dbm question - Page 1
Forum section: electronics / testgear

author: Crambone
author_rank: Frequent Contributor
timestamp: February 16, 2022, 07:07:07 pm
content: I acquired a HP 8656A and when I set the output to 0 modulation, 20MHz, 0.0dBm and look at it on my RIGOL DSA 815 I’m seeing 20.005211 MHz at -62.22 dBm. I know frequency is off a bit but my question is what should I be seeing for dBm on my 815?I’m using a 3’ double shielded cable between the 2 devices.Andrew

author: Feynman
author_rank: Regular Contributor
timestamp: February 16, 2022, 07:29:18 pm
content: In a 50 ohm system (50 ohm source, 50 ohm transmission line, 50 ohm sink) you should see 0 dBm. Have you double-checked your connection?

author: TimFox
author_rank: Super Contributor
timestamp: February 16, 2022, 07:41:22 pm
content: That looks like your source to destination connection is a very small capacitance: i.e., a broken wire. It could be in the generator before the output connector, in the cable connectors, in the cable itself, or in the destination input connections. It could also be a short circuit in any of these places. It could also be the output switch on the generator ("RF On") being broken or in the wrong position. Sometimes, the output attenuator in the generator is broken.If you set the source for 100 kHz, you should be able to see something on a cheap CRO or even a good AC voltmeter as a sanity check on the generator.I don't know if that generator includes an output fuse, sometimes built into the connector. With the power off, you can check for a DC resistance of 50 ohms across the connector.

author: Crambone
author_rank: Frequent Contributor
timestamp: February 16, 2022, 09:17:09 pm
content: Ok thank you gentleman I will take your advise and look over what you suggested.

author: nctnico
author_rank: Super Contributor
timestamp: February 16, 2022, 11:07:35 pm
content: In addition to the other suggestions: What is the RBW setting on your spectrum analyser? If it is too narrow and the frequency is off a little, you can also get a level which is too low. Start these kind of tests with the RBW set to max (1MHz or so).

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2022, 11:33:13 pm
content: Quote from: Crambone on February 16, 2022, 07:07:07 pmYou shouldn't need anything fancy or complicated. Make sure your cable has continuity. You should get 0.0dBm plus or minus a little bit. As others have said, you can set just set the output to a voltage--say 100mVRMS, and use a scope or decent voltmeter to verify the output--which should be double into high impedance. Mechanical failure of the attenuator pads is common on these and it is a VERY fiddly repair. I fixed mine but just barely--I had one broken gold finger and IIRC, I had to swap it into the one attenuator position that only uses one. Or something like that. The o-rings and ball bearings I have, but the other mechanical parts and gold finger contacts are NLA/unobtanium.https://lazyelectrons.wordpress.com/2018/09/02/hp-8656a-signal-generator-repair/

author: Crambone
author_rank: Frequent Contributor
timestamp: February 16, 2022, 11:45:06 pm
content: Rbw? I’m not sure exactly what that is.

author: Crambone
author_rank: Frequent Contributor
timestamp: February 16, 2022, 11:47:04 pm
content: Thanks for the video I’m going to look at that tomorrow it doesn’t look difficult just fragile and time consuming. I will check all my connectors and jumpers also. I still intend to do the procedure in the video either way.

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2022, 11:59:12 pm
content: As long as nobody has tried to fix it before and you have very steady hands, you'll just need grommets. They are #1070, preferably D60 hardness. If someone elsehastouched it, all bets are off and you may need other parts. Only the ball bearings are readily available, they are 3/32 G25, not stainless steel (which means you can grab them with a magnet).https://industrial-rubber-products.astrotex.com/item/all-categories/stock-grommets/m-1070

author: TimFox
author_rank: Super Contributor
timestamp: February 16, 2022, 11:59:57 pm
content: "RBW" is resolution bandwidth. If the frequency error is higher than the RBW, the level measured will be artificially low, as the input frequency falls outside the bandpass.

author: nctnico
author_rank: Super Contributor
timestamp: February 17, 2022, 12:02:20 am
content: Quote from: Crambone on February 16, 2022, 11:45:06 pmReceive bandwidth. This page explains some of the basics:https://micronix-jp.com/english/note/application/fundamentals_of_speana_2.html

author: Crambone
author_rank: Frequent Contributor
timestamp: February 17, 2022, 12:10:17 am
content: Thank you, it refers that the screw/ fasteners need to be torqued but I can’t find the torque required? I’d assume inch pounds of some sort.

author: nctnico
author_rank: Super Contributor
timestamp: February 17, 2022, 12:27:18 am
content: Don't start opening things up before ruling out a cable or settings issue. The suggestion to use an oscilloscope to check the signal level is one you should follow!

author: Crambone
author_rank: Frequent Contributor
timestamp: February 17, 2022, 01:30:54 am
content: Yes, I will do that