Is there a danger to shortening a tracking generator of a spectrum analyzer? - Page 1
Forum section: electronics / testgear

author: laus
author_rank: Contributor
timestamp: June 07, 2020, 01:44:19 pm
content: I would like to do sweeps with the spectrum analyzer on a voltage balun. Checking with a multimeter at DC there is a short between all points. Looking at the schematic it is obvious why.Do I need to worry that the tracking generator is being short circuited to ground at some frequency or are spectrum analyzers designed to handle shorts?I'm using a Siglent SSA3021X and have done some sweeps on a home made common mode choke to see the signal vs common mode response. Now I would like to compare that to a commercially made voltage balun. But I'm hesitant about breaking the spectrum analyzer.

author: aristarchus
author_rank: Regular Contributor
timestamp: June 07, 2020, 04:09:14 pm
content: Your balun provides an impedance to the tracking generator output, thus it always has a resistive component at a given frequency.Regardless, I guess that the tracking generator output can withstand shorts if not continuously at least for brief periods, after all their power is in the range of 0dBm.Better ask @tautech though.

author: coppice
author_rank: Super Contributor
timestamp: June 07, 2020, 04:36:42 pm
content: Most engineers are pretty clumsy oafs. If instruments are not designed so their low power output signals can tolerate considerable abuse, they'll be sent for repair a lot and get the product a bad reputation for being fragile. Even a lot of high power outputs are designed to detect overload and shut down quite aggressively. Don't let this encourage you to be sloppy, though. No circuit is so well protected that you can't find a way to blow things up.

author: Wolfgang
author_rank: Super Contributor
timestamp: June 07, 2020, 07:45:15 pm
content: Quote from: laus on June 07, 2020, 01:44:19 pmNot even a Siglent should be blown up by this. A tracking generator in a spectrum analyzer or a VNA port must be able to work with *all* passive impedances.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2020, 08:11:19 pm
content: Tracking generators normally achieve 50 ohm source impedance with a suitable series resistor from the source. You can check this by connecting an external voltmeter (or similar) with and without 50 ohm termination: unterminated should give 2x voltage, or 6 dB more on a voltage measurement. The source should be capable of withstanding a short after that resistor. To blow up a tracking generator or similar output from a signal generator normally requires feeding "real" power from an external source by accident. Power amplifiers, however, can be damaged by improper load impedance, and should use protection circuits to avoid smoke.

author: tautech
author_rank: Super Contributor
timestamp: June 07, 2020, 08:40:17 pm
content: Quote from: TimFox on June 07, 2020, 08:11:19 pmYep, this ^Stated clearly in the SSA3000X datasheet:TG Output maximum reverse level Mean power: 30 dBm, DC: ±50 VDC

author: laus
author_rank: Contributor
timestamp: June 09, 2020, 09:49:31 pm
content: Thanks everyone. I went ahead and did the measurements. Nothing bad has happened.Quote from: TimFox on June 07, 2020, 08:11:19 pmI tried to follow your instructions and do a voltage measurement of the tracking generator output with a digital multimeter and oscilloscope without much success. The voltage fluctuates constantly. I guess due to the TG sweeping across the frequency range. I didn't have any more luck when reducing the frequency range or set it to a fixed frequency. I just couldn't get any meaningful reading.I probably just didn't setup things properly to do the measurement.

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2020, 10:18:47 pm
content: You may be seeing the TG output "mute" during the retrace from end frequency back to stop frequency. Otherwise, that would be strange--can you slow down the sweep to a very slow sweep time?