A Novel Comb Generator for Frequency Phase Transfer

Abstract

This paper presents the design and testing of a millimeter-wave frequency comb generator developed for the Black Hole Explorer (BHEX) mission, a space Very-Long-Baseline Interferometry (VLBI) mission concept. The heart of BHEX is a dual-band receiver, centered at 90 and 270 GHz. This novel comb generator is based on a microwave phase modulator producing phase-coherent comb signals with multi-octave bandwidth, which will be used to track instrumental delays when injected into the receiver system. The comb generator was tested with astronomical receivers, which confirms its expected operation.

Figures (7)

• Figure 1: A block diagram of the comb generator setup, The radiated comb signal is sent to the receiver pair Rx-L (90 GHz) and Rx-H (270 GHz) through an optical diplexer.
• Figure 2: Measured phase change of a 2 GHz carrier when a 0.1 Hz sine wave (black dotted curve) was applied to the phase modulator for 2 different modulating signal amplitudes -- $0.6V_{pp}$ and $1.6V_{pp}$.
• Figure 3: Second and Third order combs observed when a 4 GHz carrier was modulated by a 100 MHz signal with applied power of 9 dBm (Top) and 13 dBm (Bottom).
• Figure 4: Composite spectra measured with a spectrum analyzer equipped with a WR-3.4 harmonic mixer depicting the comb coverage for Rx-L (left) and Rx-H (right). The power levels are only approximate because we do not have a calibration for the harmonic mixer. Also, we are only measuring the comb teeth falling in the 4-12 GHz IF for each LO tuning.
• Figure 5: Comb injection test setup using the ngEHT 86/115 GHz receiver (Top), wSMA receiver (Bottom Left), and 1mm and 3mm receiver (Bottom Right). For ngEHT receiver, the comb and horn are mounted on a plate, directed towards the back side of the optical diplexer, which acts as a $\sim$20 dB coupler for the comb signal. For wSMA receiver, the horn is mounted above, pointing downward toward the window. For the Kitt Peak receivers, the comb generator is mounted on the side, with the horn pointing towards a wire grid polarizing diplexer placed in front of the vacuum windows of the receivers