"kamalakar pasupuleti" wrote in message
om...

The purpose of dual conversion is for better
image rejection , improved selectivity and sensitivity .The first IF
is around
1.5 kc and the second IF usually low around 50 kc for better
amplification. Most of the good dual conversion receivers employ
atleast 2 stages of IF amplification at both levels and also provide
sensitivity or RF gain control.

How does dual conversion improve selectivity unless there is additional
filtering at the first IF? How does dual conversion improve sensitivity?
Sensitivity on the shortwave bands is highly overrated. The natural noise
far exceeds the noise figure of the receiver, to frequencies well over 15
MHz. Improved selectivity can help reduce noise _outside_ the bandwidth of
the signal, and low-noise circuitry can help even more. Many inexpensive,
synthesized receivers generate a lot of noise. The signal must be 3 dB
stronger than the noise to be audible. A double-conversion receiver, with an
additional stage of IF amplification, has more gain stages, each of which
adds its own noise. When I say improved selectivity, I mean the kind you can
only achieve with IF filtering. Antenna trimmers, tuners and unfiltered IF
stages won't do it.

Double conversion is used to reduce the possibility of image responses. If
the receiver has additional filtering at the high IF, fine. That can improve
performance (see my previous post), but is not the primary reason for the
use of two IFs. What people call "sensitivity" is very often overall gain. A
signal may be audible above the noise, but the receiver may have
insufficient overall gain to make the signal loud enough to hear well.
Adding a preamplifier at the antenna is definitely not the way to go unless
you are using a very lossy antenna and your listening environment is
electrically quiet. The preamplifier is going to amplify electrical noise
exactly as much as it amplifies the signal, resulting in no improvement in
signal-to-noise ratio. In fact, the internal noise generated by the
preamplifier may make the situation worse. If you are using a short antenna,
extra gain from a preamplifier may help, but only if the antenna is not
picking up much noise. You would do better to connect the receiver to a good
audio amplifier.

I hope this clears up some long-lived misconceptions. A trip through the
Mixers and Receivers etc chapters in a recent ARRL Handbook will give deeper
explanations, and is highly recommended. Most public libraries carry recent
editions.

"PM"