Ok Elmerlings, here is our next section. Again it's late so
I'm going to
post what you need to get started and then follow up with some more
theory. Don't wait for me to post again if you have questions. I'd
rather deal with them as they come to mind then have you wait.
Chuck will be following up with the answers to his quiz shortly, so stay
tuned. I'll bet he may even have another quiz based on this next
section. If you want a peek at "my" answers to the quiz check the web
site. http://www.qsl.net/kf4trd under the lessons section. Remember
that these are "my" answers, not necessarily the correct ones. Remember
that I'm also a student ;-)
Gather the following components.
U5 NE612 8 pin IC
PLEASE REFER TO DAVES INSTRUCTIONS FOR INSTALLING THIS PART. IT CAN BE
INSTALLED BACKWARDS AND THIS WILL DAMAGE IT!!!
Socket for U5
Q3 2n3906 pnp transistor
D11 1n5236 7.5 volt zener diode
C29 150pf (the round "ceramic" one, not the oblong one)
C110 3.3mf Electrolytic
ATTN, THE ABOVE PART IS POLARIZED AND CAN BE INSTALLED BACKWARDS IF YOU
ARE NOT CAREFUL. ELECTROLYTIC CAPS HAVE A + AND A - SIDE. THE CAP HAS A
STRIPE ON THE - SIDE.
R19 1k ohm
R20 22k ohm
R21 10k ohm
RFC2 22 microhenry RF choke
Y5 4 mhz crystal
Install all the above components after you double check the values.
Install a small wire (3") with the end striped to J3-3. This is your
Double check your soldering and component placement. (Really, you
Ok, power it up. You should measure 0 volts DC on pin 8 of U5.
Temporarily connect the wire from J3-3 to J3-1. This simulates a key
down condition. You should measure about 7.5 volts DC on pin 8 of U5.
Also, using your RF probe or oscilloscope, you should see RF on pin 4 or
U5. Disconnect the wire from J3-1 and power down the rig.
Here is a quick theory of the circuit you just built. The transistor Q3
controls the power to the transmitter. When you ground J3-3 you turn on
Q3 and allow power to flow to the transmitter circuit. In this case we
only have the transmit mixer installed.
R19 and D11 form a voltage regulator for U5 which I think isolates it's
power from the rest of the transmit chain. When power is applied to U5
it mixes it's own oscillator frequency (determined my Y5, RF32, C28 and
C29) with the VFO signal on pin 2. The resulting output is the sum and
difference products, plus lot's of other "unwanted" mixer products.
Please refer to the NE612 information in the technical information area
on the elmer101 web page. The unwanted signals will be filtered out
later. If you are looking at the output of U5 on a scope you will
probably see a very messy signal, not a nice pretty sine wave.
What we are doing in this part of the circuit is deriving the transmit
frequency from the VFO by mixing it with the equivilant of the IF
frequency. Remember that in this radio the VFO freq (3 Mhz) plus the IF
frequency (4 Mhz) equals the receiver frequency (7 Mhz). The same thing
is done to derrive the transmit frequency. The VFO (3 Mhz) plus the Y5
freq (4 Mhz) equals the transmit frequency. As our VFO frequency changes
and our receive freq. changes we want our transmit to also change along
with them. (Wouldn't be much of a radio if it didn't).
I'm sure this section will bring forth lots of questions on the
venerable NE612 mixer chip. Glen Leinweber has provided a ton of good
information on this little beauty which is on the elmer101 website at
If you would like to view the data sheet for this part it can be found
you need a pdf viewer to see it.