A CLOSE LOOK TO LOW COST NOISE GENERATORS

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glovisol
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A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by glovisol » Sat Feb 16, 2019 8:25 am

In this thread:

https://www.sdrplay.com/community/viewt ... f=5&t=3973

generic judgment was passed on low cost noise generators being on the market. In this thread I wish to look deeper into this matter.

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glovisol
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Location: Piedmont, Italy

Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by glovisol » Sat Feb 16, 2019 8:49 am

A CLOSE LOOK AT LOW COST NOISE GENERATORS

Low cost noise generators, such as Noise Generator BG7TBL, allow anyone, spending the paltry sum of 15 USD, to do many Tracking Generator / Spectrum Analyser measurements which, without them, would have a cost prohibitive to the average operator/amateur.

But before looking at the technical aspects, I wish to put on record I strongly object to contemptuous, derisive and unfair definitions, such as "Chinese junk" which through unwarranted generalizations, tend to offend other peoples of our world. The referenced text, by Mr. Owen Duffy, in comment to the performance of the BG7BL reads:
This was sold with no specifications, and for good reason… it is simply Chinese junk.
This said: but what can anyone expect to have from a Noise Generator sold at a price that in our world could not even cover, by far, the cost of the materials? Even much more accurate noise generators, sold at a price 10 times greater, do not provide the high output necessary for passive filter measurement, but for Mr. Duffy, ignoring the existence of attenuators, high power output is a disadvantage. Never the less, let us look, without comment, at the performance of the BG7BL in the screens below.
Attachments
NOISE GEN 8-15 MHz.jpg
NOISE GEN 8-15 MHz.jpg (156.97 KiB) Viewed 2943 times
NOISE GEN 11-22 MHz.jpg
NOISE GEN 11-22 MHz.jpg (156.43 KiB) Viewed 2943 times
NOISE GEN 21-29 MHz.jpg
NOISE GEN 21-29 MHz.jpg (156.01 KiB) Viewed 2943 times

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glovisol
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Location: Piedmont, Italy

Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by glovisol » Sat Feb 16, 2019 9:12 am

As we can see, we go from -50 dBm @ 2 MHz to -58 dBm @ 29 MHz, but this difference is not important, in LP / HP filter work, because what we are concerned with is the frequency range above above and below the cutoff frequency, where the noise output is flat within measurement limits, just look at the screen uploaded below. Furthermore far off the output difference is known (the Spectrum Analyser tells us) and allowance can be made. If we also have an $ 60 Network Analyser, then the Return Loss measurement will complement the far off measurement.

So, to make a long story short, to measure our filter, as hobbists, should we spend upward of $ 3,000 for a Tracking Generator /Spectrum Analyser, to test our filter?

OR

Should we spend upward of $ 600 for a more accurate, but low level, Noise Generator + an equally accurate Wideband Amplifier?

OR

Should we simply decide not to plan and build our filter as we cannot measure it?

OR

Should we spend $ 15 on a modest Noise Generator that, once all is said and done, can do the job?
Attachments
NOISE GEN 1-8 MHz.jpg
NOISE GEN 1-8 MHz.jpg (155.24 KiB) Viewed 2943 times

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Roger
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Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by Roger » Sat Feb 16, 2019 5:00 pm

One does not have to spend $600 USD for a quality noise source. Units that are properly designed and don't have heat problems are available for under $100 USD. These products have specifications for the product and correction curves for receiver sensitivity for some models. You can see the specs and pricing here >> http://g8fek.com/noise-gen.html

The BG7TBL noise source can be used for hobby applications but one should be aware of the pitfalls and limitations of the device. These have been discussed on other Internet forums and readers might find the following interesting.

Owen Duffy Blog (well known amateur radio technical writer) - https://owenduffy.net/blog/?p=11193
EEVBlog (most popular test equipment forum on Internet) - https://www.eevblog.com/forum/testgear/ ... 16-03-06_/
FlightAware Discussion Group - https://discussions.flightaware.com/t/n ... urce/18437

Modifications have been made to the BG7TBL noise source to improve the output characteristics and to prevent amplifier failure when running at the manufacturer rated voltage of 12 volts. The most extensive have been done by an amateur PA0NHC and the details are available at this site - http://www.pa0nhc.nl/NoiseGen/index.htm

.

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glovisol
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Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by glovisol » Sat Feb 16, 2019 6:12 pm

One thing is to provide generic information, just taking the trouble of giving a few references, which, once you look closer and deeper, most times do not solve the problem at hand. Another to provide the FACTUAL and DETAILED information necessary to solve a defined technical problem......By the way, who would not be aware that an $15 Noise Generator does have limitations? Any price of course has a well defined level of quality, we do not need to delve in platitudes.

This is fruitless polemising to me, I have accurately shown the performance of the noise generator in question, anybody can judge for himself if it is adequate for his job or not and therefore this is my last post on the subject. I have designed and tested filters using this much despised $ 15 Noise Generator and the filters do work, published performance makes this clear, for the simple reason that this Noise Generator was good enough for the job. The old adage: "the proof of the pudding is in the eating" is as valid today, as it was the day the pudding was invented and tasted.

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Roger
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Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by Roger » Sat Feb 16, 2019 6:34 pm

glovisol wrote:In this thread:

https://www.sdrplay.com/community/viewt ... f=5&t=3973

generic judgment was passed on low cost noise generators being on the market. In this thread I wish to look deeper into this matter.
glovisol wrote: This is fruitless polemising to me and this is my last post on the subject.
I am surprised by your response!

You posted that you wished to look deeper into this matter so I posted several references about the quality and limitations of this low cost noise source. Several of the references pointed out where this generator could be useful and where care needs to be taken in order to get good results. A way to modify this unit to get better performance was described by another amateur radio experimenter. Another source of low cost generators was also provided for those requiring a better product. I hope readers find this information useful.

Roger

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sdrom33
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Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by sdrom33 » Sat Feb 16, 2019 7:15 pm

Sorry Roger, first you write the noise gen is no good, you give a reference that says it is a Chinese junk and we must not buy it, now you say it is better and we can use it. All this very confusing to me. It seemed that glovisol did a good job using it. So must I buy or not buy. Must I buy and modify or must I spend more and buy something else? It worries me to buy junk. Pre explain more

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sdrom33
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Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by sdrom33 » Sat Feb 16, 2019 7:16 pm

I mean, did you buy one yourself? What is your result?

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Mike2459
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Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by Mike2459 » Sat Feb 16, 2019 11:28 pm

So must I buy or not buy. Must I buy and modify or must I spend more and buy something else?
I'm afraid that, like most things in life, you are the only one qualified to make that decision.

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Roger
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Re: A CLOSE LOOK TO LOW COST NOISE GENERATORS

Post by Roger » Sun Feb 17, 2019 2:14 am

sdrom33 wrote:Sorry Roger, first you write the noise gen is no good, you give a reference that says it is a Chinese junk and we must not buy it, now you say it is better and we can use it. All this very confusing to me. It seemed that glovisol did a good job using it. So must I buy or not buy. Must I buy and modify or must I spend more and buy something else? It worries me to buy junk. Pre explain more
I did not write that the noise generator is no good. This is what I said in the other topic....
Roger wrote:
The low-cost noise generators sold on ebay come in several different versions. Some of these do not have an output which is flat across the HF band unless they are modified (some owners have done that). In particular, the popular BG7TBL noise source has limitations and requires caution when testing wideband filters in the HF band. If used with a spectrum analyser or an RSP a correction table needs to be applied to the output numbers in order to compensate for this. You can read more about the limitations of the BG7TBL at https://owenduffy.net/blog/?p=11193

For narrowband filter testing or when using them with a return loss bridge (for antenna testing etc.) the lack of a flat noise output is not as much of a concern.
Yes I gave a link to Owen Duffy's blog article on the BG7TBL. Owen Duffy is well-known in the amateur radio community for writing well-researched technical articles and equipment reviews and I have learned a lot from reading his blog. He tested the product and stated that it was a poor design, had no published specs and in his opinion was not worth buying. I also gave other links that did testing on the device and found that the BG7TBL was useful for certain applications if the user was aware of the limitations. The last link described how to improve the device so that the amplifiers did not burn up under rated voltage and the noise floor was flatter. In summary I provided what I thought was a range of opinions so readers on this forum could decide if this product was something they wished to own.
sdrom33 wrote:I mean, did you buy one yourself? What is your result?
I considered buying a BG7TBL but decided that it was not something I wanted on my test bench. I prefer to buy or build equipment that is well made, reliable and reasonably accurate. I suppose that is because I spent many years as a communications engineer where I was able to use good test equipment. I would rather spend $100 for one of the noise sources I posted about earlier than $20 for a device of questionable quality and usefulness (in my opinion).

However, everyone has a budget and different technical needs. Yours will be different than mine. If the BG7TBL noise source meets your requirements and you can compensate for the limitations or they don't matter to you then it might be right for you.

I agree with Mike2459 when he said "I'm afraid that, like most things in life, you are the only one qualified to make that decision."

Roger

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