Cheap Accurate Sig Gen

[pwdrive]

.... note the frequency displayed on the SSB transmission ....

Arrrrrrrrr.
So it's got a lot of digits.
But how accurate is it?

Alan

Alan, so far as I can ascertain from the manual, it's as accurate as you set it when in calibration mode eg: you can compare and set it to a known accurate frequency eg: a WWV time signal, calibrate it to that and that's it, I assume that once calibrated then from that point on it is referenced to the clock and would only be subject to errors caused by thermal drift within the clock Xtal, which I would think would be very low, unless I'm reading the book of words wrong.

Lawrence

[pwdrive]

Just as an addition to how accurate it is:

I can only base my assumptions on the information contained on suppliers website.
My assumptions might be right, might be wrong.
The way I have read it seems to imply that if you calibrate it to a known accessible frequency standard then it is accurate as that frequency standard however accurate that standard might be.

I might be completely wrong on this, anyone else got any thoughts?

Whichever, whatever, most of the DDS VFO kits seem to be reasonably priced considering what can be done with them, with a few external bits you could have a nice modulated sig gen, I also suspect that using that technology and a bit of knowledge on PIC programing you could make yourself a nice versatile wobbulator etc.

Lawrence.

[pwdrive]

Here is a primer on DSS (as commonly used in DSS VFO kits/commercial receivers etc from one of the leading chip manufactures explaining how it works and what it can do, It has many applications....
A bit heavy in parts but I found it interesting.

http://www.analog.com/static/imported-fi...2-2-99.pdf

Lawrence.

[Skywave]

I'd like to make a contribution to this thread on the topic of accuracy.

A lot of people seem to get confused between accuracy and precision when discussing instruments and measuring devices. Even my copy of Chambers Concise English Dictionary erroneously equates the two. The point I wish to make is that 'precision' and 'accuracy' are not synonyms - least not in scientific disciplines.
I'll try to justify that claim with an example.
Suppose that I have two large-scale ammeters, each with an F.S.D. of 5 amps.

The first meter is of the moving-coil type with a linear scale. That scale is divided into 1 amp and 0.1 amp divisions in a clear manner. However, the mechanical quality of the meter is poor: cheap bearings, no attempt to compensate for temperature variations affecting the indicted readings is prone to give errors when a substantial current has been left flowing through that meter for any substantial length of time or when situated in the presence of a magnetic or an electrostatic field. The nature of the fine graduations of the scale provides for high precision, but on account of its mechanical limitations, the accuracy of any reading is low.
Hence, this meter has high precision, low accuracy.

The second meter is of the moving-iron type with the usual non-linear scale for a meter of this type. The scale has divisions in 1 amp and 0.5 amp. steps: nothing finer. The quality of the mechanics is very high: double-jewelled bearings, built-in temperature compensation and it is hermetically-sealed with a metal case for screening which carries an earthing stud. The accuracy is high: when a known current is passed through it, the needle always goes to exactly the same place on the dial, irrespective of the environment that the meter is in. But on account of the relatively coarse and non-linear scale calibrations, the precision of any reading is low.
Hence, this meter has low precision, high accuracy.

And finally: an extract from 'The Radio Laboratory Handbook', 6th. edition, 1954, page 36; author is M. G. Scroggie, B.Sc., F.I.E.E.

It is inadequate to state that the accuracy of an instrument is so-much percent. It would obviously be more sensible to state a figure for the inaccuracy, since then there would be no doubt about whether a greater accuracy meant a greater percentage or a smaller one. But other questions arise: Does the stated figure [for accuracy] mean the greatest possible difference between the value shown by the instrument [now] and the absolute value [indicated] at the time of calibration, or does it mean the greatest subsequent variation in value - and if so, over what range of temperature, humidity, frequency, etc. The latter, by itself, can be distinguished by the term constancy.

End of extract: entries such as [xxxxx] in the above are my additions to the original text in order to make the author's meanings abundantly clear.

End of post: thanks for wading through it.

Al. / Skywave / April 18, 2013 //

[pwdrive]

Thanks Al, according to the way I read the specs on these DDS VFO's they seem to be precise and accurate, precise being that the frequency can be set (on some) down to Micro Hertz levels, I am guessing that any shift in accuracy would be down to variations in the clock frequency over time eg: thermal drift within the clock xtal.
It does not seem to matter if the clock xtals natural resonant frequency is out by a few ppm from it's designated frequency as that in itself does not determine the final frequency output from the DDS VFO once all the parameters have been entered, that is done digitally within the various registers within the chip.
It is only when the xtals frequency drifts that accuracy becomes affected.
So, if the manufacturer says that the DDS VFO clock xtals frequency is 100 Mhz + - 50 ppm it does not mean that the output from the DDS VFO is tied to that + - 50 ppm or any divisor of that tolerance.
That's my initial interpretation.
Anyone else read the guff on these devices?
I did find something on the web about a bloke who had one of these and covered it in foam panels to see how far it drifted over time due to temperature shift, I seem to remember that it was less than 1 Hertz over several hours, can't remember the test frequency but if I can find the page again I will post it.

Lawrence.

[pwdrive]

Here it is, I quoted wrong in my previous post, it was not the whole unit he enclosed in foam just the xtal oscillator.

http://pensacolasnapper.blogspot.co.uk/2...s-vfo.html

Lawrence.