Need some help with designing a 3 position crystal switch.

[Techno1]

Wondering if some on the forum has made a 3 position or more crystal switch.

Looking to do this with transistors.
I tried a circuit and did not work Ill post the picture of what I tried.

I don't need an oscillator just something to switch in 3 crystals 4 xtals the most.

I tried the below and would not switch from 1 xtal to the x 2. I know this shows the x1 and x2 to oscillator but tried to read the circuit with my crystal/oscillator reader and would read x1 when powered and when x2 powered still reads x1 like its not switching.
looks like this is using diode biasing and I think a transistor circuit might work better.

Any one out there have any circuits/schematics.

The circuit im working with is using crystal switching. Its not an oscillator circuit for signal injection.

thanks

 

[nomadradio]

Here's Lou Franklin's approach:

https://www.cbcintl.com/images/160schem.gif

The Browning Golden Eagle Mark 4A receiver used an NPN transistor to provide the ground side of one crystal at a time. Just need to use a transistor that has a low collector capacitance when it's shut off.

73

 

[brandon7861]

Try making the switching diodes pass the RF instead of shunting it.
edit: looks like the link from nomad describes the same thing. sorry. looked at it after posting this

 

[Techno1]

I did try the Lou franklins setup and does work I used exact parts and it would switch from xtal 1 to xtal 2 when 12v applied. I used 1n4148 diodes.
What happens is the trim pot gives out when adjusting it. I tried different value trim pots like green- red -brown and same results. Not enough room to trim the xtal up or down.
Again this is not hooking to an oscillator. Im trying to switch xtal in and out of the 2 holes on the pc board where the original xtal was removed.

thanks also tried the picture I posted and does not work at all.
So far Lou Franklins circuit works some what.

 

[brandon7861]

What are the different crystal frequencies and how far are you trying to pull them?

 

[Techno1]

The crystals are 11.4758 and 11.1758 just need 11.3758 to 11.5758
and the 11.0758 to 11.2758. If you trim the cap im using the yellow 30pf seems to work best to try to get close to a 1kc up or down the xtal will stop, turn trim cap back and it will show frequency.
I can trim xtals to center frequency but when you put in circuit and wire lengths i keep short of course frequency of center will be off.
Again just looking for 1 kc up or down in either xtal and then the original xtal the 11.3258 the same have not tried that yet. that would be 11.2258 to 11.4258
Was just trying to set up the 2 xtals first.
This will give 40 up and down to the pll8719 radios and if using other xtals same to the mb8734 xtal.
So im trying to get a good crystal switching circuit and not an oscillator for this.
Oscillator will not work for this application.

Hope I explained what im trying to do without much drifting using an electronic switch to switch the xtals.

thanks

 

[brandon7861]

You mention wanting 1kHz up or down, and that is easily achieved with just a trim cap, but there is no way to make a crystal go from 11.4758 to 11.5758 or 11.3758. That is not 1kHz up or down, that is 100kHz up or down!

Typical HF crystals require about 30pF loading capacitance and can be pulled about 900ppm with the cap alone. You can achieve up to 5ppt by the addition of a series inductor, but typically its kept to about 2ppt so the oscillator behavior is governed by the crystal and not the LC series network after it. Above 5ppt, you no longer have a crystal oscillator, you have an LC oscillator. 2ppt on 11.4MHz is 22.8kHz, a long way from the 200kHz you are looking for.

G4OEP has a great webpage on pulling crystals if you are interested.

http://g4oep.epizy.com/pulling/pulling.htm?i=2

 

[Techno1]

nomadradio​

Here's Lou Franklin's approach:

https://www.cbcintl.com/images/160schem.gif

The Browning Golden Eagle Mark 4A receiver used an NPN transistor to provide the ground side of one crystal at a time. Just need to use a transistor that has a low collector capacitance when it's shut off.

73

I tried to pin point the Mark4A schematic on where you say it uses npn transistor to provide ground to the crystals.
I only found the 10.240 crystal to pin 4 on the pll.
Could not find anything else, I used the schematic on cbtricks not the best.

thanks

 

[nomadradio]

Receiver schematic is separate. That's the one I meant, to switch between the two tuneable receiver bands.

73