Solid State Shortwave Receivers For Beginners - The Listeners Guide

It will probably not be immediately apparent why it is necessary
to have a gain control for Tr2. There are two very good reasons for it.
Firstly, with Tr2 biased for maximum gain there is sufficient regeneration
at some frequencies to cause the set to oscillate, even with VC2
adjusted for minimum capacitance. When this occurs, VR1 can be
adjusted for slightly lower gain so that A.M. transmissions can be
properly resolved by the receiver.
Secondly, the high gain of Tr2 makes fine setting of the reaction
level rather difficult using VC2 alone. VR1 can be used as a fine
regeneration control, rather in the same way as the regenerative receiver
described in Chapter 2 had a fine regeneration control. Note however,
that there is a slight difference in the use of the fine reaction controls
on these two sets. On the regenerative receiver of Chapter 2, the fine
reaction control was kept turned well back in order to give efficient
detection. Here this control has no effect on the detection efficiency,
and it should be kept well advanced (where possible) in order to give
a high level of gain.
The use of a dual gate MOSFET produces a circuit which can
handle quite high signal levels without overloading. On the other hand,
the set is extremely sensitive, and it is not completely immune to
overloading. If an overload should occur, it will probably be possible
to cure this by backing off VR1 and advancing VC2 somewhat.
Detector and A.F. Stages
The output from Tr2 is fed to detector diode D1 via D.C. blocking
capacitor C4. C5, R5 and C6 form a very effective R.F. filter.
The audio stage is a high gain common emitter amplifier using
Tr3. This has R7 as its collector load resistor and R6 as its base bias
resistor. C7 couples the output signal to SK3, which can be used to
feed any type of medium to high impedance headphones.
S1 is the on/off switch and C7 is an A.F. supply decoupling
capacitor.
Some readers may be wondering why it is necessary to use a
separate diode detector, rather than simply use Tr2 as a regenerative
detector. It would be quite possible to do this, and such an arrangement
would work very well, but using a separate detector does have a
couple of advantages. Furthermore, using a separate detector does not
greatly increase the cost or complexity of the receiver.
The bigger of the two advantages is that MOSFETs generate
rather a lot of noise at A.F. If Tr2 were to be used as a regenerative
detector, this noise would be coupled to the audio stages of the set.
With the set up used here, the diode detector blocks virtually all the
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