How to mix outputs of two function generators? - Page 1
Forum section: electronics / testgear

author: calzap
author_rank: Frequent Contributor
timestamp: March 24, 2013, 01:54:37 am
content: What's the best way to mix the outputs of two, separate function generators? The two I have are a very nice Instek (mains powered) and an el cheapo Elenco (battery powered). Max frequency is 5 MHz; max Vpp is 10. A cheap, off-the-shelf solution would be best, but I'm not adverse to DIY.Mike

author: nctnico
author_rank: Super Contributor
timestamp: March 24, 2013, 02:11:44 am
content: For low frequency stuff you can build a summing circuit using an opamp. Otherwise create a 50 Ohm combiner (a splitter connected in reverse):http://www.microwaves101.com/encyclopedia/Resistive_splitters.cfm

author: JackOfVA
author_rank: Unknown
timestamp: March 24, 2013, 08:41:58 pm
content: The object is usually to keep energy from one generator from being coupled back into the second generator where the two signals can mix in the output stages and produce intermodulation products.If for some particular purpose you don't care about intermoduation products in the output, you can likely just parallel the generators assuming you are not doing some potentially dangerous things like running one generator with a high DC offset and/or high output level, as these might damage the generator output.As one poster said, for lower frequencies, an op-amplifier mixer will provide good isolation, as each generator operates into a load with a virtual ground.Once you are above the op-amp frequency range, a resistive splitter/combiner can be used, but it has significant drawbacks. The isolation between ports is obtained by the resistive loss, so the greater the isolation you want/need between the two generators, the larger the attenuation between generator output and common port. The good features of a resistive mixer are (a) cheap and (b) can be built for a wide frequency range.In RF engineering one normally uses a hybrid coupler / splitter to combine multiple signal sources. These are (in the lumped form configuration) a transformer that couples the input ports to the output port with little loss (usually 3 dB or 6 dB depending on the coupler design) whilst providing high isolation from generator to generator, 30 to 40 dB or more.I've written about this athttp://www.cliftonlaboratories.com/6_db_hybrid_combiner.htm-- the circuits explored include resistive and hybrid couplers, both transformer based and the Wilkinson coupler.

author: calzap
author_rank: Frequent Contributor
timestamp: March 25, 2013, 01:09:30 am
content: Jack,Thanks for the reply. Great website! But it will take a while to explore and digest it. Right now my needs are primarily educational with frequencies of a few hundred Hz to a few hundred thousand. Power loss is not a big deal either at this point; just so the scope can detect it.Mike

author: MasterOfNone
author_rank: Regular Contributor
timestamp: March 25, 2013, 09:15:42 pm
content: Silly question (probably) but since one of the generators is battery powered couldn’t the outputs be wired in series?I.e:Instek-Gnd -> Scope-GndInstek-Signal -> Elenco-GndElenco-Signal -> Scope-SignalIf you did this with two mains generators you would short the outputs, but in this case since the battery generator is isolated wouldn’t its waveform sit on top of the mains generator?

author: David1
author_rank: Contributor
timestamp: January 14, 2018, 09:09:08 am
content: Hi MasterofNone and calzap.Did you ever find out whether connecting the function generators in series as MaterofNone suggested works?My interest is similar to calzap, educational. I want to combine signals as input for practice designing RC filters to filter them out again. So most important for me is just not blowing things up!My main generator is a BK Precision 4017A (earth grounded) plus a little battery powered XR2206 kithttps://www.amazon.co.uk/gp/product/B01HMBFSPQ/ref=oh_aui_search_detailpage?ie=UTF8&psc=1.I've seen EEVblog video 279so believe the earthing issue should be fine, but what what about damaging ICs or polarised capacitors in the function generators?Gonna try it now!ThanksDavid