Here is part 9 for your building enjoyment ;-)
Just an update, I only got a handful of responses regarding the pace of
the class. I'll assume that there are more than 5 students left, and
those who did not respond were ok with the pace.
I'm going to give the project a little "boost" this weekend to attempt
to get done before the summer is over.
The section we are going to build next is the product detector. Remember
where we last left off, we had mixed the receive signal with the VFO
down to the IF frequency. From there it is "magically" filtered by the
IF crystal filter. Now we want to take that signal and turn it into
something we can hear with our ears. This is the job of the product
detector. It mixes the filtered IF with it's own oscillator that is
running at the IF frequency. In this case, if the VFO frequency is 3Mhz,
the transmit frequency is 6.9992Mhz (VFO + xmit mixer freq 3.9992Mhz).
Remember that the crystal frequency in the oscillator of U5 is pulled
800 Hz lower by the addition of RFC2. This provides the proper xmit
signal offset of 800Hz on the upper sideband. Remember that we are using
a small part of the transmit signal as our receiver source, so the
receive signal frequency is 6.9992Mhz. Once it is mixed with the VFO we
get VFO - receive=3.9992Mhz (look familiar?). This is within the 500 hz
bandwidth of the IF filter so this 3.9992 Mhz signal is presented to the
product detector. The product detector mixes this signal with its own
internal oscillator running at the IF frequency of 4Mhz. The resulting
signals are 7.9992Mhz and 800Hz. The 800Hz signal is in the audible
range for our ears to hear.
So far in the receiver we have been able to tune a signal with our VFO
that was 6.9992 Mhz and turn it into an 800Hz tone on the receive end.
Gather the following components.
U3 NE612 mixer chip
Y4 4mhz xtal (the case of this one is not grounded)
Install on the board and check your work.
Those of you with a scope can look at the output of U3 on pin 4 or 5.
You should see an interesting mix of high frequencies (7.9992Mhz) and a
low audio frequency (800 Hz).
For those without a scope, or if you just want to "hear" your receiver
work, try this. Power off the rig. Get one of the 47 Microfarad caps
left that has not yet been installed. Don't cut the leads! Tack solder
the + lead to the C19 pad that is closest to U3. Now using clip leads
connect one side of a speaker to the - lead of the capacitor and the
other side of the speaker to ground. Make sure the room is really quiet.
Power up the rig and hit the key. You should hear a very faint 800Hz
tone coming out of the speaker. That is your superhet receiver at work!
Once you hear the tone, shut down the rig and remove the capacitor and
put it back in the parts bag (you need it later).
There is not much to describe in this portion of the circuit. Just a
standard implementation of an NE612 mixer.
Stay tuned for the next section, the first audio amp.