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xtal issues?

n7gtb@...
 

I've just begun to characterize an batch (40 pieces) of the 3.2768Mhz xtals to use in the measurement receiver, and am seeing unexpected frequency plots.  I've uploaded two examples of the curves, to the files area under N7GTB stuff/xtals for measurement Rx...

Has anyone here seen this sort of response, and if so, know how to correct (or is this simply a bad batch of crystals?)...

Thanks & 73,
N7GTB - Vern
Kennewick, WA.


Vern VanZ <n7gtb@...>
 

btw, I see this same response curve regardless if I use my homebrewed CTF, or the N5IB version (i.e., PCB with amp).

I've also noticed, that the high frequency side of the curve is less distorted with the crystals having a 3db bw > 100hz.  As the 3db bw approaches 80hz, things get considerably 'bumpier'...

I'd like to determine if this is my test setup, or simply bad crystals.  Any help would be greatly appreciated.

73,
N7GTB

N5IB
 

The fact that the odd behavior is on the high side is a clue. That's where you're passing through the parallel resonance of the crystal. The only things that pop to mind right away are drive level and sweep speed. Any chance the drive level might be too high? I think the level is adjustable in the Midnight SNA. Pretty sure Dave has the sweep speed set in firmware appropriately for crystals

If you haven't already,you might also pose the question over on the SNA group to see if it's been observed by other NAT/SNA users.

Jim, N5IB

David Collins
 

Hi Vern,

The attachment shows my results after grading 30 crystals for the Measurement Receiver.  This is typically what I see.

Your plots are new to me.  It looks like your crystal test fixture is the same as the one I use with one  difference.  I originally used mine with the PHSNA and didn't space the BNC connectors for direct connection to the MSNA so I had two short (about 18") pieces of coax in the circuit.  I'm not sure what difference that would make but you might try it to see if it makes any difference.

The max level looks about right so the power level is probably OK but you could try adding an external attenuator, re-calibrate, and see what that does to the plot.

BTW, if you set the plot format (Setup > P) to one, the firmware will connect the dots and you will get a better picture of curve shapes.

73,
Dave Collins - AD7JT

Vern VanZ <n7gtb@...>
 

Hi Dave,

Thanks for the tip on plotting config.

When I first began looking at this batch of crystals, I was using the CTF described here in the PHSNA group; built with the board Jim made up.  At that time I was using commercial coaxial cables between the MSNA and the CTF.  The response curves were the same.  Not knowing if the problem was with some potential mistake I may have made populating the board, decided to build the 'bare bones' version you see in the photos...But as I mentioned, the response curves are the same no matter which CTF is used.

I did try a 10Mhz crystal, from a batch I had purchased about a decade ago, and the response from those units looked pretty much normal.  That's what led me to suspect these newer crystals (?)

I do have a couple of surplus adjustable attenuators, made by HP; one is 0 to 12 db, the other 0 to 120.  I'll throw the 0 to 12 inline between the MSNA and the CTF, and see what comes of it.  I'll also post my findings here for any future interest...

Thanks and 73,
N7GTB - Vern

Vern VanZ <n7gtb@...>
 

...try(ing) this again, but with smaller attachments.  -V.


On Friday, April 29, 2016 7:41 PM, Vern VanZ wrote:


I added additional attenuation in 10db steps, up to 20db and aside from the lower drive level, the response curves appear unchanged (see attached photos). 

Following Jim's (N5IB of PHSNA group) suggestion, I'm cross-posting this to nat-sna in the hopes that others may have encountered this before...

For the folks in nat-sna, I recently purchased a batch of 3.2768Mhz crystals, to select six for the purpose of constructing the PHSNA measurement receiver.  The attached photos are representative of the sort of responses seen using both the PHSNA(built with N5IB PCB) and the K8IQY CTF.

Any help would be greatly appreciated.

Thanks and 73,
N7GTB -Vern






Jack D. Generaux
 

Vern,

Have you tried extended the sweep range upward to include the Fp.  I wonder if the phenomenon manifests on the high side of it?

Jack(W0FNQ)


vasilyivanenko@...
 

Hi Vern

Since this doesn't happen with other xtals, we'll assume your measurement deck is OK.

Spurs in crystals occurs more often than not, however, some crystal batches show poorly. I had 1 set of 4.952 MHz xtals where multiple spurs lie 50 KHz apart up from center when swept in a tracking gen and spectrum
analyzer set-up.

 My sweeps skirt looked jagged on the high side of Fs in my homebrew sweep system.  I ordered another batch. They were fine and I threw the others in the rubbish.

V

Vern <n7gtb@...>
 

Hi Vasily,

Thanks for your feedback. 
It turns out there was an issue with the latest beta version firmware in the SNA I'm using.  At Dave's  (AD7JT) suggestion, I rolled back to an earlier version (4.2) and then ran through the batch of crystals again.  This time the response curves looked normal.  I now have two sets of six crystals, that can be used to complete the measurement receiver I've been working on. 

73,
Vern - N7GTB 



Sent via the Samsung Galaxy S®6 active, an AT&T 4G LTE smartphone

-------- Original message --------
From: "vasilyivanenko@... [PHSNA]" <PHSNA@...>
Date: 5/2/2016 4:44 PM (GMT-08:00)
To: PHSNA@...
Subject: [PHSNA] Re: xtal issues?

 

Hi Vern

Since this doesn't happen with other xtals, we'll assume your measurement deck is OK.

Spurs in crystals occurs more often than not, however, some crystal batches show poorly. I had 1 set of 4.952 MHz xtals where multiple spurs lie 50 KHz apart up from center when swept in a tracking gen and spectrum
analyzer set-up.

 My sweeps skirt looked jagged on the high side of Fs in my homebrew sweep system.  I ordered another batch. They were fine and I threw the others in the rubbish.

V

vasilyivanenko@...
 

Great news Vern.  Now with MCUs both fast and cheap,  the double edge sword -- the  software and hardware parts of instrument fidelity looms.

I know you'll enjoy your lab receiver.

Measurement receivers provide great learning. I spent much of last week studying perhaps the holy grail "measurement receiver" from circa 1980  ---  the HP3586A/B/C.  A Programmable Selective Level Meter (Wave Analyzer) with synthesized tuning. From the synthesizer [ pretty good phase noise for a fractional synth back in the day] to the 20 Hz and 400 Hz wide xtal filters, I still marvel at this engineering and have copied some of the circuits into my notebook for future consideration.

Best!
V/T