Based on a single-level YBCO thin film topology and seven bicrystal grain-boundary junctions, a high temperature superconductor delta modulator for analog-to-digital converters has been designed, fabricated, and tested. A single bit quantizer based on a quantum flux parametron (QFP) comparator is used for the modulator. A large superconducting inductor is used both for the signal input and for the feedback. A small resistor is chosen for connecting the comparator to the read-out section. The oversampling provides large input signal bandwidth with high dynamic range. Circuit simulations prove the concept of the design. The effective LSB and MSB of the modulator are discussed in terms of input inductance, junction critical current, and QFP noise current. The density of latched output pulses is proportional to the change of input amplitude. Au wires were employed to make crossover of bias lines. Although this modulator did not function as expected for low-speed measurements, it shows the proper latching behaviour of the output section at 4.2 K and 35 K.