What is a comb generator?
Comb generator output (harmonics)
A comb generator is a type of signal generator that produces multiple harmonics of an input signal. The output signal of the comb generator resembles a comb; hence it is called a comb generator. Comb generators are used as sources for EMI test sites to validate the measurement environment including antennas, cable losses, reflectivity, ground planes, etc., and to determine the resonance of cables. Other applications include investigation of reverberation (mode stirred) chamber behavior, characterization of filter performance, Cable loss measurements, and Inter-laboratory test programs.
How does a comb generator work?
A simple comb generator circuit consists of a clock generator (usually crystal oscillator), impulse generator, wave shaping circuit, and output impedance matching circuit. Different types of comb generator circuits are available. Below is an example of a simple comb generator circuit.
A simple comb generator circuit
A crystal oscillator delivers the square wave output (another wave-type also works) at a certain input frequency (MHz). The square wave is fed through a capacitor (C1) and shunt resistor that together forms a passive differentiator (impulse generator circuit). The impulse circuit delivers the output signal with positive and negative impulses (figure).
Oscillator output (time domain)
Differentiator output (time domain)
Comb generator output (time domain)
Oscillator harmonics (frequency domain)
A key component of this comb generator circuit is a step recovery diode (SRD), also known as snap-off diode or charge storage diode. When forward-biased, this diode exhibits low impedance and stores charge in the intrinsic region. When suddenly reverse-biased, the SRD will continue in conduction (low impedance state) until all of the stored charges are removed, at which time the diode will abruptly change to a high-impedance state.
Differentiator output (frequency domain)
Comb generator output (frequency domain)
This diode operation will result in the positive impulses of the impulse generator being sharpened, while the negative ones are nearly eliminated. This successfully breaks the even symmetry of the input signal and generates both odd and even harmonics at the SRD output. A comb generator circuit may use other non-linear elements such as non-linear transmission lines (NLTL), bipolar junction transistor (BJT) instead of SRD. The resistor (R2) helps to provide output impedance matching, and the capacitor C2 helps to block the DC bias current from the output connector.
Specifications details of comb generator:
Frequency range: Represents the useable frequency range of the comb generator. It is usually in MHz to GHz range.
Output frequency range: Represents the output signal (harmonics) frequency range. It is usually in MHz to GHz range.
Input power range: Represents the required input power level in dBm of the comb generator
Minimum output power per picket: Represents the minimum output power per picket in dBm of the comb generator
Frequency Step Size: Represents the frequency step size of the comb generator. It is usually the in MHz range.
Frequency Stability: Represents the output frequency deviation from the actual output frequency. It is represented in ppm (1/106 part of a nominal frequency). For example, For example - If a quartz oscillator has an output frequency of 2 MHz (2000000 Hz) and it has a Frequency Stability of 5 ppm, it will vary in frequency by 10 Hz.
Amplitude Stability: Represents the output amplitude deviation from the actual output amplitude. It is represented in ± dB.
Connector type: Represents the connector type available with the comb generator. Usually, it is an N-type.
VSWR: Represents the measure of how efficiently radio-frequency power is transmitted from the comb generator into the input terminal of the load (For example, antenna). A low input VSWR is the indication of a good comb generator.
Typical current consumption: Represents the current consumption of the comb generator at a certain frequency. Usually, it is in the mA range.
Pulse width: Represents the pulse width of the output waveform. Usually, it is in the ps (picoseconds) range.
Pulse frequency: Represents the number of pulses per second that can be delivered by the comb generator. Usually, it is MHz range.
Input return loss (S11): It is the measure of how much power is reflected back in the input side of the comb generator due to impedance mismatch between the source and a device (comb generator). It is measured in dB.
Output return loss (S22): It is the measure of how much power is reflected back in the output side of the comb generator due to impedance mismatch between the comb generator and a load (antenna). It is measured in dB.
Charging Adapter Output: Represents the power supply rating in VDC, mA.
Battery Type: Represents the battery type and its rating (associated with the comb generator). For example, 6V NiMH, 1Ah.
Operating time with a full charge: Represents the operating time of the comb generator with a full charge. It is represented in Hours.
Weight: Represents the weight of the comb generator in Kg.
Operating temperature: Represents the safe operating temperature limit of the comb generator in °F (°C).
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