Auto Carrier Frequency / Phase

When Analog Demod is selected, two different types of auto carrier can be selected depending on the type of demodulation, as shown in the following table.

For this demodulation:	Use this auto carrier:
AM Amplitude Modulation - CW modulation using amplitude variation in proportion to the amplitude of the modulating signal. Usually taken as DSB-LC for commercial broadcast transmissions and DSB-SC for multiplexed systems.	None
FM Frequency Modulation	Auto Carrier Frequency
PM	Auto Carrier Frequency and Auto Carrier Phase

Auto Carrier Frequency cannot be selected with AM demodulation. This is because carrier amplitude estimates are always performed. AM demodulation does not require carrier frequency estimation because the AM calculation is based on the carrier envelope. The carrier amplitude estimate is based on data within a single time record and is updated every measurement sweep.

Auto Carrier Frequency

Accurate angle demodulation (FM or PM) depends on precisely identifying the carrier frequency. Errors result in phase ramping. The arc tangent of the complex time record is the basis of both PM and FM demodulation. Hence, correcting for the phase ramp is the goal of auto carrier frequency.

When the Auto Carrier Frequency check box is cleared, the VSA uses the current center frequency as the carrier frequency of your signal. When Auto Carrier Frequency is selected, the VSA uses an algorithm to estimate the carrier frequency. If the measurement hardware can be locked to an external reference which is coherent with your carrier, no carrier frequency estimation is needed and Auto Carrier Frequency does not need to be selected.

When Auto Carrier Frequency is selected, the carrier frequency estimate is calculated independently for each time record and is used in the demodulation calculation to take out FM offsets or PM phase ramps due to error between your carrier frequency and the VSA's center frequency.

If auto carrier frequency is selected, Analog Demod Carrier can be selected to display the estimated carrier frequency for FM demodulation. Cross channel results show the estimates for both channels. For PM demodulation, both Auto Carrier Frequency and Auto Carrier Phase must be selected to show the estimated carrier frequency.

Auto Carrier Frequency and Averaging

The operation of auto carrier frequency is modified if averaging is enabled. For FM measurements, the carrier frequency estimate from the current time record is fed into an exponential average of estimates from prior time records. The resulting, averaged carrier frequency is used to compensate the current time record for carrier frequency offsets from the center frequency.

Considerations When Using Auto Carrier Frequency

The following situations can bias the phase ramp estimation:

Low frequency modulation, such as a periodic signal with fewer than 10 cycles over the time record.
Phase discontinuities present in digital communication formats.
Transients, such as carrier turn-on in the middle of the time record.

In cases where biases are unavoidable, Auto Carrier Frequency should be disabled. Where possible, lock the measurement hardware's external reference to a reference coherent with the carrier to eliminate frequency errors.

Auto Carrier Phase

PM demodulation also has the option of correcting for phase offsets. A phase offset does not affect FM results, because the offset differentiates to zero.