Undersampling (or bandpass sampling) phase modulated signals directly at high frequency band, the harmful effects of the aperture jitter characteristics of ADCs (Analog-to-Digital converters) and sampling clock instability of the system can not be ignored. In communication systems the sampling jitter brings additional phase noise to the constellation pattern besides thermal noise, thus the BER (bit error rate) performance will be degraded. This paper examines the relationship between the input frequency to ADC and the sampling jitter in digital IF (Intermediate Frequency) downconversion receivers with undersampling scheme. This paper presents the measurement results with a real hardware prototype system as well as the computer simulation results with a theoretically modeled IF sampling receiver. We evaluated EVM (Error Vector Magnitude) in various clock jitter configurations with commonly used and reasonable cost ADCs of which sampling rates was 40 MHz. According to the results, the IF input frequencies of QPSK (16 QAM) signals were limited below around 290 (210) MHz for wireless LAN standard, and 730 (450) MHz for W-CDMA standard, respectively, in our best configuration.