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Introduction
This application note describes the theory and use of the comb/marker generator (CMG) product line produced by High Sierra Microwave. The CMG is also known as 1) a crystal calibrator, 2) a weak signal source and 3) a crystal marker.
Some Theory
The typical High Sierra Microwave comb/marker generator (CMG) consists of an novel, very low current impulse generator design, which generates a narrow impulse containing high orders of harmonics. Also included is an internal crystal oscillator clock or provision for an external input signal to drive the impulse generator circuit and the input voltage conditioning and regulator circuits.
Traditional impulse generator design uses a step recovery diode (SRD) to generate high harmonic content. While SRDs can generate harmonics with high power levels they typically require 0.5 to 1 watt of drive power at the clock frequency.
The impulse generator we have developed can provide usable harmonics to beyond 18 Ghz with just milliwatts of drive power. While the power levels in the harmonics are lower than those obtained with the traditional SRD approach, it still provides usable harmonic power levels, but with the distinct advantage of requiring very low currents, which open up portable and power sensitive applications.
For example, the Model CMG-16-NNN, with an internal 16 MHz temperature compensated crystal oscillator, provides a typical harmonic power level of -95 dBm at 10.368 GHz, while running from 9Vdc at 15 mA.
Except in applications requiring high harmonic power levels, our high efficiency impulse generator design can provide economical, low power consumption solutions for:
B.I.T. (Built-In-Test) Generator
Crystal Controlled MHz/GHz Signals
Antenna Measurements
Spectrum Analyzer Calibration
Xn Multiplier
Weak Signal Source
Choice Of Comb/Marker Frequency
The standard High Sierra Microwave comb/marker generators are available with crystal controlled comb frequencies from 1 MHz to 32 MHz. Specials are available up to 100 MHz. The frequency selected will depend on the intended application. To determine if a particluar comb/marker frequency will have a harmonic on the frequency you desire, just divide the frequency you desire by the comb/marker frequency and if the result is a integer then the frequency will work.
For instance, if you desire a marker on 2304 MHz and you are considering a 16 MHz comb/marker generator, then 2304/16 = 144, will work.
Power Level Considerations
As a general rule, the lower the frequency of the comb, the lower the harmonic power will be. For example, a 16 MHz comb will provide more power in the 10.368 GHz harmonic, typically -95 dBm, versus a 1 MHz comb which will have a typical output of -110 dBm at 10.369 GHz.
While -110 dBm will provide a very adequate signal for testing and tuning a low noise 10 GHz downconverter when applied directly to the input, it may not be adequate if the CMG will feed an antenna used as a source when making antenna gain and pattern measurements or checking system operation through a coupler.
Comb Frequency Considerations
The desired harmonic must hit the desired frequency. The 32 MHz frequency will generate 10.336, 10.368, 10.400 Ghz (signals at 32 MHz intervals). If used for Amateur applications, the 32 MHz CMG with provide “markers” in the Amateur bands at and above 1296 MHz. Notice that it will not provide signals in the common converter/transverter IF bands of 144 and 432 MHz.
This is important since input signals generated by the CMG in these IF bands could feed through the converter/transverter with inadequate filtering and be seen in the IF as a false signal, especially since the power level of the CMG harmonic at the IF frequencies will be significantly higher than the 10.368 Ghz signal. Naturally, the use of adequate filtering on the input to the device under test will alleviate this problem.
But, if it desired to provide a marker at all the Amateur bands at and above 144 MHz, then the 16 MHz frequency will be a good choice, but the user must recognize the potential for IF signal feedthrough.
Choosing the 1 MHz comb frequency will yield many more “markers.” For instance, 1 MHz will provide harmonics at 10.367, 10.368, 10.369 Ghz, etc, but with the penalty of significantly lower power, as discussed above.
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