VCXO frequency isn't high enough

It's interesting that I don't see that oscillator circuit arrangement at

Thanks for the reply. I should have been more clear, the desired frequency is 3581kHz so yes I need to pull the crystal up in frequency about 1.5kHz. Right now I'm only able to get to just under 3580. It seems that much inductance should have more effect than moving it up only about 500Hz, but this isn't my bag.

By tank I guess you mean to rid myself of the crystal and use an ordinary LC circuit? I think I can come up with something simple, but I'd rather just get this working if at all possible. Do you have any idea what impedance the 602 wants to see on it's oscillator? I have a couple more chips and I could try and cobble something together if I knew what impedance to target.

Try going smaller than 10 pF (you can put a 10 pF fixed capacitor in series with the trimmer).

Get rid of the series choke and just use the small value variable capacitor to go higher in frequency. I believe the inductance will lower the frequency--the combination works to give an increased VXCO range if I am not mistaken.

Pete k1zjh

In message , Uncle Peter writes

If the series capacitor is very small, the oscillation will stop. In these circumstances, I have successfully pulled VHF crystals (3rd, 5th and 7th overtone, between 70 and 200MHz) HF by adding an inductor IN PARALLEL with the crystal. The value will be somewhat less than that required to neutralise (ie parallel resonate with) the parallel capacitance of the crystal. The lower the inductance, the higher the frequency. Ian.

You have a capacitor and inductor in series. Capacitive reactance = -j/(wC) Inductive reactance = +jwL The net reactance [jwL-j/(wC)] may be capacitive (negative) or inductive (positive).

Reducing the inductor makes it less inductive i.e. more capacitive. Reducing the capacitor also makes the net reactance more capacitive.

You turned your trimmer all the way down to 10pF and it wasn't enough, so try taking a few turns off the inductor.

Use less inductance or replace it with a wire link.

I did this and now I'm up to 3580.050; an increase of about 70Hz or so.. I don't understand how this was designed to work. What is making the crystal oscillate above its design frequency? I thought the inductor had something to do with it, obviously not. Is it being operated in series mode instead of parallel?

I tried putting small cap in series with the adjustable one and that just greatly narrowed the tuning range and only slightly raised the frequency. It seemed to be getting unstable at that point as my old frequency counter started showing variations of quite a few hertz each update instead of remaining steady. It could be the amplitude is too low and the counter is missing ticks.

You need to reduce the inductor.. that is causing a down swing in your freq.

If you want a VXCO, you use the capacitor and inductor to achieve maximum range of the crystal frequency.

An inductor in series will lower the frequency. A capacitor in series will raise the frequency.

Eliminate the coil and just use the series trimmer to see how how high the crystal frequency can be raised before it becomes unstable or quits oscillating.

As Ian suggested, you can add an inductor ACROSS the crystal to raise the frequency. Think of the crystal as being a parallel LC circuit (over simplification) and you'll be better able to visualize how this works.

Try a few other color burst xtals in the circuit for the heck of it. You may not be able to pull the crystal that far.

Thanks, I took it out and the frequency increased by only about 70Hz, but it did increase. :-) Do you know of anything else I can do to increase the frequency by about another couple of kcs?

maybe the Q of you inductor is too low, try an air core, needs lots more turns, a trimmer core as a core is good too.

Also you should test it with the crystal shorted, then when you get close to the frequency you want put the crystal back in.

Colin =^.^=

try a different Xstyle, it sounds like your at the limits.. it's possible the chip has too much capacitance on the osc legs, i suppose you could try a stand alone osc with a high freq transistor to lower the cap. use a buffer on the stage to drive the chip . Also, have you considered the board mounting? you could have to much cap via your board construction.

My W1AW receiver uses the crystal with just a 5-47 pf variable capacitor...no inductor, and it nets right on frequency. Try shorting the inductor and see if this gets you closer to the required frequency. Not all color burst crystals were created equal. If you have another crystal you might want to try it.

Your ferrite will not be saturating at the small amount of signal you are sending through it as part of an SA-602 oscillator.

Arv - K7HKL _._

Looks like you are quite likely correct. Without the inductor and with a

I did this and it will go slightly higher in frequency, but it gets a tad unstable (could be the counter too, I wish my new scope would get here). I guess I'm at the limits of that particular crystal.

It hasn't stopped, but it has gotten weak or unstable (I can't be sure right now) with capacitance below 10pF.

Are you referring to the capacitance inside the crystal, or the 100pF external cap? Do you have a ballpark number on the inductance value I should try?

I understand that, but I don't use values like j and w, just 2, f and PI. ;-)

I took it out, and it's still too low in frequency. :-( Back to the drawing board I guess.