US 4,063,169 A · Method and apparatus for microwave frequency counting

Abstract

A microwave input signal whose frequency is to be determined is mixed in a mixer with a local oscillator (LO) signal and the resultant difference signal is applied to a low pass filter. The frequency of the LO signal is controlled by a digital processor that samples the output of the low pass filter. Initially the LO signal is at zero. Thereafter, the frequency of the LO signal is sequentially stepped through the fundamental and, then, the harmonics of a principal oscillator signal having a fundamental frequency F.sub.R. When the difference frequency falls within the direct counting capability of a digital counter, the digital processor opens a main gate for a short period of time, and a first difference frequency (F.sub.1) count is made and stored. The frequency of the LO signal is then offset by a known amount, and a second difference frequency (F.sub.2) count is made and stored -- if this F.sub.2 count is within the direct counting capability of the digital counter. Alternatively, if this F.sub.2 count lies beyond the direct counting capability of the digital counter, the frequency of the LO signal is stepped to the next F.sub.R offset harmonic and, then, an alternate F.sub.2 count is made and stored. The relationship between F.sub.1 and F.sub.2 determines whether or not the LO signal frequency, when the counts were made, was above or below the frequency of the input signal. Subsequent to the F.sub.1 and F.sub.2 counts being made and stored, the main gate is again opened, this time for a precise period of time related to the requisite resolution required and the digital counter makes and stores a third difference frequency (F.sub.3) count. F.sub.3 is algebraically summed with the LO signal frequency produced when F.sub.3 was determined, to obtain a displayable count equal to the frequency of the input signal. A jitter generator is provided to randomly frequency modulate the LO signal to preclude phase lock between the LO signal and the input signal, when the F.sub.1 and F.sub.2 counts are being made and stored during a counting sequence.

A frequency counter suitable for determining the unknown frequency of an input signal over a wide frequency range up to and including at least a portion of the microwave frequency range comprising:

1. a local oscillator means suitable for selectively producing one of:
a. a sequence of principal local oscillator signals, said principal local oscillator signals comprising a fundamental frequency and a predetermined number of harmonics of said fundamental frequency;
and,
b. a sequence of offset signals, one offset signal related to each of said principal local oscillator signals each of said offset signals being at a frequency offset from its related principal local oscillator signal by a predetermined amount;
2. a two-input mixer means, having one input connected to receive the signal whose frequency is to be determined and a second input connected to said local oscillator means for receiving a selected one of the signals produced by said local oscillator means, for producing a signal having a frequency equal to the difference between the two signals applied to the inputs of said two input mixer;
3. a controllable gate having a signal input connected to the output of said mixer for passing the difference signals produced at the output of said mixer when said controllable gate is enabled;
4. frequency counting means connected to the output of said controllable gate for counting the frequency of signals passed by said controllable gate, said frequency counter having a maximum frequency counting capability;
5. a control and summation means for controlling the operation of said local oscillator means and said controllable gate and for summing a predetermined count made by said frequency counting means with the frequency of the signal produced by said local oscillator means when said predetermined count was made, said control and summation means:
a. connected to the output of said mixer means, for receiving the output thereof and determining whether or not said output includes a signal whose frequency is below the maximum frequency counting capability of said frequency counting means;
b. connected to the enable input of said controllable gate for enabling said controllable gate when the output of said mixer means includes a signal whose frequency is below the maximum frequency counting capability of said frequency counting means;
c. connected to said local oscillator means for controlling said local oscillator means such that said local oscillator means:
i. produces, one-at-a-tine, said sequence of principal local oscillator signals until the output of said mixer means includes a first signal whose frequency is below the maximum frequency counting capability of said frequency counting means;
and,
ii. subsequent to the occurrence of said first signal being included in the output of said mixer, produces one of said sequence of offset signals, said one of said sequence of offset signals being the one related to the one of said sequence of principal local oscillator signals produces when said first signal was included in the output of said mixer;
and,
d. connected to said frequency counting means for receiving the counts made by said frequency counting means.

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Method and apparatus for microwave frequency counting

Life of this patent

Method and apparatus for microwave frequency counting

Life of this patent

Abstract

Claims25