We in this blog post want to discuss about CDR. In Digital Communication Clock Role is very important in Receivers and Transmitters. we know that any element in Digital Communication has internal or external clock that work with it as a reference clock. So for overcome lost data as unsynchronization between TX and RX we sould have a solution. Even in same Conditon is hard find two clock has same freqency. Without synchronization we will garbage data in sampleing in Receivers.
For example you can see in below image that clock in TX and RX is not same, so data that we sampled is incorrect in receivers.
Fig1:Unsame Clock TX and RX cause we read wrong data
So I have a question?When i want to synchronize TX and RX which parameter of clock shoulb be same? I myself answer:
- Frequency Detection
- Phase Detection
So we should with a mechanism detect frequency incoming bit stream, that usually first data is called recovery stream that we should use these data stream for detection frequecy.So we may in read frequency had inaccuracy, that most important are:
- Jitter:
Jitter is a shift in the edges of a periodic signal. This breaks the periodicity of the signal.
Fig2:Jitter in Frequency Detection
- PPM (parts per million):
PPM is an inaccuracy of certain components (quartz crystal in case of clock generator) in a circuit which leads to generation of a signal with inaccurate period. PPM does not break the periodicity of a signal.
For Phase Alighment this link related good nostalgie :
You may have seen an analog radio. There are two knobs namely coarse tune and fine tune on analog radio. When one wants to listen to any audible signals, coarse tune knob is used to lock onto a frequency where signals are audible but with some disturbances. Here coarse tune is as good as frequency detection and disturbances are jitter and PPM. To remove these disturbances and make the voice audible, fine tune knob is used which adjusts the pre locked frequency a little bit here and there to get a perfect audible signals. Here fine tuning is akin to phase alignment.
So phase alighment is important that arising edge or falling edge clock use for reading data.With using these mechanism stimulaneously we have clock Recovery method that is very important is communications.
we can Transmit in gnuradio with HackRF One DVB-T signal successfully, we can see video with dongle DVB-T player in hardware base method.But for receiving DVB-T signal with SDR based in gnuradio we have these errors:
DVBT project receiver probelm OOrestart aquisition d_freq_offset: -5 Od_freq_offset: -5
My Qeustion is In SDR Hardware SDR should do these task or in Software Enviroment In gnuradio we should myself do clock recovery methods? Any one that same problem welcome to discussing for solving DVB-T signal receiving with HackRF one.
Best regards stackprogramer
http://blog.stackprogramer.xyz/en/2017/06/09/clock-recovery-in-communication-and-optic-electronic/