with the California Dept of Motor Vehicles to renew my drivers license and obtain a Real-ID endorsement. I passed both but am still recovering and might need a few more days to calm down.

Interesting spectrum analyzer display and description. The most interesting part is how you managed to produce the photo, without bothering to mention the:

1. Network analyzer test Frequency
2. SA (spectrum analyzer) horizontal MHz/div.
3. SA vertical full scale, dBm per division, and dBm full scale.
4. Whether you were misusing a scope probe (again) or feeding it with 50 ohms. Also, any attenuators or couplers in the RF path.
5. Did you remember to turn of the 1, 10, 50Mhz markers on the HP8754A?
6. Any other numbers which might be useful, especially those you previously mention, but which I'm too lazy to dig through your old postings to find.

Had you supplied some numbers, I would have dived into the HP8754A network analyzer service manual, and determined the frequencies of the two oscillators (3.6 - 4.3GHz minus 3.6 - 3.0GHz) used to produce your unspecified output frequency. I would then compare the possible intermodulation mixes of these two frequencies, with the mess on your spectrum analyzer screen, and see if any of the spurs are the result of oscillator feedthrough, intermod mixing, or overdriving the spectrum analyzer and creating the spurs in the SA. However, since your photos were not accompanied by any numbers, I can't do this for you. Incidentally, you might want to ask in one of the HP test equipment forums for what a working HP8754A RF output looks like on a spectrum analyzer. I would not be surprised if the mess on your screen is quite normal.

Incidentally, if you download and read the HP8754A service manual, you'll find a large number of Performance Tests. Performed properly in the manner specified, you should get a better idea if your network analyzer is functioning as expected.

Also, this is your second warning about providing insufficient information and numbers. Provide numbers with your problems or prepare thyself for the wrath of the radio gods. The recent failure of your Tek 2645A scope was a poorly aimed warning shot (I was aiming for your Tek 2232A scope) and a hint of things to come.

My sympathies. I've heard about the queues in those places and they must be pretty bad indeed to make the international news! Is it true they recruit retired-disabled ex-NYC taxi drivers for their counter staff?

Yeah, I know. I'm sorry but you know about as much as I do! If I'd zoomed out on those shots there's a bezel around the outside of the screen on that SA which is *supposed* to show all that info. Unfortunately, it's bust. This is a partly working unit I picked up at a ham rally and am in the process of re-commissioning. Unfortunately, my No.1 RF SA (the 8566B) is currently out of action with a recent "YIG oscillator unlocked" error message, so there's nothing to fall back on. It's kind of tough to do stuff with all this busted equipment. Even the equipment I picked up in prime working condition is letting me down lately. All I *can* say about those photos is the SA is set on the 0.01-1.8Ghz display range, I'm sniffing the VNA output with a few turns of wire rather than a direct connection, the 40dB attenuation is set (and seems to work!) and the VNA output frequencies in each picture are the filenames for each of those pictures. So the situation is "not ideal" but you have to work with what you've got, I guess.

I just noticed some points you mentioned I forgot to cover in my earlier post...

Shown in picture title for example 10Mhz.jpg means the VNA was set to

Full band mode so ~180Mhz/div.

No reliable info.

Neither - sniffing with few turns of wire.

Nope. Just the 40dB attenuation setting of the SA.

Yep, no markers.

Good suggestion.

I do actually have the original physical hard-copy of the service manual and was working my way through those performance tests when I got side-tracked by something else whilst waiting for the required phase-matched power splitter and some new terminations to arrive. I really MUST get back to that as they have since turned up in the post.

I also need to try out some other poster's suggestion re sweeping the VCO ramp with DC from batteries, too. So I have plenty to get on with whilst you digest my replies, Jeff. :-/

The VNA was putting out 10dBm in all those photos, btw. I'm reasonably confident that much at least is 'pretty accurate.'