The AA8V 6x2C Crystal Controlled Converter
by Greg Latta, AA8V

Choosing Crystal Frequencies

 

Crystals
Click on the image for a larger view.

Introduction:
When I built my 6x2 receiver and used it on the air I found that it was a great performer, especially when combined with a Timewave DSP 599 digital audio filter. Even though I had a modern transceiver in the shack I found that I preferred to use the 6x2 receiver with my DIGITAL VFO/6AG7 AMPLIFIER/6146B AMPLIFIER transmitter system over the modern transceiver. When used with the transmitter system, the station was then completely homebrew, and I really liked that.

Though completely homebrew, the receiver limited me to two bands, 80m and 40m. The transmitting system could, however, also work on 30m and 20m, and it would have been nice to operate on those bands as well. Modification of the receiver itself was not an option. However, the addition of a receive converter that could take signals on the 30m and 20m band and convert them to 80m or 40m would solve the problem.

I found a converter design in the 1965 Handbook for a 20m/15m/10m converter which could be modified into a 30m/20m converter. The problem was finding the correct crystals.

Back in 1965 there was no problem having custom crystals made. There were companies like International Crystals and JAN Crystals that could do the job, but they are now long out of business. For several years I looked for surplus crystals or for crystals at hamfests that could do the job, but with no luck. As it turns out, there may have been crystals I could have used, but just didn't know would work at the time. It turns out that there are quite a few crystal frequencies that will work in a given situation, especially if you are willing to compromise.

Choosing A Crystal Frequency:
For a given input frequency and output frequency there are two possible crystal frequencies. They can be found as follows:
a. Add the input and output frequencies.
b. Subtract the input and output frequencies and ignore any minus sign.

Example: The input frequency is 10.1 MHz. The output frequency is 7.0 MHz.
Adding: 10.1 MHz + 7.0 MHz  = 17.1 MHz
Subtracting: 10.1 MHz - 7.0 MHz =  3.1 MHz
The two crystal frequencies are 17.1 MHz and 3.1 MHz

To determine whether a chosen crystal will work over a band of frequencies, perform the calculations below.

Determining Whether A Crystal Will Work:
If you have a particular crystal and have a chosen input band and desired output band, see below and calculate the output frequency bands given the crystal frequency and your chosen input frequency band. If a calculated output band lies within your desired output band, then the crystal will work If the calculated frequency band lies partly within the desired output band then the crystal is conditionally usable by limiting the input band. Remember, if the crystal frequency is higher than the input frequencies the output band will be inverted.

Example #1:
The crystal frequency is 3,146 kHz. The input band is the 30m band, 10,100 kHz to 10,150 kHz.
The desired output band is 6,900 kHz to 7,250 kHz.

To get the first output band, add the crystal frequency to the lowest input frequency:
3,146 kHz + 10,100 kHz = 13,246 kHz
Then add the crystal frequency to the highest input frequency:
3,146 kHz + 10,150 kHz =  13,296 kHz

The first output band is 13,246 kHz to 13,296 kHz which is not in the desired output frequency range.

To get the second output band, subtract the crystal frequency from the lowest input frequency and ignore any minus sign:
3,146 kHz - 10,100 kHz =  -6,954 kHz which, ignoring the minus sign, is 6,954 kHz.
Then subtract the crystal frequency to the highest input frequency and ignore any minus sign:
3,146 kHz - 10,150 kHz = -7,004 kHz which, ignoring the minus sign, is 7,004 kHz.

The second output band is 6,954 kHz to 7,004 kHz, which is within the desired output band. This crystal will work.
(This is the crystal used in the 6x2C converter.)

Note that it isn't necessary for the input and output band edges to "line up".

Example #2:
The crystal frequency is 10,240 kHz. The input band is the 20m band, 14,000 to 14,350 kHz
The desired output band is 3,500 kHz and 3,850 kHz.

To get the first output band, add the crystal frequency to the lowest input frequency:
10,240 kHz + 14,000 kHz = 24,240 kHz
Then add the crystal frequency to the highest input frequency:
10,240 kHz + 14,350 kHz =  24,590 kHz

The first output band is 24,240 kHz to 24,590 kHz which is not in the desired output band.

To get the second output band, subtract the crystal frequency from the lowest input frequency and ignore any minus sign:
10,240 kHz - 14,000 kHz =  -3,760 kHz which, ignoring the minus sign, is 3,760 kHz.
Then subtract the crystal frequency to the highest input frequency and ignore any minus sign:
10,240 kHz - 14,350 kHz = -4,110 kHz which, ignoring the minus sign, is 4,110 kHz.

The second output band is 3,760 kHz to 4,110 kHz, which is partly within the desired 3,500 kHz to 3,850 kHz output band. If the input band is reduced to 14,000 kHz to 14,090 kHz, the output band will be completely within the desired 3,500 to 3,850 kHz output band. This limited input band is where most of the CW operation on the 20m band takes place, so if only CW operation is planned, this crystal will be fine. (This is the crystal used in the 6x2C converter.) Thus, if part the calculated output band lies outside of the desired output band, the crystal can be used if you compromise and appropriately limit the input band.

Calculating the Output Frequency Bands Given An Input Frequency Band And A Particular Crystal Frequency:
Note: There will be two output bands for one input frequency band and crystal frequency..

To get the first output band:
Add the crystal frequency to the lowest input frequency and ignore any negative sign. This will give one edge of the first output band.
Add the crystal frequency to the highest input frequency and ignore any negative sign. This will give the other edge of the first output band.

Example:
The input band is 14.0 MHz to 14.3 MHz. The crystal frequency in 3 MHz.
14.0 MHz + 3.0 MHz = 17.0 MHz
14.3 MHz + 3.0 MHz = 17.3 MHz
The output band is thus 17.0 to 17.3 MHz.

To get the second output band:
Subtract the crystal frequency to the lowest input frequency and ignore any negative sign. This will give one edge of the second output band.
Subtract the crystal frequency to the highest input frequency and ignore any negative sign. This will give the other edge of the second output band.

Example:
The input band is 14.0 MHz to 14.3 MHz. The crystal frequency is 17.0 MHz
14.0 MHz - 17.0 MHz =  -3 MHz which, ignoring the minus sign, is 3 MHz.
14.3 MHz - 17.0 MHz  =  -2.7 MHz. which, ignoring the minus sign, is 2.7 MHz
The output band is thus 3 MHz to 2.7 MHz.

Note that if the crystal frequency is higher than the input frequencies, the second output band will be inverted. A higher input frequency yields a lower output frequency. This is often a point of confusion. In addition, if the input signal is an upper sideband signal, it will come out as a lower sideband signal and vice versa.

Calculating the Output Frequencies Given an Single Input Frequency And Particular Crystal Frequency:
Note that for a single input frequency and crystal frequency there are two output frequencies.
Two obtain these frequencies, add and subtract the crystal frequency from the input frequency and ignore any negative sign.

1st Example:
The input frequency is 14.0 MHz. The crystal frequency is 10.5 MHz
14.0 MHz + 10.5 MHz =  24.5 MHz
14.0 MHz - 10.5 MHz = 3.5 MHz
The output frequencies are 24.5 MHz and 3.5 MHz

2nd Example:
The input frequency is 14.0 MHz. The crystal frequency is 16.0 MHz
14.0 MHz + 16.0 MHz  =  30.0 MHz
14.0 MHz - 16.0 MHz = -2.0 MHz which, ignoring the sign, is 2.0 MHz.
The output frequencies are 30.0 MHz and 2.0 MHz

Where To Buy Crystals:
It is next to impossible to buy a crystal for the exact frequency that you might need in a converter circuit. However, it is usually possible to find a crystal close enough that it will work. One place you can buy crystals from is Mouser Electronics:

Mouser Electronics

Just type the word "crystals" into their search box and you will be taken to their crystal search program. You will be confronted with a choice of tens of thousands of crystals. To reduce the search, select "HC-49" (without any suffix) as the Package/Case and select "Apply Filters". The HC-49 case is one of the standard case sizes and is large enough that it won't be overdriven by the oscillator in the 6x2C. (The cases with a suffix, such as HC-49U, are smaller and may be overdriven by the oscillator in the 6x2C.) You can then scroll down through the many frequencies available.

As of July, 2023 the following frequencies were available from Mouser in an HC-49 holder that could be used to convert all or part of the bands indicated to the 80m (3.5 MHz to 4.0 MHz) or 40m (7.0 MHz to 7.3 MHz) bands::
20m in to 80m out: 10.245 MHz, 10.1783 MHz
30m in to 80m out: 6.176 MHz, 6.36406 MHz, 6.4 MHz, 6.5636 MHz
30m in to 40m out: 3.0 MHz, 3.072 MHz

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu