Presto QO-100 Transverter (Work in progress)

Description

Presto is a transverter that allows you to use a standard 10 m capable transceiver to communicate via the QO-100 satellite. It provides an S-band uplink (2.4 GHz), while the downlink can be received directly on your HF radio, since Presto also processes the signal from the Ku-band LNB.

It's all about signal quality

Presto employs several techniques to achieve a clean uplink signal. This is challenging with the low input frequency of the 10 m band compared to the desired 2.4 GHz. Directly mixing 30 MHz with a ~2.4 GHz local oscillator would result in spurious signals very close to the desired carrier. To solve this, Presto first generates an intermediate frequency at 433 MHz. At this frequency, it is easier to implement sharp band-pass filters that remove unwanted mixing products. Additional separation is achieved by following the first mixer with a triplexer. This circuit has passband characteristics for the IF and can also be considered a directional filter, since its input is well matched to 50 Ohms across a wide spectrum. As a result, the IF port of the first mixer does not experience impedance mismatches at frequencies where spurious products occur. Instead, these spurious emissions are dissipated in resistive loads connected to the other two ports of the triplexer. This approach reduces intermodulation in the mixer. The choice of mixers was also critical. Both mixers feature low LO leakage and high intercept points. To drive the mixers with a high IP3, a local oscillator with substantial output power is required, about 13 dBm in this case. For maximum flexibility and upgradability, Presto’s local oscillators are mounted on separate modules.

Modular construction

Presto uses a modular construction, with the motherboard (MB) PCB serving as the foundation of the entire system. Each functional section of the schematic is implemented in a physically distinct area of the PCB. To improve isolation between RF stages, shielding is used extensively. The power supplies are located at the bottom of the motherboard. The local oscillators are designed as swappable modules, allowing future upgrades for improved phase noise performance or operation with different frequency plans. The control head serves as the device’s front panel and is also replaceable.

LNB front end

The advantages go beyond downconversion, as the signal is preconditioned before reaching the downconverter. The LNB front end includes two band pass filters to remove out of band signals, an attenuator to reduce signal levels and minimize distortion, and a 13 dB LNA with bypass functionality. The signal from the LNB is then split between the built in downconverter and an output for an external SDR. This allows continuous monitoring of the downlink even while transmitting an essential capability for microwave operation.

Presto includes a bias-Tee and a programmable power supply, allowing two selectable DC voltage levels to be applied to the LNB coaxial connector. This feature enables polarization control in most LNBs, where the applied voltage determines vertical or horizontal polarization.

Digital control

Project in progress

Except for the 2nd LO module the PCBs are assembled in 90%. I am currently starting to write a software for the STM32 in order to test the full RF path. So far the signal path was tested piece by piece.

Schematic