[R-390] 2nd LO xtal measurement

[Gordon Hayward]

I agree.  Typical 9 MHz AT crystals are less than 10 Ohms in air at series resonance.
Maybe a tiny (really tiny) amount of something gooey leaked in and landed on the crystal
surface. The gooey stuff would also push the frequency down a bit. In liquid water the
resistance with a really clean crystal (cleaned with chromic acid) is about 135 Ohms and
the frequency drops by about 1500 Hz (this is with only 1 side wet. Its about twice that
with both sides wet. I'm recalling most of this from memory and sketchy notes.  The
paper is Hayward and Thompson, J. Appl. Phys, vol 83 page 2194, 1988.

Cheers, Gord VE3EOS.  

----- Original Message -----
From: "Bob Camp" <kb8tq at n1k.org>
To: "Alan Victor" <amvictor at ncsu.edu>
Cc: r-390 at mailman.qth.net
Sent: Saturday, September 27, 2014 7:55:44 PM
Subject: Re: [R-390] 2nd LO xtal measurement

Hi

Assuming that you have done a full circle fit and pulled everything out of the model, the 45 ohms on the 9 MHz crystal is way high. It should be much closer to the resistance of the 10 and 13 MHz crystals. There’s something more wrong with that crystal than just the frequency.

Bob

On Sep 27, 2014, at 5:18 PM, Alan Victor <amvictor at ncsu.edu> wrote:

> So for completness and maybe secondary information, I measured the bad vs.
> the good R-390A 2nd LO xtals. The bad is a 9 MHz unit and some good are 10
> and 13 MHz. All are fundamental xtals and I would have to look up their
> spec but, I assume the correlation C for these xtals is ~ 32 pF. In any
> case, measure them on a VNA results in the following:
> 
> *xtal                       Z at desired frequency   R at series resonant
> freq*
> 9 MHz                   500 +j 5000 ohm           45 ohm
> 10 MHz                  7.5 +j 505 ohm             7.5 ohm
> 13 MHz                  9.0 +j553 ohm              9 ohm
> 
> If you search for the frequency where the 9 MHz xtal correlates with 32 pF,
> its 8.99441 MHz or about 5500 Hz off.
> Now, I would need to check if 32 pF is the right number,  but this error is
> about what I saw for this unit when
> operating on the 7 MHz band, about 15 KHz error. So the xtal is not
> fractured, its not broken and it functions. That
> is to say it looks like a xtal, just not a very good one and not usable in
> the receiver.
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