The Receive Mixer is the section of the receiver that mixes incoming signals from the antenna with the 5 MHz VFO to convert the incoming signal to 9 MHz, our IF (intermediate frequency). Then the signal passes through the crystal filter to eliminate all but the desired signal. Then it’s off to the product detector (U1) where it gets converted to audio. Let’s say that we’re interested in tuning in a signal at 3.900 MHz. So, we tune the VFO to 5.1 MHz, and the receive mixer (U2) combines those two signals to produce a 9 MHz signal. If I want to tune in a signal on a lower frequency, I have to increase the frequency of the VFO, right? So the Survivor can be said to tune backwards. Indeed, when it’s all built, if you turn the tuning knob clockwise, the frequency of the radio actually goes down. It takes a bit of getting used to, but it’s easy enough.
We’ve already seen U2, the VFO, and the BFO in action in previous installments of this series, right? They all do double duty (along with U1) in this circuit, meaning they are used in both the transmitter and receiver. So, all that’s needed to complete the receive mixer is to install the parts that connect it to the antenna. Install the following parts on the board:
C1, C2, C5, C6, C7, C10
Next, follow the instructions in the Survivor manual to wind and install T1, T2, L4, and L5. (Strictly speaking, I’m not sure we need T2 at this point, but since you’re doing the others, why not do them all, eh?)
I haven’t figured out any interesting tests to do at this point, but let’s talk about the circuit. L4 and L5, along with C1, C2, C6, and C7, make up the low-pass filter that’s primarily responsible for removing any harmonics from the transmitted signal before it gets to the antenna. I guess it probably keeps the receiver from having to deal with as many unwanted signals, too. From there, the signal travels to transformer T3, which we’ll ultimately adjust so that it’s most sensitive somewhere in the middle of our tuning range. It also helps to eliminate unwanted signals.
Before the signal gets to T3, it passes through Q1, which serves as a switch controlled by Q5. Normally, the gate of Q1 is held high by virtue of its connection to the DC input voltage through R5. This allows it to freely conduct signals through its drain and source. However, when the transmitter is in use, the voltage VTX at the gate of Q5 is high, causing it to conduct freely. Its drain is connected to Q1’s gate, meaning that when Q5 is on, Q1’s gate gets grounded and it shuts off. This action prevents any transmitter signal from getting back into the mixers through the receive chain. Q7 and C19 (installed in a previous segment) help to ensure that nothing leaks past Q1.
Once the received signals get past T3 they’re fed to U2 where they’re mixed with the VFO signal and sent to the crystal filter. One thing to realize is that there are potentially many signals coming in from the antenna and getting mixed with the VFO. However, only those signals that, combined with the VFO frequency, end up in the bandpass of the crystal filter make it through to the product detector. So, the crystal filter is providing nearly all of the selectivity in this radio.