Keysight-Agilent 5100B

HP 5100B  frequency synthesizer DC-50MHz in 0.01Hz steps

A frequency synthesizer is an instrument which translates the frequency stability of a single frequency to any one of many other possible frequencies usually over a broad spectrum. It s single frequency generator is usually of frequency standard quality; thus, the synthesizer may be called frequency standard capable of furnishing a large number of frequencies. The two basic approaches to frequency synthesis are known as Direct and Indirect. Direct synthesis performs a series of arithmetic operations (multiplication, division, and mixing) on the signal from the frequency standard to achieve a desired output frequency. The indirect method uses tunable oscillators, phase-locked to harmonics of a standard frequency, to derive a desired output frequency.

One advantage of HP direct synthesis type of instruments is that they can be switched (frequency changed) much faster than indirect synthesis where the switching speed is governed by the phase-locked circuitry. With direct synthesis the limitations on switching speeds are set by the time constants in the filtering circuits on the control lines to the switches and by the circuit bandwidths. Direct synthesis is also fail-safe in that if any part of the circuitry becomes inoperative the output signal is not present.

On the other hand, the indirect method of synthesis can be less costly for an equal frequency range as less circuitry is normally required in the synthesis process. It then follows that for an equivalent cost the indirect synthesizer can include modulation and attenuation functions that the direct synthesizer does not.

Hewlett-Packard direct synthesizers are particularly noted for the following:

1. Complete guaranteed specifications on tested noise and stability characteristics for all instruments—spurious signals, phase noise, a.m. noise, rms fractional frequency deviation, long term stability, and harmonic signals.

2. Frequency switching speeds of 4 ms for 2 digits and typically 20 ms for a change of many digits.

3. Rugged and modular construction designed for reliable, long life operation.

4. Customized modifications for special requirements often to achieve a cost savings.

The basic principle that conducts the 5100 development was the following: "The sources that provide the highest frequency stability are single frequency sources, commonly called "Frequency Standards." Having these high quality standards to hand, it is natural to look for some method to translate their stability to other desired frequencies. This translation when the operation is something more than a single arithmetic operation, is commonly know as frequency synthesis. Hence, a variable frequency synthesizer is an instrument that translates the frequency stability of a single frequency standard, to any one of many other possible frequencies, usually over a broad spectrum."