How to test a signal generator HP 8647A? - Page 1
Forum section: electronics / repair

author: ttx450
author_rank: Regular Contributor
timestamp: February 07, 2022, 07:32:39 pm
content: Hi, I picked up a used HP 8647A from ebay and I am trying to see if it works. Have a 30 day return option.The unit power up fine with self test and no errors. I am using a 75ohm cable, as I don't currently have a 50ohm cable. The SG has 50ohm output. Also the amplitude is in dB, which is newer to me.My frequency counter goes to 120MHz, my dig scope 100MHz. Running the SG at 50kHz and at 0 dBm I am not able to get a frequency reading. My scope shows a weird pattern at around 5.0mV Pk-Pk and varying around 33kHz-50kHz.This SG will take a while to learn on how to use it, I am just trying to see if it works or send back. Appreciate any help.

author: w2aew
author_rank: Super Contributor
timestamp: February 07, 2022, 08:18:04 pm
content: A 0dBm output should give you 632mVpp into a 50ohm load, and double that into a high-impedance load (such as your counter or scope input) - it should certainly be easy to see. Be *careful* with your 75 ohm cable - the connectors used for 75 ohm are not always compatible with their 50 ohm counterparts. Mating them can either cause no-connection (best case) or connector damage (worst-case). The issue is typically centered around the diameter of the center pin and socket...I'm assuming that you've read the manual so you know how to set the frequency, modulation, amplitude and to enable the RF output...

author: ttx450
author_rank: Regular Contributor
timestamp: February 07, 2022, 11:19:10 pm
content: Quote from: w2aew on February 07, 2022, 08:18:04 pmThanks for the info, I did not know that. I have ordered a 50ohm cable. I found out I had the RF turned off. I have looked at basic startup functions of the 200 page manual, which are pretty straight forward. The amplitude limits (pre-set limits) I don't really understand. They seem to limit the amplitude at different frequency?I cannot get my frequency counter to see anything. It has a 2V pk-pk min, and can't get everywhere close to that voltage. I might try another FC, but has the same voltage requirements or close to it.Here's a picture feeding my scope with it @ 1.4V 250kHz. I dont understand the ghosting, nor the unclear sine wave? The scope shows around 20mV pk-pk, 268.9-270.1 Khz

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2022, 11:31:49 pm
content: You might want to post a good photo of front face of the unit in operation so we can look at it and spot anything you may not have set correctly.I have not repaired or disemboweled this line of HP RF generator yet, but I've heard that they are more difficult to repair then earlier boat-anchors. It does sound like yours may not be working, and on other models the first thing I'd suspect is an attenuator issue. Try varying the output from the max (is it 13dBM?) downwards by 10dB at a time down to -47dBM or so and see if you ever pick up a signal. Also try it at 10 and 100MHz, which is well within the range of your scope. Don't bother using anything but your scope at this point. If you don't have right cable, don't go wrecking anything--just use a 1X probe with the scope to the center of the connector. Obviously don't bother with 100MHz in that case.If it doesn't work, I'd suggest returning it if you didn't buy it 'for parts or not working'.

author: ttx450
author_rank: Regular Contributor
timestamp: February 09, 2022, 12:12:33 am
content: Quote from: bdunham7 on February 07, 2022, 11:31:49 pmHi, thanks for the help. Just using the probe is probably best way to tell, being I have iffy cables etc. I used probe into SG at 10x. After 60MHz I could not get reliable reading. At 50MHz it drops off fast around 9.1dBm, it is a error on the spreadsheet. (fixed it)Here are some voltages at specific frequency and amplitude. The unit has a cut off of 10dBm. The chart shows where frequency starts dropping off at a given dBm. It drops off fairly fast at a .1 dBm or a few more 10th on the low side. The MHz looks a little more erratic then the KHz. Let me know what you think of these numbers and if I should do more test? Thanks.

author: bdunham7
author_rank: Super Contributor
timestamp: February 09, 2022, 12:37:27 am
content: Quote from: ttx450 on February 09, 2022, 12:12:33 amI can't figure out exactly what you are doing. Can you simply try this set of steps:Is the probe properly compensated? if not or if you aren't sure, set the scope up with the probe attenuation to 10X, connect a probe set to 10X to the scope and then connect the probe to the scope calibrator output. Dial it in as needed (or press Autoset) and then verify that you have a clean square wave--if not adjust according to the manual.Once you're sure you have a properly set up probe, set the scope to 10X, 500mV/div vertical and 1us/div horizontal.Set the SG for 0 dBM and 1 MHz. Connect the probe to the SG.If you can, post a photo of the scope and the SG in that configuration.

author: ttx450
author_rank: Regular Contributor
timestamp: February 09, 2022, 12:52:04 am
content: Quote from: bdunham7 on February 09, 2022, 12:37:27 amWhat I did is set various frequency on the SG & read the voltage at 10 dBm and turned down the amplitude until the frequency started falling off. Is not that what you asked? Yes the probes are compensated. I will post up what you asked. Thks

author: ttx450
author_rank: Regular Contributor
timestamp: February 09, 2022, 01:04:24 am
content: Quote from: bdunham7 on February 09, 2022, 12:37:27 am

author: ttx450
author_rank: Regular Contributor
timestamp: February 09, 2022, 01:05:27 am
content: Hang on made a mistake and had the RF off

author: bdunham7
author_rank: Super Contributor
timestamp: February 09, 2022, 01:52:39 am
content: OK, that looks reasonably good.Staying at 1 MHz, take it down 10dB at time. You'll have to adjust the scope vertical as you go, you can refer to this chart to see what you should expect if you don't already have this info.http://wera.cen.uni-hamburg.de/DBM.shtmlSo you should end up at -40dBM and 5mV/div, you should see roughly 14mV peak-to-peak although it may be a bit noisy. Any lower than that and the scope will be near its limits.If you are using the probes that came with the SDS1104X-E and they are compensated properly, you can repeat this set of tests at 10MHz and 100MHz. If you consistently see roughly what the chart predicts, than as far as can be determined, the SG is working.

author: ttx450
author_rank: Regular Contributor
timestamp: February 09, 2022, 02:14:20 am
content: Quote from: bdunham7 on February 09, 2022, 01:52:39 amThanks for that chart I've been reading on how to calculate it. I will check those different settings and get back.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 09, 2022, 03:16:49 am
content: The advice from bdunham7 is good. The units "dBm" are a power level referenced to 1 mW. To know what voltage voltage is present when indicating a power level, the load resistance must be specified. The signal generator is assuming a 50 ohm load. Most everything with RF test equipment (generators, analyzers) is assuming a 50 ohm load. The chart in the link bdunham7 showed assumes a 50 ohm load. If the load is large (like the 1 Mohm input of a scope) the voltage will be twice what is shown on the chart.To have the signal generator display in units that are more familiar, try setting the amplitude by pressing: AMPLITUDE 500 mV (I haven't used this model, but I am guessing it will work). Instead of dBm, hopefully it will show mV (or some voltage scale). I just tried this on my HP signal generator (a different model) and the amplitude shows "499.441 mV". So if you have a 50 ohm load, that will be the RMS voltage at the output assuming a 50 ohm load. The peak output is sqrt(2) times the RMS output. So 500mV RMS would give a 707mV peak output. If the load was not 50 ohms, but a high impedance, the peak output would be twice that, 1.414V, a nice hefty signal to see on a scope. Hope this isn't too confusing.

author: ttx450
author_rank: Regular Contributor
timestamp: February 10, 2022, 02:47:45 am
content: Thanks everyone.. should have 50 ohm cable soon. see how many files I can upload. few picts

author: ttx450
author_rank: Regular Contributor
timestamp: February 10, 2022, 02:48:44 am
content: 10Mhz

author: ttx450
author_rank: Regular Contributor
timestamp: February 10, 2022, 02:49:26 am
content: 100Mhz

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2022, 05:16:35 am
content: Are you using a ground clip on your probe?Obviously those signals look rough, but I wouldn't want to conclude anything without getting a proper cable on there.

author: ttx450
author_rank: Regular Contributor
timestamp: February 10, 2022, 05:37:00 am
content: Quote from: bdunham7 on February 10, 2022, 05:16:35 amNo not using, I tried a ground clip for a bit and did not change. Maybe should test more.