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I see in the review in the Jan/Feb 2020 QEX that the minimum resolution is 1 kHz and scanning a crystal might give a close approximation of its resonant frequency but not much else.

I used nanoVNA Saver to scan a 4.915 MHz crystal with a resolution of about 4 Hz and it looks like I'd have the info necessary to calculate the motional parameters. I'm not sure if I'm trying to fool myself in going around an inherent limitation or not though. I measured a 3 dB BW of about 100 Hz but in another system I got 69 Hz. So that's a pretty big difference.

I'm still in the early stages of understanding the thing though.

73-

Nick, WA5BDU

I see in the review in the Jan/Feb 2020 QEX that the minimum resolution is 1 kHz and scanning a crystal might give a close approximation of its resonant frequency but not much else.

I used nanoVNA Saver to scan a 4.915 MHz crystal with a resolution of about 4 Hz and it looks like I'd have the info necessary to calculate the motional parameters. I'm not sure if I'm trying to fool myself in going around an inherent limitation or not though. I measured a 3 dB BW of about 100 Hz but in another system I got 69 Hz. So that's a pretty big difference.

I'm still in the early stages of understanding the thing though.

73-

Nick, WA5BDU
==================================

Nick,

Newer firmware than QEX tested may give better resolution. I thought there was a simple table of what was available, but take a look at:

/g/nanovna-users/message/8087
/g/nanovna-users/attachment/8087/0/NanoVNA_firmwares_20191210.pdf

Bear in mind that the stand-alone device has a 101 point scan, so if you need 4 Hz resolution limit the scan to 400 Hz, Alternatively, use the NanoVNA-saver software which allows multiple scans to be combined as a number of segments, so you might use 10 segments to over a 4 kHz scan, or perhaps 10 segments of 1 kHz each to get a 10 kHz scan with 10 Hz resolution. It's easier than it sounds.

73,
David GM8ARV
--
SatSignal Software - Quality software for you
Web:
Email: david-taylor@...
Twitter: @gm8arv

However, do not forget the time base accuracy is 0.5 ppm.

Alan

Nick

Keep in mind that the loaded Q of the crystal + measuring circuit will be affected by the source and load impedances the crystal is embedded in. The crystal itself will probably have an equivalent series resistance of 10 ohms or less, so connecting it directly to a 50 ohm signal generator and a 50 ohm measurement device will dramatically affect the Q, and thus the bandwidth, of the measurement. You can make some simple resistive terminations to do this if you can tolerate the loss (shouldn't be a problem considering that the NanoVNA has around 70 dB of range at HF), or you can make some transformers, or a combination of both.

If you're characterizing crystals for use in a crystal filter, keep in mind that frequency matching is far and away the most important consideration and, for best filter shape, you should try to match all the crystals within 5% or less of the intended filter bandwidth.

Joe
W3JDR

The timebase might not be within 0.5 ppm but the variance will not effect the motional Lm, Cm calculations much. The TCXO's have a temperature compensated drift spec of +/-0.5 ppm and the make tolerance is +/-2.0 ppm at 25 deg C after solder reflow stress. I am not aware if there is any synth offset freq calibration to correct for initial tolerance. My nanoVNA-F measured +0.16 ppm off freq (surprised me). My nanoVNA measured -0.40 ppm off freq. They both took less then 10 minutes to settle down on freq with charger attached. Measured with oven controlled timebase Fluke counter with <0.1 ppm uncertainty.

If you are looking to build a super narrow bw crystal filter you may need good freq accuracy. Relative zero-pole spacing will not change for 2 ppm freq measurement accuracy. The biggest source of error is usually getting an accurate value for the shunt capacitance, Co, of the crystal which is used in the parameter calculations. It is better to measure Co at low frequency with an accurate capacitance meter at less than 10% of expected fundamental crystal resonant freq. The VNA will not be accurate for Co measurement as the Xc is very high.

Perhaps you can measure Fs, Rs at phase offset caused by expected Co value and -3 dB bandwidth around Fs.. At 4.9 MHz the Fs and Fa may be spaced far enough apart so the Fa does not effect Q slope too much. I usually don't use this method for higher freq xtals because Fa pulls in the freq response slope on high side of Fs yielding a higher Q calculation result then actual..

Thanks for the responses.

The absolute accuracy of the frequency source isn't generally much of an issue for me. It does need to be stable during the measurement process though.

The formulas used to determine Lm and Cm do take the source and detector resistances into account. It's common to use 12.5 ohms and have either 4:1 transformers on both ends or use minimum loss pads of 50 to 12.5 ohms on both ends, which is what I'm doing.

I usually find Rs based on the difference in attenuation with the crystal installed, at resonance, and attenuation with the fixture shorted. I suppose this VNA would give me a direct reading of Rs though ...

I should have mentioned that the QEX article was probably pretty out-of-date by publication time, for example the "saver" software wasn't discussed. But it appears that the author's "resolution" issue was not with the precision with which the synthesizer chip is programmed but had more to do with the measurement techniques, which sort of went over my head.

I did do a crystal measurement on a similar VNA device at one time and got good agreement with other measurements. The device is described to an extent here, scroll down to product #25:



It uses similar hardware to the nanoVNA but what goes on inside its brain (in either case), I don't know.

I've generally not found Co to be critical in filter design but it does seem to come into play more when designing voice BW filters. I either use rule-of-thumb values or measure with my AADE type instrument.

My belief is that matching frequencies closely is a good technique if you aren't going to measure the crystal parameters, but otherwise it's of lesser importance. Of course if one deviated by a large amount, I wouldn't use it in my filter.

73,

Nick, WA5BDU

On Fri, Jan 3, 2020 at 02:56 PM, Nick Kennedy wrote:

" … I should have mentioned that the QEX article was probably pretty out-of-date by publication time …"
============================================================================
I was just wondering the same thing as QRP's most recent firmware has the frequency resolution down to 1 or 10 Hz, and using NanoVNA-saver's increased measurement point feature, crystal testing should be in-line with the VNWA 3 below 300 MHz. With 101 measurement points, characterizing a crystal in the NanoVNA standalone mode would not be my first choice.

I still haven't received my first 2020 QEX issue. Overall what was Dr, Stebers impression regarding the NanoVNA?

- Herb

His overall impression seems pretty favorable - like many of us amazed that
such performance could be available at such a price.

He found impedance measurements to be very good in the range of 12 to 200
ohms and acceptable from 5 to 500 ohms.

He cautions potential buyers that clones appearing on the market might be
of varying quality, and that available manuals might be poorly translated
from the original Japanese or Chinese.

Otherwise, " ... if you like to play with hardware and software you may
find this little gem an wonderful toy for experimentation."

73-

Nick, WA5BDU

I find it interesting that some very educated folks like Dr Stebers are quick to critique things but you rarely see them helping with the creation of improved documentation or helpng others on the forum for something like the Nanovna.?

At least there are other very knowledgeable experts on this forum from around the globe who are willing to contribute to the increasing sum of knowledge on this forum with value added tips, techniques, documentation and personalized help in many instances and all the participants are to be heartily commended.?

Cheers to all for 2020!
Looking forward to the new V2 forum (?) Maybe coming soon ?


On Sat, 4 Jan 2020 at 9:49 AM, Nick Kennedy<kennnick@...> wrote: His overall impression seems pretty favorable - like many of us amazed that
such performance could be available at such a price.

He found impedance measurements to be very good in the range of 12 to 200
ohms and acceptable from 5 to 500 ohms.

He cautions potential buyers that clones appearing on the market might be
of varying quality, and that available manuals might be poorly translated
from the original Japanese or Chinese.

Otherwise, " ... if you like to play with hardware and software you may
find this little gem an wonderful toy for experimentation."

73-

Nick, WA5BDU