This invention relates to systems and methods for minimizing harmonic in a communications circuit.
Frequency conversion is the process of changing one frequency to another. This may occur in instances when one signal is multiplied with a second signal to produce, a sum and/or difference of the signals. Frequency conversion circuits are commonly used in transmitters/receivers to convert an outgoing/incoming Intermediate Frequency/Radio Frequency (IF/RF) signal to the RF/IF signal. In case of receivers the IF signal is further processed by circuits following the frequency conversion circuit to develop a suitable signal for passing to a demodulator. The demodulator recovers the information encoded into the IF signal. The frequency conversion circuit produces the IF signal by mixing the RF signal with one or more local oscillator (LO) signals using a mixer.
Wireless receivers usually use several mixers to take the RF signal from the antenna and shift it to the lower IF frequency. This procedure usually will help on sampling the data and going to the digital domain. The frequency of the IF signal will be determined by the separation in frequency between the radio frequency signal and the local oscillator signals. The mixer receives separate input signals and combines the signals to produce an output signal. The mixer allows very high frequency to be down-converted to baseband or IF so that signals may be evaluated using analog or digital signal processing techniques. In another example, the mixer up-converts a low frequency to a high frequency.
Potential detrimental interference can arise in the IF circuit. One form of detrimental interference includes harmonic interference, also referred to as “harmonic”. Harmonics are generally spectral components of a signal that can interfere with proper operation of the wireless receiver. These digital transceiver ICs typically deploy many clocks in the same IC running at different frequencies. The problem becomes more pronounced for single chip digital RF transceivers since such single chip solution for wireless receivers requires combining the RF and Base Band processing. The effect of the harmonic of any of these clocks on the mixed signal part of the IC becomes significant. As an example of this problem, the third harmonic of a 1 MHz clock at the output of the last mixer appears as an IF signal at 3 MHz. This third harmonic of the 1 MHz clock will show up as a 3 MHz signal at the output of the IF mixer at 3 MHz and during the demodulation process this will look like a DC offset, which is undesirable.