Do all GPSDO's radiate 10MHz from Coax? - Page 1
Forum section: electronics / projects

author: NY2KW
author_rank: Regular Contributor
timestamp: July 13, 2019, 03:18:23 am
content: I use a Thunderbolt GPSDO plus TADD-1 distribution amp as a reference for my spectrum analyzer, signal generator, scope and counter. These all share a common earth chassis ground. I usually work on boatanchors but recently wanted to get into SDR and I bought the UBitx HF SDR transceiver kit. Calibration is needed and in firmware is done by zero-beating to WWV or Signal Gen on 10.000000 MHz carrier. I couldn't do it because of a loud birdie around 10.000.000 Mhz. The UBitx has 12MHz and 25MHz crystals so I didn't think the radio was the source. I quickly found that with UBitx powered off, and just using a broadband antenna into my SA, there is a -60 dBm signal at 10.000.000 MHz. I got rid of the TADD and focused on just one coax Freq Ref between Thunderbolt and any device on my bench other than the SA. If coax is disconnected from Thunderbolt, signal disappears (< -130 dBm) so coax is acting as antenna. I thought the Thunderbolt must be poorly shielded but I also have a Lucent GPS XO/Rb pair shielded like a tank and also a Leo Bodnar GPSDO reference and I see the same 10 MHz -60 dBm spur on my SA with these also. I changed to RG-402 semi-rigid shielded coax and same thing. I have a FLEX radio about 10-15' away from my workbench and it's noise floor goes down to -120 dBm and no spur there. Is this expected for any GPSDO as frequency references on the workbench?Jerry

author: ArthurDent
author_rank: Super Contributor
timestamp: July 13, 2019, 04:07:49 am
content: WWV at 10Mhz doesn't exist in my little lab. I have so many 10Mhz cables running around, any radio is dead quiet at 10Mhz because of all the separate sources adding together. I have 3 GPSDOs, multiple 10Mhz xtal/rubidium standards, and a 10Mhz distribution amplifier with about 20 outputs and the unused outputs aren't terminated. I wouldn't even try to eliminate the radiated 10Mhz. I suspect even ground loops and the center/shield of the coax acting as a poor transformer may add to the problem as well.You shouldn't need to get WWV at 10Mhz when you have a far more accurate source available. You might have to run a long length of coax to another part of your house where the 10Mhz is near zero and calibrate the receiver there. I assume this is something that has to be done once or rarely so this may be the simplest way out.

author: NY2KW
author_rank: Regular Contributor
timestamp: July 13, 2019, 11:01:20 am
content: Thanks for replying to my post. WWV doesn't exist on my bench either :-) It was the recommendation of the UBitx user manual but I always turn to my Rigol signal generator.Have you ever looked on your SA for any buried signal around 10MHz ?? Yes, for the UBitx calibration is infrequent (hopefully) but not sure how moving location will help.... I would need to move my UBitx AND Signal Generator so I still would need a long coax from one of the GPSDO to the SigGen and I dont see why that won't act as an antenna much the same as on my workbench.Jerry

author: NY2KW
author_rank: Regular Contributor
timestamp: July 13, 2019, 11:03:27 am
content: I was thinking to try to add common mode chokes on both ends of the coax coming from GPSDO.Jerry

author: thinkfat
author_rank: Unknown
timestamp: July 13, 2019, 01:56:30 pm
content: If the output of the gpsdo and the inputs are proper 50 ohms impedance, the cable will not radiate. Since it does, you have an impedance mismatch, most likely at the inputs of your test equipment. You can try common mode chokes, like, winding the coax around a ring core. But I doubt it will be effective.Gesendet von meinem Nokia 6.1 mit Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: July 13, 2019, 04:23:25 pm
content: Quote from: NY2KW on July 13, 2019, 11:03:27 amThat is where I would start. Also run a test where just the shield of the coaxial cable is connected without the center conductor.Quote from: thinkfat on July 13, 2019, 01:56:30 pmBy itself an impedance mismatch will not cause radiation. But driving the shield of an unbalanced transmission line with a common mode signal will. Some distribution amplifiers use transformer isolated outputs or include common mode chokes (current mode baluns).

author: duak
author_rank: Super Contributor
timestamp: July 13, 2019, 04:42:53 pm
content: NY2KW, a coax cable can radiate if there is an alternate ground connection between the source and destination units. This is because the signal current's EM field is only fully cancelled out if the signal return current in the shield is exactly the same. Sometimes adding ferrites will help - try them on power and control cables too. Often the best is to use the shortest cables to minimize loop area. Other things to check are that the shield crimps on the cable's connectors are solid and that the cable has the correct impedance and is properly terminated.