stm32f303RE PWM输出波形变形的原因？

我使用STM32 nucleo开发板，使用tiM3 CH4输出PWM方波，发现波形变形，整个工程只有这个PWM输出的功能，其他外设都没开启。工程使用的是标准库。请大神支个招。
void TIM_Config(void)
{
    GPIO_InitTypeDef  GPIO_InitStructure;    TIM_TimeBaseInitTypeDef  TIM_TimeBaseInitStructure;    TIM_OCInitTypeDef  TIM_OCInitStructure;    TIM_BDTRInitTypeDef TIM_BDTRInitStructure;    RCC_AHBPeriphClockCmd(RCC_AHBPeriph_GPIOB,ENABLE);    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_7;    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;//GPIO_Mode_AF    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;    GPIO_Init(GPIOB, GPIO_InitStructure);    GPIO_PinAFConfig(GPIOB,GPIO_PinSource7,GPIO_AF_10);    RCC_APB1PeriphClockCmd(RCC_APB1ENR_TIM3EN,ENABLE);        TIM_TimeBaseInitStructure.TIM_Prescaler = 71;    TIM_TimeBaseInitStructure.TIM_CounterMode = TIM_CounterMode_Up;    TIM_TimeBaseInitStructure.TIM_Period = 499;    TIM_TimeBaseInitStructure.TIM_ClockDivision = TIM_CKD_DIV1 ;    TIM_TimeBaseInitStructure.TIM_RepetitionCounter = 0x00;    TIM_TimeBaseInit(TIM3, TIM_TimeBaseInitStructure);    TIM_OCStructInit( TIM_OCInitStructure);    TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;    TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;    TIM_OCInitStructure.TIM_OutputNState = TIM_OutputNState_Disable;    TIM_OCInitStructure.TIM_Pulse =250;    TIM_OCInitStructure.TIM_OCIdleState = TIM_OCIdleState_Reset;        TIM_OCInitStructure.TIM_OCNIdleState = TIM_OCIdleState_Set;    TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;    TIM_OCInitStructure.TIM_OCNPolarity = TIM_OCNPolarity_High;    TIM_OC4Init(TIM3, TIM_OCInitStructure);    TIM_Cmd(TIM3,ENABLE);    TIM_CtrlPWMOutputs(TIM3,ENABLE);}