Re: Connectong two or more varicaps in series to reduce minimum capacitance?

I thought so. But it tunes nicely. It is this design https://www.youtube.com/watch?v=WREwJQVE8xI
I have verified it by placing a probe at the resonator and watching the oscillation of the regenerative stage on a scope/frequency-meter and a shortwave receiver.
As said, with the varicap connected it tuned about 80KHz indeed. With the varicap disconnected the resonator tuned to 7.105 (natural frequency) probably due to breadboard parasitic capacitance.

I suspect that was a typo. 7.003 -7.105MHz is more likely.

I agree, there is a real danger of pulling the frequency so far the mechanical resonant properties can no longer work properly and making oscillation less predictable and possibly unstable.

Brian.

Yes it was a typo.
Accorging to the datasheet
"As CSTLS series does not require externally mounted capacitors, the number of components can be reduced, allowing circuits to be made more compact."
Maybe it is the internal calacitors that cause this large pulling without negative effect

In that case, we see a tuning range of about 2%- quite reasonable. I presume the Q will be quite low, 100-1000 range.

When I measured the self resonant frequencies of my collection of resonators, they were all over the place by several Khz, none were at exactly 500 Khz.

The highest one was about 4.5 Khz high without any extra capacitance. But with a varicap I could only get it up to about 4Khz at full maximum varicap voltage.

Which leads me to think selecting resonators for highest natural frequency may be a more productive path than trying to reduce minimum capacitance.

Another thought was that these cheap Chinese ceramic resonators are probably all out of spec parts.
I bought a quantity of cheap 32.768Khz watch crystals from e-bay, and discovered all were either slightly high or slightly low in frequency.
Each fell into two distinct frequency groups with typical bell curve distribution with a dip right at 32.768 Khz. These were obvious reject parts.

With 7Mhz resonators you might find the same thing, and get lucky finding a rogue really high frequency one.