This paper presents RF-Transformer, a unified backscatter radio hardware abstraction that allows a low-power IoT device to directly communicate with heterogeneous wireless receivers. Unlike existing backscatter systems that are tailored to a specific wireless communication protocol, RF-Transformer provides a programmable interface to the micro-controller, allowing IoT devices to synthesize different types of protocol-compliant backscatter signals in the PHY layer. By leveraging the nonlinear characteristics of the negative impedance, RF-Transformer also achieves a cross-frequency backscatter design that enables IoT devices in harmonic frequency bands to communicate with each other. We implement a PCB prototype of RF-Transformer on 2.4 GHz ISM band and conduct extensive experiments. We leverage the software defined platform USRP to transmit the carrier signal and receive the backscatter signal to verify the efficacy of our design. Our extensive field studies show that RF-Transformer achieves 23.8 Mbps, 247.1 Kbps, 986.5 Kbps, and 27.3 Kbps throughput when generating standard Wi-Fi, ZigBee, Bluetooth, and LoRa signals.