Simple Offsets For Crystal-based Oscillators


 
 

Discussion

This page is mostly experimental and discusses shifting the frequency of crystal oscillators in simple transceivers that use a "rock-bound" oscillator for both transmit and receive. VXO applications are not discussed.
For all-weather usage versus stability it is hard beat the crystal oscillator when comparing it to an LC based VFO. During transmit mode, it is necessary to shift the oscillator frequency during transmit to provide an AF beat note for the other station to copy. A shift of 600 to 800 hertz is normally used but can ve varied to suit the builder. For crystal controlled oscillators, it is convenient to shift the oscillator frequency down the necessary offset during transmit.

Oscillator Experiment

Figure one shows a partial schematic of the basic crystal oscillator used for the tests. The buffer is not shown for clarity. Q1 is a 2N3904 and D1 is a 9.1 volt zener diode. A crystal was ordered with the following specifications: 7.0400 MHz, AT-cut in a HC-6/U metal holder with 30 pF load capacitance. The crystal was inserted into the basic oscillator and the frequency was measured. Refer to figure 4A. The measured frequency was 7038371 Hz.
Refer to figure 4B. A 7.5 pF cap was used for C1 and the measured frequency was 7043288 Hz. Next a 68 pF cap was used for C1 and the frequency measured was 7039293. The 68 pF cap was removed and replaced with a 220 pF cap resulting in a frequency measurement of 7038650 Hz. Finally a 0.001 uF ceramic cap was tried and the measured frequency was 7038432 Hz.
Refer to Figure 4C. Adding just a 22 microhenry inductor as shown resulted in a frequency of 7033091 Hz.
Refer to figure 4D. The 22 uH choke for L1 was retained and a 68 pF cap was used for C1. The measured frequency was 7039293 Hz. With a 220 pF cap for C1 the frequency was 7033757 Hz. You can draw your own conclusions from these tests.

Offset Circuits

The two offset circuits were built using 2N3904 transistors as the switches. Refer to figure 2. I wanted to my transceiver to transmit on 7040 MHz , the 40 meter band QRP frequency. To achieve this goal , two variable capacitors were used. One to set the receive frequency at 7040600 MHz and the other to set the transmit frequency as mentioned. The circuit in figure 2B allows this. C1 is used to set the receive frequency while C2 sets the degree of shift during transmit. I was able to get a 7040001 Hz transmit and a 7040609 Hz receive frequency by tweaking the variable caps. There is interplay between the two caps plus all the stray capacitance in the circuit. The capacitor values needed will vary with each and every circuit however for interest sake, my C1 measured value was 39 pF, so I swapped it with two 20 pF NP0 ceramic caps in parallel. This resulted in greater frequency stability for my oscillator. Using two variable caps will result in a frequency stability penalty however using quality high-Q variable capacitors will give acceptable results. The circuit component selection is always somewhat experimental and provides a good learning tool.

Please refer to figure 3.
Figure 3 shows another method to shift oscillator frequency down during transmit. It was developed by Wes Hayward, W7ZOI and is used by permission. Like the offset circuit shown in figure 2 , the frequency offset is activated by applying positive voltage at the point labelled VCC switched. This circuit when activated effectively bypasses the capacitor C1 and connects the crystal to ground, shifting the frequency down for transmitting. Using the before-mentioned 7.040 MHz crystal, three tests were conducted with 3 different values for C1. A 68 pF cap gave 7039202 Hz during RX and 7038366 Hz during TX. A 100 pF cap gave 7038974 Hz during RX and 7038372 Hz during TX. Finally, a 168 pF cap gave 7038744 Hz during RX and 7038374 Hz during TX. The results show a pattern which can be used to obtain the desired offset that you are looking for. The 100 pF cap gave an offset of 602 Hz, which is perfect! In my personal transceiver, I used a 100 pF NP0 ceramic cap and the oscillator has very, very good stability. It is unlikely that the transmit frequency of such a transceiver will be spot-on a standard frequency such as 7040 MHz, however, this has not stopped the rig from providing many enjoyable hours of use.

Conclusion

The oscillation frequency of a given crystal is dependant on many variables. Factors such as crystal type and frequency along with oscillator architecture are all at play. In addition, the frequency shift available to a given crystal generally goes down with frequency and this must also be factored. Effective offset circuit component values can be determined through experimentation and I hope this web page will inspire you to explore the many possibilities available.

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