I describe a simple direct conversion receiver,
thinked for QRSS and DFCW communications, as companion of ARGO or
programs. I don't pretend to beat the performances of professional or
ham radio receiver, the scope is to suggest an easy way for beginner to
listen (of course I mean look at) the signals in this poorly populated
band. The receiver is useful also as portable receiver, fast turn-on
for a quick look an the band or as very low cost (and performances)
analyzer. I currently use this equipment for tests purposes (e.g.
frequency meter etc.).
The front end is described elsewhere prea136.htm
The scope is reduce as most as possible the image frequency. I have
tried a different front end, based on a low pass filter with frequency
cut-off of 180 kHz. In my QTH I had a lot of undesirables signals on
screen of the PC. The filter is tuned with a signal generator for a
of 300 Hz, centered on 136.650 kHz. The coupling capacitor between the
coils is also tuned for the desired bandwidth.
The mixer is the well known NE602. It's not a monster of performances
but it provides some gain, it's cheap and easy to find. The rest of
amplifier is common to other direct conversion receiver.
The local oscillator is the original part of my project. The frequency
is fixed at 135.500 kHz, so that the image frequency for QRSS portion
the band (137.600 - 137.800 kHz)drop about 4 kHz lower. This permitted
to attenuate the image using a simple front end filter. The oscillator
is controlled by a 27.100 MHz crystal, the frequency is divided by 200
and the square wave is injected into the mixer. Adjust the trimmer V1
the best signal to noise ratio (or the best sensitivity). CV1 is tuned
for a secure start of oscillations, tune CV2 for the exact frequency of
27.100 MHz. The transformer T1 is winded on an Amidon T50-5 toroidal
19 turn primary, 4 turn secondary.
There is not an audio amplifier, the output must be connected to the
audio board of the computer. If you prefer an audio feedback use an IC
amplifier (e.g. an LM386).
I usually set-up ARGO offset frequency to read the actual frequency
on the screen of the PC.
I suggest to use a low noise operational amplifier as U2.
The stability is good since the drifts of the
oscillator are divided by 200. The sensitivity is also good since the
and artificial noise on the band is high. The high selectivity filter
the front end reduce the possibility of overloading of the mixer. In my
station the receiver is connected to the antenna through the low pass
of the transmitter, helping in reducing overload from broadcasting
I think that a tuned loop antenna is the optimal companion for this