tunable Xtal oscillator circuit

IvanHi,
experimenting with analog and mixed circuits is often replaced by computer modelling nowadays...  

73 Ivan

in the factory it always come down to the soldering iron , 
for now i have SMOVPO's circuit oscillating at right frequency  to detect bad labels.
Before finding out that "pulling " cristals is not so easy i had the idea that this combination of my previous experiments could work :

My thinking was the following (at least what i expected to happen): The oscillator is working on the right frequency . If i put a RF label under the coil from the drain , if the label is ok , it will start stealing energy (like some neighbors    ) from the circuit because of the magnetic coupling . And  this will produce a variation in the voltage drop on the 1K rezistor connected in the source.
The control voltage of the pseudo-varicap could be generated by a microcontroller or an other oscillator (inclined saw tooth generator) like Harry's time base from his scope , triggered by a  label counter sensor or a simple fork sensor ( that's an other motive why i'm so interested in Harry's time base generator and trigger circuit which is not drown yet     , i don't want to bother now with that , but i have a big arsenal of questions around this ) .I'm interested in this analog stuff because it seams so reliable .
After my experiments with coils and condensator i found that there is a certain balance between L and C that must be kept . For a given frequency fo (= 8 Mhz) and  for  some given condensator set (10 pF .... to 100pF in my case) there is a multitude  of L's that will give the exact same f0 (=8 Mhz ), just that only a special LC pair will give the best result. 
Now i got clever on this . 
The solution was under my nose all the time just that i was stupid enough not to see it from the beginning : The Rf label . 
The RF label is the proper LC circuit i need .
I don't need to know about LC resonance or have a frequency meter or even a scope .
All i had to do is to take a good label , carefully wipe off the paper to gain access to the circuit , then  solder 3 thin wires to it   and connect it in the circuit . So actually i'm testing a label with a label
I'm turned away from cristals and ceramics . The synthesizer thing is interesting me now mostly

Hi,
experimenting with analog and mixed circuits is often replaced by computer modelling nowadays...  

73 Ivan

Hi , 
analog stuff is always interesting, is a lot of experimenting. I will study how the square signal affects  RF label detection . This is what i'm seeking for with the oscillator for now  ..

Hi,
I found an article about ceramic resonators here.

73 Ivan OK1SIP

Hi Zsolt,
"Why are there 3 terminals , and how do i connect that in my oscillator ?"
Resonators with 3 terminals are intended as IF filters. The terminals are input, ground and output. It is necessary to terminate both ends with the proper impedance (see the filter datasheet). You should better find a ceramic resonator with 2 terminals only.

"Or could there exist an other way to sweep frequency around 8.2 MHz with +- 200 Khz"
Xtals can be pulled in one direction only, never "around". The frequency range is limited. If you need more, a well stabilized VFO or frequency synthetizer becomes the (much more complicated) solution.

"Can cristals or ceramics be added or subtracted ?"
Generally, NO. Each crystal has a series and a parralel resonance on two distinct, but near frequencies.
But I remember an internet article about a "tunable" oscillator, which employed three identical Xtals, a coil and a variable capacitor, all in series. According to the author it had a remarkable tuning range, while still keeping excellent stability.

BR from Ivan

Ivan wrote:Hi Zsolt,
much depends on the Xtal properties, its cut etc. Pulling a 8,0 MHz Xtal to 8,3 MHz seems too much to me, namely with a quartz resonator. Look at the RX here.  According to the author the oscillator covers almost whole 80 m band (3,5 - 3,8 MHz). The resonator is a ceramic one, not a quartz one, 3,558 MHz. The ceramic resonators' frequency can be pulled in a wider range.

BR from Ivan

interesting,  i did not know about this . I think i have seen one ceramic resonator , a brown capsule like a condensator and with 3 terminals . Why are there 3 terminals , and how do i connect that in my oscillator ? The middle and side terminal ? 
Or could there exist an other way to sweep frequency around 8.2 MHz with +- 200 Khz   ( 8 .... 8.2 ...... 8.4 Mhz)
Can cristals or ceramics be added or subtracted ? like one of 8Mhz in series with one of 200 Khz ? I have not seen this or heard of anywhere  , just curious what could happen .

Hi Zsolt,
much depends on the Xtal properties, its cut etc. Pulling a 8,0 MHz Xtal to 8,3 MHz seems too much to me, namely with a quartz resonator. Look at the RX here.  According to the author the oscillator covers almost whole 80 m band (3,5 - 3,8 MHz). The resonator is a ceramic one, not a quartz one, 3,558 MHz. The ceramic resonators' frequency can be pulled in a wider range.

BR from Ivan

Hi, 
i have a question to  '' clever  old foxes "   about this :

The Xt = 8MHz . Can this be tuned with C2 to 8.1Mhz, 8.2Mhz and 8.3Mhz ?