As promised here is the next section of the Elmer 101 project.
This is a
short one and fairly simple so we should get through it quickly.
I have updated the elmer101 web site at http://www.qsl.net/kf4trd
with the part 4 information and mixer discussion. The web site is
getting a little big so I need to do some work on it to try to organize
Anyway, where we left off we had a hodgepodge of frequencies coming out
of the NE612 mixer chip. Only one of the frequencies is the one we want
to transmit, so we need to filter it out and stop the other unwanted
signals from reaching our final amplifier. Dave uses a bandpass filter
implemented with 10.7 Mhz IF transformers and some caps.
Gather the following components.
C33 .01 microfarad
T2 IF transformer
T3 IF transformer
Install these components in the appropriate places on the circuit board.
Be sure to solder the "tabs" of the transformers to the board as they
act as a shield and need to be grounded.
Double check your component values and parts placement (Really, you
should do this ;-)
You will need your RF probe, or an oscilloscope for the adjustment of
this section. Connect your RF probe to pad of the base of Q4 (not
installed yet). Turn on the power to the board and connect your
temporary wire key between J3-1 and J3-3. You should see a reading on
your RF probe. First adjust T3 for maximum reading on the RF probe.
After you peak T3, adjust T2 for maximum reading. Go back and forth a
few times to get the highest reading possible.
If you have an oscilloscope you should see a nice clean sine wave at
about 7 MHz. Mine was 3 volts peak to peak at 7.111.42 MHz.
Here is what is happening in the circuit. The outputs of the NE612 (pins
4 and 5) are the differential outputs of the mixer. Remember the mixer
discussion? You have the VFO signal at about 3 MHz and the internal BFO
signal at 4 MHz being mixed at U5. The two main frequency components on
the output are 7 MHz (desired signal) and 1 MHz (difference mixer
component). Look at the mixer output frequency components at
http://www.qsl.net/kf4trd/ne602.html . Notice that there are frequency
components from the mixer as high as 25 MHz. We have to get rid of all
of them except the 7 MHz one.
T2 and T3 are configured as a band pass filter, tuned with C30 and C32
for 7 MHz. The fine tuning is done by turning the slugs in the
transformers. These IF transformers are designed to resonate at 10.7 MHz
and contain internal capacitors, but the resonant frequency is lowered
to 7 MHz by paralleling C30 and C32.
I'd like to get into a discussion regarding the characteristics of the
IF transformers and exactly how they are used, relating to impedances
and the use of C31.
Don't be shy and ASK QUESTIONS!
Also, when you finish up and adjust T2 and T3, you can prepare the
primary of T4 as per the instructions on page 13. Don't install it in
the board yet and don't install the secondary winding. We will need this
for the next step.