[DSP-10] Grounding of X101 case (10 Mhz xtal), grounding of mixer U3 / U15

[Bob Larkin]

Hi Marius and all,

I would really expect the grounding of the crystal case to be perfectly
fine. The added capacity would be small---we need more capacity with the
Epson crystal to get it on frequency, anyway!  The added ground helps with
the mechanical mounting, whichever side of the board it is on.

I played with added grounding to the U3 case and could see no difference. It
would not hurt to do so, though.

In the case of U15, the added grounds increases the RF-to-IF isolation. The
crystal filter impedance, along with the mixer, can act as a FM
discriminator and convert the phase noise on the 19.68 MHz LO to the 15 kHz
range. Reducing the level of LO getting to the filter helps a lot. However,
in the case of U3, the LO and RF signals are so far apart in frequency that
it makes no difference.

Good luck with the transceiver. It sounds like it is really close!

73,
Bob  W7PUA

At 09:16 PM 7/24/02 +0000, you wrote:
>Hi list,
>
>in my kit there was a thin cylindrical cased 10 MHz crystal instead
>of the traditional case.
>I found this to be a little "unstable" mechanically if mounted
>on top of the board without support. I soldered it below the board instead. 
>I soldered the case of the crystal to the groundplane to
>avoid microphonics when operating portable from my car with the engine 
>running in the winter to keep warm.
>I guess there will be some more stray capacitance to ground because
>of this because the xtal leads now have a closer path to ground to
>set up an electrical field in the air dielectricum . Do anybody have 
>experience with this mounting form ? Do you have experience if it will  
>affect the stability of the xtal osc in a any way ?
>(I plan to use a portable GPS diciplined TCVCXO external reference later for 
>some low power EME experiments so this will be a temporary arrangement).
>
>Mixers :
>========
>I saw that the RX sensitivity could be affected if U15 was not grounded 
>properly on Bob's page. I took some copper litz and soldered the case to 
>ground as suggested. While I was at it, I also grounded U3 (the first mixer) 
>the same way. Do anybody have objections to help U3 with grounding as well ?
>
>73 de LA9EEA
>
>
>
>
>
>
>
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