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APPLICATION NOTE AN-CGM001

Theory, Selection, and Applications for

a Comb/Generator Module

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

 EMI - RFI and 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 particular 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 feed through.

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.

Applications

EMI - RFI - Antenna Measurements

Using the CMG as the portable signal source with a sensitive, calibrated tuned receiver, various antenna parameters such as gain and front to back ratio can be measured.

As shown in Fig. 1, a battery powered CMG is connected to the source antenna and placed up to a hundred feet away from the antenna being measured. An antenna with a known gain is first measured with the measurement receiver. The antenna under test is then substituted for the known gain antenna with the differences noted.

Fig. 1 - Antenna Measurements

B.I.T. Oscillator

As shown in Fig. 2, the CMG module is coupled into the input of the receiving system providing a test signal used to measure the integrity of the receiving system. If the amplitude of the CMG is calibrated, the signal can be used to verify or measure system sensitivity.

Because battery operation is practical with our design, the CMG can also be attached to an external antenna and placed in front of a systems antenna to verify system integrity in the field.

Fig. 2 - Built-In Testing

Calibration/Marker Oscillator

With internal crystal controlled oscillators available from 1 MHz to 125 MHz, the CMG can be used as a low cost alternative to a signal generator to provide accurately spaced harmonics which can calibrate spectrum analyzer displays and receivers or to measure and adjust voltage controlled oscillator linearization.

The CMG can be used to calibrate the frequency dial of older shortwave radio receivers lacking internal crystal calibrators.

Alignment Signal Source

The desired harmonic or harmonics from the CMG can be used to align filters and tuned amplifiers. Depending on how accurately the test configuration is calibrated, filter and amplifier gain can also be determined, and depending on the harmonic spacing, estimates of bandwidth can be made.

In fact, over relatively narrow bandwidths, the CMG harmonics are relatively flat in power amplitude and can therefore provide a good estimate of filter bandwidth and can act like a sweeper to do a “coarse” tuning of the filter.

Figure 3 shows the response of a 432 MHz filter when measured on a spectrum analyzer and driven with a 1 MHz CMG at the input. While not intended to compete with a calibrated sweeper or a network analyzer, it will provide a “quick performance estimate” in many applications.

Fig. 3 - Filter Alignment

Xn Multiplier

Depending on the frequency and harmonic power level, the CMG can be used as a frequency multiplier. Using appropriate filtering and the use of low cost MMIC amplifiers, a frequency multiplier can be designed which is lower in cost and more efficient than conventional SRD comb generator designs.

Weak Signal Source

Since the harmonic signal level from the CMG in the microwave region is typically below -60 dBm, it is well suited as a weak signal source to determine system sensitivity and can be used as an alignment signal source. One CMG can replace several signal generators.

Contact High Sierra Microwave if you have any questions.

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XN Multipliers

Fig 4 - Typical 10 Ghz Level versus External Input Frequency

(Rev 05/31/2015)