1
\$\begingroup\$

I'm not having much luck searching this out.

I need a circuit that will allow me to select which of two parallel resonant crystals is operating with a common on-chip gate oscillator. I vaguely remember seeing circuits like this back in the 70's. I can't pull a single crystal far enough or accurately enough. Fop is around 14 MHz

\$\endgroup\$
2
  • \$\begingroup\$ One challenge you'll have is that each time you switch crystals, the newly connected crystal will take 0.4-2ms to begin to oscillate and stabilize. Depending on the characteristics of your gain block, the output may become noisy, or pin at rail, or exhibit a "chirp" while the frequency stabilizes. The higher the "Q" of the crystal, the longer it will take to start, each time. If you are switching rapidly between crystals, residual mechanical vibration will enable a faster startup. \$\endgroup\$
    – elchambro
    Jun 2 '19 at 22:50
  • \$\begingroup\$ Understood. The switchover time is irrelevant, even if it's 100s of mS. \$\endgroup\$
    – user121934
    Jun 13 '19 at 17:54
0
\$\begingroup\$

From the 1976 ARRL Handbook. My apologies for the poor focus. Note that the inductor and cap values are for 455kHz-ish, not 14MHz. A search on "diode switched crystal oscillators" may prove helpful.

enter image description here

\$\endgroup\$
1
\$\begingroup\$

One option that is quite robust is to have two oscillators and feed square wave clock to the chip. Another what I have seen is to use an analog multiplexer to switch one pin from the chip to one of the crystals.

\$\endgroup\$
3
  • \$\begingroup\$ Dual oscillators is my fallback plan but that's probably more board space and components. \$\endgroup\$
    – user121934
    Jun 2 '19 at 19:36
  • \$\begingroup\$ @user103218 I haven't experimented with using two external xtals. So I can't help there using "off the top of my head" info. But I think SiLabs (si532?) has a chip where you specify the two frequencies you want and they supply the result. That might meet your board requirements, but perhaps not your cost or availability targets. Just another thought to consider. \$\endgroup\$
    – jonk
    Jun 2 '19 at 19:46
  • \$\begingroup\$ I know Silabs, and that's one path though not preferred for the reasons you stated. \$\endgroup\$
    – user121934
    Jun 2 '19 at 19:54
-1
\$\begingroup\$

How about this

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
2
  • \$\begingroup\$ critique the circuit, not the answer???? \$\endgroup\$ Jun 3 '19 at 16:37
  • \$\begingroup\$ Interesting.. I got distracted by a few other issues and a small heart attack, but I'll be checking these both out in LTSpice, and I'll get back to you. The BFO schematic seems to be very similar to my foggy memories of what was used in scanner circuits of the 70's and both have their advantages. I'll let you know! \$\endgroup\$
    – user121934
    Oct 17 '19 at 20:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy