Ramsey Electronics AR2WT, Manual

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AR2
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9
have to be set at 118.300 MHz + 10.700 MHz or 129.000MHz. It would then
be sent to the mixer and on the output there would be 129.000MHz (LO),
118.3MHz(Fr), 247.3MHz(Sum) and 10.700MHz (IF). (See Figure 1)
After the mixer we use a narrow-band filter (E) to reject everything but the
10.7MHz IF signal output from the mixer. This works well because 10.7MHz is
far from the next highest frequency, 118.3MHz, so it’s easy to reject
everything but the signal of interest. However, there is another signal you can
receive that is 10.7MHz
above the LO frequency. This is called the image
frequency, and can be a real hassle in radio designs. In this case we could
receive not only 118.3MHz but also 139.7MHz (2x IF + Fr). In this case this
“image” is only 700kHz outside the band of interest and is almost impossible
to filter out.
The AR2 reduces the reception of image
frequencies by using good band-pass filters
on the input to reject the image frequencies
before they make it to the mixer (
A, C ).
Figure 3 to the left shows what the RF input
to the mixer looks like. Notice the Fi is
greatly reduced by the input band pass
filter, but not completely eliminated. The
band pass filter certainly helps and gets rid
of all but the strongest image signals
outside the band.
Now that we have our filtered 10.7MHz IF at the output of the IF filter we can
do some simple amplification on it (
F ), then send it to be demodulated into
audio.
The IF signal is then sent to the AM detector part (
G ), where it is mixed down
to yet another IF frequency of 450kHz by combining 10.7MHz with 10.25MHz.
450kHz is used due to the large array of components available at 450kHz, and
it is also the frequency at which our particular AM demodulator works best.
The signal is then amplified greatly to a consistent level using “slow” AGC
within the detector part. The detector is able to receive quality AM signals in a
90dB range, since the AGC can amplify the signal up 90dB. The AGC is
important in that all received signals within its range will be of the same audio
amplitude. Otherwise weak signals would be very quiet, and nearby ones
would be very loud. The reason it is “slow” is to allow changes in level due to
audio to be left alone, but longer-term signal level changes due to distant
signals and close signals to be compensated for.
The 450kHz IF, now that it is a consistent level, is demodulated using a full-
wave rectifier and filter to remove the 450kHz, and leave the AM level behind
as shown.
IF LO Fr Fi Sum
F
Figure 3