Tracking Generators - Page 1
Forum section: electronics / testgear

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: August 17, 2013, 02:55:13 am
content: I am passively shopping around for a spectrum analyzer. Perhaps I would buy used on Ebay if see one but problem is that at some point in the past I decided that if I will buy spectrum analyzer then I will certainly buy one with tracking generator installed. I have seen few really good units but all without tracking generator, so I started thinking of alternatives. I never owned spectrum analyzer so I am wondering if I get one without tracking gen what will be my options then? I understand that if I get used equipment I may never be able to add that option and even new ones may require replacement of whole unit. I have seen some external, stand-alone tracking generators, but not many. What makes and RF signal gen a tracking generator? Does it have to have something like standard signal out to spectrum analyzer to tell it where it is on sweep sequence or something like that?

author: w2aew
author_rank: Super Contributor
timestamp: August 17, 2013, 03:13:57 am
content: Quote from: Alexei.Polkhanov on August 17, 2013, 02:55:13 amGenerally, it's the other way around. The spectrum analyzer usually supplies an LO signal to the tracking generator, so that its output tracks where the SA is sweeping.

author: cyr
author_rank: Frequent Contributor
timestamp: August 17, 2013, 05:50:52 am
content: You can find some DIY tracking generators on the web for older analyzers, such as:http://www.ke5fx.com/tr503.htmlBasically, the spectrum analyzer is a radio receiver, and the tracking generator is the matching transmitter. The tuning LO signal(s) are shared (output by the analyzer) so they are always in sync.More modern analyzers, which have the option of an internal TG, don't generally have LO outputs for an external TG.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: August 17, 2013, 07:47:37 pm
content: Quote from: cyr on August 17, 2013, 05:50:52 amThis one seems to have LO output only as an option:http://www.home.agilent.com/en/pd-1000002159%3Aepsg%3Apro-pn-8595EM/emc-analyzer-9-khz-to-65-ghz?cc=CA&lc=eng. - option 009 that is. I found no external tracking generators anywhere.Looking at what is available on used equipment market I found that majority of Spectrum Analyzers don't have tracking gen option installed. So I started wondering why people buy $5-10K gear without option that cuts its usefulness in half? I would not be able to test a RF filter for example or find a self-resonant frequency of a capacitor or tune an antenna ... many other thing I don't know about.Everyone is just busy checking what center frequency of their neighbor's wifi signals and microwave ovens?-Alexei.

author: G0HZU
author_rank: Super Contributor
timestamp: August 18, 2013, 12:42:55 am
content: I guess a lot depends on the quality of measurement you want to make but I'm not a fan of using tracking generators apart from using them for very basic swept measurements.There are some analysers I can remember that had decent TG performance but many TGs are a bit ropey and I include some expensive HP models in this ropey category.Also, I would recommend you run a mile from any HP analysers in the HP859x range because you can buy far better HP models for just a bit more money. eg the HP856x range looks the same at a casual glance but is markedly better.I have only used HP859x analysers a few times and they are not good by today's standards and I class them as a field technician's analyser. eg many of them only have 70dB log range and even then the performance of the log section isn't great over the full 70dB. Also the synthesiser used in them is very basic compared to similar models from HP that only cost a bit more on the used market. Of course they are still arguably way too good for basic hobby use but why buy a technician's analyser when you can have a proper lab grade analyser for just a bit more money?The HP8560A is a nice (used) 2.9GHz analyser and often comes with a TG option fitted but it can give strange results when you alter the RBW as the TG doesn't always track truthfully. Or at least the ones I used didn't.The alternative would be to buy a decent spectrum analyser without a TG fitted and do any swept measurements with something like a standalone N2PK based VNA kit. Some variants of this run up to over 1GHz and the RF performance will easily outclass a spectrum analyser with a TG.The other option is to try and find something like an old HP4396A spectrum analyser as this instrument can also act as a decent VNA as well and it operates up to 1800MHz. But these tend to attract high prices even though they are typically 20 years old. The B version can be a lot newer and has an LCD display instead of CRT but these are often very expensive on the used market.However, if you still decide to go for a spectrum analyser with a TG fitted then look for one that offers a decent normalisation function

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: August 18, 2013, 06:22:04 pm
content: Quote from: G0HZU on August 18, 2013, 12:42:55 amAs I mentioned before from user manual for HP859x SWEEP OUT is an option and, rarely available one as far as I can see. I am speaking only about used analyzers where I cannot add new options when I buy. I considered VNA option and I am unsure if Vector Signal Generators can double as external tracking generators - something can go wrong, like type of tracking or triggering signal won't match or something like that. Besides all Vector Signal gens I have seen are obscenely expensive or obscenely old (like 1980 s).

author: PA4TIM
author_rank: Super Contributor
timestamp: August 18, 2013, 07:38:45 pm
content: I go with G0HZU, I have no experience with TGs on SA's but I(;n repairing a Tek 2710 with TG. That has a big dynamic range an has a normalisation function.But I have 3 VNA's. An old HP upto 100 MHz from 1968 and two VNWA sold by SDR-kits. They are super upto 500 MHz, can be used very well upto 1400 MHz..http://sdr-kits.net/VNWA3_Description.htmlI have written a series tutorials about using a vna.http://www.pa4tim.nl/?p=1594http://www.pa4tim.nl/?p=676picture from my newest vnwa

author: w2aew
author_rank: Super Contributor
timestamp: August 18, 2013, 07:57:22 pm
content: Quote from: Alexei.Polkhanov on August 17, 2013, 07:47:37 pmKeep in mind that it is possible to use a free-running sweep oscillator with your spec an to make many of the same measurements, with a few caveats:- it will take a lot longer since you'll have to setup at least an order of magnitude difference in the sweep speed of the generator and analyzer, and use a max-hold trace.- in-band distortion products from the full sweep will be added to the response in the max-hold trace.

author: G0HZU
author_rank: Super Contributor
timestamp: August 18, 2013, 08:15:52 pm
content: QuoteI think you are perhaps looking at this the wrong way around if you want to make good quality swept measurements.From my perspective, a decent VNA (Vector Network Analyser) is a very versatile tool for RF design work. You can use it to analyse lots of linear circuits and you can measure complex impedance and also export 1 or 2 port data models for the circuits to an RF simulator and this helps with computer modeliing.A VNA also has various error correction (calibration) routines that allow it to correct for its own limitations and this reduces measurement uncertainty. This takes it way beyond the performance of a spectrum analyser with a TG option fitted.All of the above uses of a VNA are a great way to learn about RF design. A TG on a spectrum analyser is really just a toy for a technician to play with. You are in a great position to get the best of both worlds if you buy something like one of the VNA kits shown by PA4TIM for swept work and then buy a spectrum analyser without worrying about it having a toytown tracking generator optionAs I said in my last post you can also consider a combined VNA and spectrum analyser like an old school HP4396A/B but I would recommend the VNA kit and something like a used HP8560A/E or even the Rigol 815 if you only want to do basic spectrum analysis. I know the Rigol has a TG option but it is one of the worst performing TGs I've ever seen.

author: KJDS
author_rank: Super Contributor
timestamp: August 18, 2013, 08:34:55 pm
content: I'm another one to give a thumbs up to a VNA, however a decent one is very expensive, beyond the means of most hobbyists, especially if you want to use it above 1GHz. Having said that, if you want a decent spec ana, and it has a tracking generator too then there's still a lot of measurements that can be made with it. Just be aware that most RF circuits are tuned more for return loss than the through characteristic so a good quality directional couple is also very beneficial.

author: PA4TIM
author_rank: Super Contributor
timestamp: August 18, 2013, 08:39:06 pm
content: The VNWA has several modes/ VNA, VNA with external coupler, SA, SA+TG (and offset possible for mixers or converters), LCR m,eter, very versatile TDR, very high res and accurate counter, signal generator and powermeterThe SA funtion is not to be compared with a good SA because dynamic range is not big, the noisefloor is only around -100dBm (but with slow sweeps and smaller filter setting it can go deeper. But the other functions are very good.) but still very usefull.The N2PK can outperform the VNWA, can use the same software but as far as I know you must build it yourself and to get it so good is not a simple task. I builded 2 VNWA's, one for me and one for a friend, that was a complete kit but still very much work.You are not there with a SA and TG, you need a directional coupler or bridge too..

author: G0HZU
author_rank: Super Contributor
timestamp: August 18, 2013, 08:59:19 pm
content: I agree that a decent VNA is normally very expensive but if you are happy to to buy used (and >20 yrs old) then the key to success is to be patient and wait for a bargain.Over time I've bought:An HP8714B 3GHz VNA ex rental (Livingstone Hire) with ALL options fitted for £250 that had a simple fault to repair.An HP8568B spectrum analyser for £125 (fully working)An HP8566B 22GHz spectrum analyser for £1000 (ex rental fully working from Microlease)An Advantest TR4172 1800MHz spectrum analyser with TG and VNA option fitted for £400 (fully working)So all of that lot cost me just a bit more than the cost of a new Rigol 815 with a few options fitted.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: August 19, 2013, 01:35:37 am
content: Quote from: w2aew on August 18, 2013, 07:57:22 pmI have Agilent E4421B, 250kHz-3GHz RF generator, it is about 9 y old analog version. It has sweep out which generates 0 to 10V output depending on where it is on sweep sequence. It can sweep over a list of frequencies or do automatic sweep /up or down in fixed steps, but minimum dwell time is 1 (one) millisecond.I configured it to go from 1 MHZ to 200MHz with 1 millisecond (minimum allowed) dwell time - it was pretty slow sweep, nowhere near 200 milliseconds plus it is not continuous - it is in discrete increments. I have whole thing captured on the scope - green is sweep out signal and red/purple is actual RF signal. (I am not sure why amplitude fluctuates, I use nice, brand name 50ohm feed through terminator on the input of the scope so it should be flat. Also I just noticed that there are gaps in between - one around 25 MHz, probably when it jumps between internal ranges. I am not sure how tracking gens do it but they probably don't have gaps even on entire range up to 3G?

author: KJDS
author_rank: Super Contributor
timestamp: August 19, 2013, 07:46:09 am
content: A tracking generator is created in a spec ana by mixing the first local oscillator with another fixed oscillator. The first local oscillator is what is used to scan the spectrum analyzer. If you have a good sig gen, and your spectrum analyzer has a local oscillator output then it would be best to make up a mixer board to create your tracking generator.

author: rollingstone
author_rank: Contributor
timestamp: August 18, 2014, 05:39:31 pm
content: Hi,this topic is old, but maybe You are still alive?So even if cheap SA does not have LO output, but anyway somwhere in SA there is LO and is it possible to "steal" that signal and atach a new connector somewhere on the backplate and connect output from LO to it? I think it is possible. What do you think?

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 18, 2014, 07:22:06 pm
content: Quote from: rollingstone on August 18, 2014, 05:39:31 pmIt is possible but you have to be very careful not to alter the LO (output level, impedance, spurious etc) of the SA, so you need a good amplifier followed by a good splitter, both able to do the full range of the SA's LO.