jhon1 said:
thanks Jan
do you have any simple circuit that can perform that.
can I add selective amplifier for a definite frquency to any superhet.
receiver (may be commercial radio), to isolate the desired high
frequency carrier without mixing?!
thanks
You need to undertsand what is going on a bit. On a normal radio, you are
not interested at all in the 'carrier', except as a 'carrier', of the
modulated data (hence the name). It is harder to amplify high frequency
signals, than those at lower frequencies (limitations of the amplifiers
themselves), and to really accurately tune them, so what was done, was to
start by amplifying the signal a little, then immediately mix to produce a
lower working frequency (the I.F. - intermediate frequency), and
amplify/tune this. This is the 'superhet'. This also has the big advantage
that for a 'tuneable' radio (that can be used to receive different
adjacent bands), the RF tuning can be quite wide band, and by changing the
mixing frequency, the _same_ IF can be produced for different incoming
frequencies. The IF tuning can be accurately set, and quite narrow band.
to give good selectivity.
Now, if you want to look at the incoming carrier itself, there is
absolutely no point at all, in getting involved in a superhet. After the
first stage, the signal will already be at a lower frequency, and will not
give you access to the 'carrier'. Instead do a search for 'T.R.F.' (tuned
radio frequency). This is a much simpler radio design, which relies on
just a tuned RF stage, and amplification at the RF frequency. Normally, a
envelope detector is then used to get the amplitude modulation on the
carrier, but without this, such a radio, will give direct access to the
incoming 'carrier'. Historically, getting good gain/selectivity with such
designs, was quite hard, but a number of basic IC's exist to do this. The
ZN414, and it's latter descendants, was probably the most famous, and
though this is no longer available, the MK484 is an almost direct
replacement.
Best Wishes
