Frequency resolution improvement of microwave measurement using down-convert technique

In this paper, we show that a frequency down-convert technique in conjunction with a SR620 universal counter could measure microwave frequency signals with better frequency resolution than that provided by most commercial microwave counters. Precisely speaking, while carrier frequency is from 300 MHz to 26.5 GHz, the measurement capabilities for frequency resolution using the above method could be promoted to 0.0001 Hz. This is helpful for us to accurately calibrate a DUT (device under test) when performance of its reference frequency is better than an OCXO (oven-controlled crystal oscillator). The microwave frequency measurement system was then established at TL between the year 2012 and 2013 after many tests. We have also compared frequency stability of the system with two different modes of a SR620 counter including direct frequency measurement and phase comparison. The results from the two modes showed a good agreement with each other at 1 second averaging time when carrier frequency of 1 GHz and 26.5 GHz were tested.