Systron Donner 762 series (very ancient) portable SA - RF emitted from input? - Page 1
Forum section: electronics / rf-microwave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: March 25, 2025, 03:39:26 pm
content: I have a 1970s era Systron Donner 762-2A (similar to 762-1 model) portable RF/microwave spectrum analyzer. It was a contemporary of the Tektronix 491 I believe, and was intended to cover up to 12 GHz on coaxial input, and to 40 GHz using the (typically included) pair of waveguide mixers. It is a non-preselected design (so uses "signal identification" procedure to validate a signal among the typical spurs.) Needless to say, no microcontroller!Today I decided to try defining a calibration table of sorts, by using a signal generator (an old Wiltron 610/6237) as source, 3-port 18 GHz weinschel DC-RF splitter (N connectors), and HP 436/8484A power meter. Idea is to place the splitter at the analyzer input port, source to one of the splitter ports, power meter to the other. As the loss through the splitter is about 6 dB (it's just three resistors in Y configuration), in principle the indication on the analyzer is the power level seen by the power meter, hence a way to generate a calibration table. This seems to be a method used to calibrate more contemporary spectrum analyzers.It turns out the analyzer is emitting about +6 dBm (presumably the 2-4 GHz YIG LO's power into the simple diode internal mixer). I wasn't expecting this. This made identifying the generator's power (passing through the same splitter on it's way both to the analyzer and the power meter) with any accuracy impractical (generator output about +10 dBm leveled.)Anyone have any experience trying level-calibrate an ancient spectrum analyzer with this sort of LO leakage (or does this leakage indicate a problem with the otherwise-seems-to-be-working analyzer?)Thanks for any suggestions (unfortunately, a new Keysight signal analyzer would be a little more expensive.)

author: jwet
author_rank: Frequent Contributor
timestamp: March 27, 2025, 11:27:59 pm
content: I see that you got no responses- I'll try to start a discussion with a comment at least.No direct experience with this instrument- all I can say is wow!, +6 dBm on the INPUT of any instrument is a lot of power! This can't be right. I can't even imagine 0 dBm, even way out of band. Something this egregious would have to be documented somehow.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: March 27, 2025, 11:42:00 pm
content: Thanks for the comments...Something about a spectrum analyzer that was built when I was in high school - an adventure!I've got a second analyzer (of this type) - they sell for about nothing and are comfortable to use - and first order of business I think is to measure it's input with the power meter.

author: jwet
author_rank: Frequent Contributor
timestamp: March 28, 2025, 12:51:52 am
content: I love this old gear that I lusted after in my younger years. I have a lot of mid 80's gear that I love and use daily. The newer stuff doesn't have any service data so I've kind of settled in this mid area. The truth is that I think a nano VNA or a Tiny SA have comparable specs! Good luck- I'll watch this thread, 6 dBm ain't right...

author: jonpaul
author_rank: Super Contributor
timestamp: March 30, 2025, 12:06:28 pm
content: input mixer leakage, common issue

author: wb0gaz
author_rank: Frequent Contributor
timestamp: March 30, 2025, 12:40:24 pm
content: Thank you, johnpaul - presume then this would be a problem if the device being tested is an active device and presence of the unexpected (and strong) RF alters or otherwise responds to the DUT operating conditions?

author: AFTORF
author_rank: Contributor
timestamp: April 16, 2025, 04:29:23 pm
content: The use of a tunable stop band filter to attenuate or cancel LO leakage will be a solution ?

author: TimFox
author_rank: Super Contributor
timestamp: April 16, 2025, 05:14:01 pm
content: What type of input mixer is used in the 762?If it isn't a "doubled-balanced mixer", the LO to RF (input) leakage can be substantial.If it is supposed to be a double-balanced mixer, you might have a blown diode in the four diodes usually used in a DBM.You stated a "simple diode mixer" in the original post.Would it be possible to substitute a modern packaged DBM (e.g., from Mini-Circuits) in its place?

author: jonpaul
author_rank: Super Contributor
timestamp: April 17, 2025, 04:37:37 am
content: Bonjour WBOGaz, had used these types of SA in 1960s...1970s for defense sigint, the old HP.Not familiar with the SD but assuming that they're similar....The mixer emission of LO and harmonics is an old story. Agree t check if mixer has blown or damaged diodes, eg due to previous overloadEasy solution is add an external LPF, to pass desired spectrum band and stop LO emission above band.Mini Circuits has excellent LPF in convenient connectors like BNC, SMA, N in all Zo and band cut configurations.Just the rambling of an old retired EE. Bon chance !J '68