Zero Beat operation by K3NG

K3NG wrote a good article about An 80 Meter CW Five Watt QRP Tranceivert. In the article, there is the section about the opeartion:

Operation
As this rig has a direct conversion receiver, signals can be heard on both sides of zero beat. In other words, when tuning up the band, you will hear a signal with a high frequency beat note, decreasing as you tune up. The beat frequency continues to get lower until reaching zero beat (dead on the transmit frequency of the other station). Tune further up, and the beat reappears as a low frequency and increases frequency as you continue to tune upwards. This characteristic of direct conversion receivers require a little more thought when tuning on stations and attempting a QSO. Since the transmitter frequency is negatively offset (TX frequency is 600-700 Hz below the RX frequency), one needs to tune on the “high side” of zero beat in order to have the transmit frequency fall within the other station's receiver pass band.


This is a good description of zero beat on the direct receiver.

The article of "Listen to AMATEUR RADIO Signals!" also describes the zero beat by direct conversion on the receiver.