UBitx Crystal filter capacitor Q


ohwenzelph
 

Larry N2AJX Wrote “I swept the crystal filter and found its 3dB bandwith at only 1.2 kHz, less than half of what it should be. So I experimented a little and found that by changing the 5 capacitors from the crystals to ground from 100 pF each to 82 pF each, the bandwidth increased to 2.4 kHz”

(sorry for the size change with
Pasting)

On uBitx.net under Crystal Experimentation it says
3) As suggested by Allison KB1GMX, 82pF is working just fine in the filter and achieves a bandwidth of around 2.2Khz. Capacitor value is bit touchy between 82 to 100pf.

4) The Q of capacitors used has a major impact on the filter response. Parallelling up two values to arrive at a desired overall capacitance value results in a better response.”

so
do we want hi or low Q?
how is the “response” better?
does paralleling capacitors increase or lower the Q?
what does “touchy”mean when talking about values between 82 and 100pF?

Thanks!

For ever clueless,
jer aa1of

(really, I do much better if you spell it out...)


ajparent1/KB1GMX <kb1gmx@...>
 

The caps should have a high Q but there is a point of little return for ones effort to go better.
If you use quality caps its generally not an issue at 12mhz, bigger deal at 1296mhz.
 
So for this its more a matter of using the right value for that set of crystals.

Paralleling gets better Q but for this it allows you to exactly hit the desired value.
as capacitors in the 30 to 100pf range are generally standard values only.

For example your could put 56pf and 33pf to get 89pf, or 47and 47 for 94.

I believe the optimum value centers around 91pf but, 1% caps are scarce and far from cheap
especially AVX and other MLCC types.  So hand picking (with measurement) and paralleling
is the way to go.  For a given set of crystals as they also vary so the "correct" capacitors is
in the range of 82 to 100.

Of course one could extract the crystals measure them and then calculate the exact value.
That's a lot of work and a simple change the values and try them is easier.

How wide should it be?
Other than my personal preference, a SSB filter is considered very tight at 2khz and wide
at 3khz so there is plenty of room to allow for anything in that range.  Also I have two radios
that have adjustable digital bandwidth (IF DSP) that goes down to 200hz and up to 2800
so 2300 to 2500 is fairly normal sounding 2000 or less is really narrow sounding.

For radios where I've built the filter from the ground up I shoot for about 2400hz
a variation 100hz either way is hard to hear and still very good. 

For CW nothing beats 500hz or so unless tuning then wider is easier.

Hope that helps.
Allison


Rahul Srivastava <vu3wjm@...>
 

so
>>do we want hi or low Q?

It is  advisable to use High Q capacitors in filter circuits as well as use of high Q crystals.

>>how is the “response” better?

Response is better in terms of filter skirt resulting in a better shape factor, lower losses and over all a better performing filter.. In my experiments I have noticed a major impact on the rounding off at the filter bandpass edges. They are much better when Q of capacitor is increased.

>>does paralleling capacitors increase or lower the Q?

Paralleling the capacitors will result in increase of Q.

>>what does “touchy”mean when talking about values between 82 and 100pF?

If you will notice as mentioned by others for a change of just about 18pF in capacitor value bandwidth changes almost by a factor of 2. So touchy here refers to capacitor values being critical, a couple of pF change has a significant impact and if you had been using a trimmer to tune this filter then the adjustment would be very sensitive and critical or Touchy...

If making filters I would highly recommend reading and archiving this paper by Wes Hayward:



Rahul VU3WJM


George
 

Caps should be NP0 to have stable result. Filter itself very simple with flat responce but but still the skirt or shape is too great - it is better to do real 8-crystal filter as it is not so difficult. I did so and got 2.5 filter/6dB near 2 shape/60 db. Caps were 68 to 82 on small experimental board. Best regards George RX3ARG/UR4CRG


Ashhar Farhan
 

We use NP0 anyway. I would look at what your termination impedance is and how strong it is. ideally, you must use series resistors to get a good, strong impedance at both ends.

- f

On Sun, Aug 12, 2018 at 9:04 AM, George via Groups.Io <rx3arg@...> wrote:
Caps should be NP0 to have stable result. Filter itself very simple with flat responce but but still the skirt or shape is too great - it is better to do real 8-crystal filter as it is not so difficult. I did so and got 2.5 filter/6dB near 2 shape/60 db. Caps were 68 to 82 on small experimental board. Best regards George RX3ARG/UR4CRG



George
 

Dear  mr Ashar the impedance didn't changed greatly and one can use the same transformes. My experiment was in several steps:
1. Repeat 12 MHz filter with crystals I've got here. Rezult: just the same filter as original one with flat responce 2.18 losses but too great skirt. Transformes-43-2402
binocular 1:4
2. On the second step I deleted caps on one side of the fiter (as it is simmetrical) and by help of vary cap 10/200 p got the very exeptable form of the filter. Filter lost its flatness but still was good not more than 2 dB fluctuations in band.
3. I did it with the other side and at last got: 2 filers 2.5 kHz and 2.7 with  shape 6/60 near 1.8 with losses 3.17 and 3.57. the input/output transformes were 1:4 - that is strange but close to optimum.
Best regards 
P.S  Still I think mBITX is very good interesting and simple transceiver! If you have time look at UR3LMZ SW-2010 to SW-2017 radio.