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Crystal Pulling Range

1. Modern miniature crystals in the HC18/u or HC49/s enclosures display a significantly lower pulling range than the much older versions at the same frequency in HC/6u enclosures, probably due to the smaller physical size to allow them to fit in the smaller enclosures and the resulting much higher L/C ratio of the equivalent motional values.

I therefore decided to do some simple measurements on my Type 193a crystal tester to see if the pulling range could be increased by paralleling multiple crystals. The following table shows the results:


No of Crystals

Frequency at 20pF

Frequency at 50pF

Pulling Range

1

3.276817MHz

3.276629MHz

188Hz

2

3.277150MHz

3.276822MHz

328Hz

3

3.277377MHz

3.276958MHz

419Hz

20pF and 50pF are standard load capacitor values provided by the crystal tester. You can see that the resonant frequency is increasing slightly as more crystals are connected in parallel. This is easy to achieve as being a standard frequency and made in large quantities they are very cheap to purchase – typically $2 for 10 crystals.

In principle, similar results may be obtained if parallel connected crystals are used in ladder filters but ladder filters use the series resonance and I have yet to test that assumption.

There is a table of crystal frequencies and ladder filter maximum bandwidths for single crystals on this technical web site - see here.

At 3.2768MHz the maximum bandwidth that can be achieved with these crystals in a standard ladder filter is typically 1.6KHz which is fine for CW but too narrow for SSB.

2. Single Crystal Pulling Range

I carried out some measurements on the series resonant frequency of a single IQD 9.0MHz HC18/u crystal while connecting a range of components in series with the crystal with the following results:

Series component

Series Resonant Frequency MHz

7uH inductor

8.9955

4u7 inductor

8.9966

Short circuit

8.9984

220p capacitor

8.9989

100p capacitor

8.9994

47pF capacitor

9.00064

22p capacitor

9.00268

10p capacitor

9.006