Common Crystal & Oscillator Frequencies
Radios That Use Crystals for setting Tx and/or Rx Frequency
When a crystal is installed into an oscillator circuit a number of factors determine the actual frequency of oscillation.
Initial frequencyThe initial frequency is specified for either series mode or parallel mode with some load capactance. In either case becasue of stray capactance either the frequency will be off some or there will be a variable capacitor to adjust the frequency.
A tolerance of 0.002% on a crystal is a little better than normal. That is equivalent to 20 parts per million (ppm). Low cost unit oscillators have a frequency specification of 100 ppm that covers initial frequency, power supply variations and aging for something like 90 days.
Note that 100 ppm translates to 2,000 Hz for a 20 MHz oscillator or crystal.
Temperature effectsThe frequency vs. temperature curve is a strong function of how the crystal was cut. For narrow temperature range applications the turning point can be controlled so that the slope of the curve is about zero at the operating temperature. For wider temperature ranges the turning point can be set to be in the center of the temperature range to minimize the variation over the complete temperature range. The temperature variation can be characterized and is repeatable.
Lab standard crystals are typcially SC cut and the turning point is set for the double oven temperature.
AgingWhen operating (vibrating) the crystal will change frequency over long time periods. If the crystal was manufactured in a modern clean factory and is sealed using a cold weld process then the aging process follows a formula whose constants are serial number tied to the crystal. Once the aging function is known for a crystal then it can be predicted. Very high quality oscillators have an aging function that appears to be linear for time frames of many months. You could just turn the fine tuning adjustment a small number of counts each day, once you know what the drift rate is.
The best laboratory grade double oven crystal oscillators have an aging rate of about 1E-10 per day after they have been burned in for many weeks. That's 0.002 Hz in 20 MHz.
A Rubidium oscillator might have an aging rate of 1E-10 per month. The Stanford Research PRS10 uses GPS to correct for long term aging and uses an improved version of their SC10 crystal oscillator to provide very good short term stability (Allan Variance).
A problem with aging relates to the pullability or Q of the crystal. The aging rate is very low and gets better with age like a fine wine. But at some point the coarse trim capacitor can no longer pull the oscillation frequency back to the nominal value. This happened to me with a Gibbs lab standard that was very old. I got a little more life out of it by adding some glass caps to the coarse circuit, but in the end still could not pull the Bliley glass cased crystal back to nominal. When a timing output is desired, like a 1 Pulse Per Second output a microcontroller can continue to correct for a non nominal frequency so the only thing that matters is that the oscillator is running. The old crystal would have a much lower aging rate then newer crystals.
Allan VarianceIf you look at how the frequency varies as a function of how far from the carrier you test the parameter is called phase noise.
If you look at how the time varies as a function of the time interval the variation is called Allan Variance or modified Allan Variance.
A low cost unit oscillator may have a frequency variation of a few Hz when measured using a 1 second integration time counter whereas a high quality oscillator will have a one second frequency stable to a small fraction of a cycle.
Table as pdf document with factors that are typically 2, 3 and 5.
Radios like the PRC-68 family or the PRC-25 or PRC-77 or NRD-545 use some crystals and a frequency syntheizer to set the operatiing frequency and do not need to be recrystaled to change frequency.
The following radios use individual crystals to set their operating frequency. Typically one crystal for the transmit frequency and another crystal for the receive frequency.
PRT-4 & PRR-9
Series Reasonant Frequency
Parallel Reasonant Frequency with a specified CL
Operating Temperature - Temp Comp Patents
Equivalent Circuit Values: Ls, Lm. Cm, Rs, Cp
The mechanical reasonance shows up as Lm, Cm and Rm whose equivalent electrical values are much larger than conventional parts.
Military Crystal Impedance (CI) Test Sets
RFL (Radio Frequency Labs) 541C
10 to 1,100 kHz
same as TS-683?
MIL-T-12333C Drive Adjustment Procedure for Crystal Impedance Meter TS-710/TSM MS91446A TS-537
75 to 1,100 kHz
TM 11-5052 MIL-T-12628
RFL Model 459A
1 to 15 MHz
Drive Adjustment Procedure for Crystal Impedance Meter TS-330/TSM MS90167B TS-531
10 to 110 MHz
RFL Model 531
10 to 140 MHz
11-6625-456-14, -40P and TM 11-5051 ?
Drive Adjustment Procedure for Crystal Impedance Meter TS-683/TSM MS90168B Drive Adjustment Procedure for Crystal Impedance Meter TS-683/TSM (for Testing CR-33/U Quartz Crystal Unit) MS91415A AN-TSM/15
> 200 MHz
Saunders 200VHF 10 to 200 MHz
Commercial Crystal Test EquipmentThe Agilent (formerly HP) E5100 network analyzer with LCD screen is used by almost all the manufacturers for engineering and some production testing for crystals above 10 kHz. This is a swept instrument where the crystal is connected in series using a test fixture as described in app note 5967-6048E.pdf
The Agilent E4915 & E4916 crystal impedance meters are production meters with numerical output. These two are difficult to use because you must know the rough parameters before you test.
The Agilent 4194 is still used for low frequency (32,768 KHz) crystals where the impedance can get into the hundreds of Meg Ohm range.
Agilent App Note: Measurement of Load Reasonance Frequency of Crystal Reasonator 5967-6048E.pdf Note This is all about a test circuit that will supply the correct load capactance.
Crystal Acivity Meter - used to see if a crystal is alive and get a rough idea of the frequency. Amateur Radio operators use simple circuits with known CL values and a radio and/or counter to test crystals.
It is possible to use the 4395A to measure the crystal equivalent circuit by using a series connection, like in the E5100 and curve fitting the reasonance data in a computer program.
Victor VC2000 Crystal & Frequency Meter
Sanders 150C -
These are radios that use a crystal to rectify the RF signal and provide modulation to headphones. Almost all of these do not use any power source, some may use an amplifier. I put this here because if you do a web search on "crystal radio" you will get many hits on this topic. A Galena crystal was used in the old days.
Quartz crystals can be combined (or just one) to make a filter with very steep skirts.
Some companies make only crystal oscillators. They combine the crystal and the oscillator circuit in a small packate that has power, ground and output.
To save money a ceramic reasonator may be used in some applications where a quartz crystal was used.
These are ceramic parts that work with Surface Acoustic Waves. Some can by used to replace crystal filters.
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About FT-241 "Channel" Crystals -
Agilent (HP) policy on aging rate -
IEEE Ultrasonics, Ferroelectrics, and Frequency Control Society - Frequency Control History -
YouTube - How It's Made: Quartz Used in Military Radio Communication - Vintage Technology Documentary (40 minutes)
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