A 30 Mtr Direct Conversion Receiver.
The receiver described here is based on a design built and used by Hans
Summers (G0UPL) and Paolo (I1DFS) for the reception of QRSS signals in
the 30 Mtr band. The receiver can be easily duplicated from the details
which follow or simply used as a source of ideas for a design of your
own. An example of the original receiver upon which this design is based
appears on Hans (G0UPL) QRSS web pages here.
(G0UPL) 30m QRSS Receiver.
It is a simple Direct Conversion design and while it does not yield
state of the art performance it does provide a useful
introductory receiver for qrss reception. In this version a number of
enhancements have been incorporated to improve the basic performance.
These enhancements include the following...
A modified SA602 circuit
to improve the short term frequency stability of the crystal oscillator.
2) A crystal "oven" to ensure
excellent long term frequency stability of the crystal oscillator.
separate signal frequency crystal bandpass filter to improve front-end
selectivity and remove A.M. broadcast "breakthrough".
4) A separate audio frequency bandpass filter to further improve
selectivity and reduce interference (QRM) from in-band signals.
receiver is comprised of three modules, this was done in order to
provide a quick method of
replacing or upgrading sections of the receiver without having to fully
rebuild the entire receiver each time the circuit was modified. The three
modules in the system are...
Conversion Receiver Module.
The first module to be described is the Receiver. This is a direct
conversion design built around the popular SA602 device followed by an
NE5534 low noise Op-Amp. The NE5534 is not one of the lowest noise
devices available but it is very cost effective in this application.
Because of the relatively high level of atmospheric and man made noise
(QRM) in the 30 Mtr band a lower noise Op-Amp would not give much
over the 5534 used unless you happen to be using a very small or
inefficient receiving antenna.
An image/link to the full schematic of the receiver module
The circuit does require some explanation and astute readers will
notice that the configuration of the crystal oscillator is somewhat
unusual. An additional transistor has been added to the SA602
oscillator circuit, the BC238B transistor in
conjunction with the transistor/resistor already present in the SA602
form a Darlington pair, this configuration offers a number of
advantages. Firstly, the lower O/P impedance of the Darlington
configuration makes it possible to increase the value of the capacitors
in the feedback network such that they will "swamp" any stray or
parasitic capacitance more effectively. Secondly the BC238B
transistor forms a very effective buffer stage permitting the
of a frequency counter (or QRSS TX etc) to the oscillator stage without
"pulling" the frequency of the crystal oscillator. The idea for this
modification arose after previous success with a crystal oscillator
design given to me by Peter (DL6NL) which is currently used in my 30
Mtr QRSS TX. Looking at this oscillator circuit I realized it may be
possible to incorporate elements of the circuit in this receiver design
using some of the components which are internal to the SA602.
An image/link to
the schematic of the crystal oscillator from DL6NL appears
below for reference.
The circuit for the DC-RX has been constructed "ugly" style over a
ground plane. Both
the SA602 and the NE5534 are mounted in sockets just in case of
"accidents" and to permit easy replacement in case of failure. An image
of the DC-RX board appears below (left hand side image) and an internal
view of the complete DC-RX module appears below on the right. The
yellow foam (left hand image) is thermal insulation around the crystal
to improve the operation of the crystal oven while the pink coloured
foam shown in the right hand image is an additional layer of thermal
same compartment as the DC-RX is the crystal oven temperature control
board, this part of the circuit does not require a ground plane
therefore it has been constructed using strip board. The schematic for
the crystal oven temperature controller is shown below (left hand side
image) and an image of the oven controller
below on the right.
Frequency Crystal Bandpass Filter/Amplifier Module.
The direct conversion
receiver module can be used on its own but I encountered frequent
problems with A.M. broadcast stations "breaking through" from the lower
frequency S.W. bands. Part of the problem is with the SA602 which is
its poor large signal handling. Because of these "breakthrough"
it was decided that a very narrow bandpass filter would be constructed
using a quartz crystal as the bandpass element. This proved to be
highly successful in removing the A.M. breakthrough. The design of this
unit became a highly rewarding project in its own right and is the
focus of a separate web page which can be found here.
The schematic for the crystal
bandpass filter/amplifier board appears below (left) with images of
the assembled and boxed unit appearing below center and right.
The signal frequency bandpass filter is constructed over a ground
plane and housed in its own enclosure measuring 75 x 75 x 50 mm. The separate enclosure ensures
freedom from unwanted signal "leakage" around the filter. The crystal
bandpass filter module also includes a small R.F. amplifier of modest
gain to compensate for the small loss in the crystal filter. The module
is fitted with phono sockets for Antenna I/P and R.F. O/P. The R.F. O/P
of the filter/amplifier module is connected to the antenna I/P of the
QRSS DC receiver. The current drawn by this module is about 35 mA @ 12.5 Volts which includes the L.E.D.'s.
The direct conversion receiver and filter/amplifier modules together
provide a very effective QRSS receiver, a further improvement can be
made with the addition of the A.F. Buffer Amplifier/A.F. Filter
A.F. Buffer Amplifier/A.F. Filter Module.
25/10/06) The unit about to be described was added to the DC-RX in an
attempt to further improve the performance though I have to admit the
improvement was hardly noticeable except under conditions of heavy QRM
when signals close to the QRSS sub band would confuse or desensitize
Argo. Since this unit was built I have had some time to evaluate it
more fully and would suggest that using op-amps with a lower noise
figure would give improved performance. My feeling is that the
A.F. filter is a worth while optional extra for those times when QRM is
The A.F. Buffer Amplifier/A.F. Filter
module was added to the receiver to ensure that the software A.G.C. in
was not "confused" by unwanted signals which appear in the audio
passband of the DC-RX. These signals arise from other users of the 30
Mtr band (CW, RTTY etc) which are within the bandpass of both the
front end (RF) filter and the audio stages of the receiver. These
unwanted signals can cause "desensitization" of Argo
such that wanted
signals appear weaker than they actually are. Initially a low pass
audio filter was considered but it was soon realized that with little
or no additional complexity an audio bandpass filter could be used
which would offer better performance. The low frequency roll-off of the
bandpass A.F. filter helps to reduce any possible power line noise from
reaching the P.C. sound-card and also offers attenuation of unwanted
signals below the 100 Hz window used for QRSS.
It was also felt that the original DC RX design was perhaps lacking in
AF gain. To much gain risks overloading the sound-card on strong
signals but my feeling was that a little more audio gain could be
tolerated. I came to this conclusion for two reasons, the atmospheric
noise was barely audible which made me wonder if I was achieving
optimum sensitivity and secondly the additional gain from another low
noise amplifier stage would lift the wanted signal level far above the
noise level which may result from the Op-Amp's used in the active
bandpass filter. A full
schematic for the A.F. Buffer Amplifier/A.F. Filter is shown below.
The center frequency of the A.F. Filter
was chosen to match the center of the QRSS sub-band as it appears in
. This frequency may not be the same on your DC RX and may require
adjustment to suit the frequency of the crystal you use. For use with
frequencies between 1 and 2 kHz should be suitable. Fine tuning of the
A.F. filter can be performed by adjustment of the two resistors (Rx2
and Rx3 in the circuit diagram), adjusting these resistors
individually can also permit "stagger" tuning of the two cascaded
filter stages. This can be useful if a wider B.W. is required. In my
version of the filter I found a "Q" of 8 to be more than enough with
higher "Q" values causing excessive "ringing" of the filter.
Note: With the component values shown in the schematic the filter may
appear to "ring" a little with CW or RTTY signals but this causes no
problems with QRSS signals due to the slow nature of the QRSS modes.
If you wish to build this filter and want to "trim" to a different
center frequency or wish to experiment with different values of "Q" or
filter gain then the formulas below may be of interest.
The A.F. Buffer Amplifier/A.F. Filter
module is constructed on strip board and mounted in a small metal
enclosure measuring 130 x 70 x 35 mm which provides screening. The I/P
and O/P connections are via phono sockets. This module draws about 22
mA @ 12.5 Volts which includes the "Power-on" L.E.D. Two images of the
completed module appear below.
With all three modules interconnected the current drawn is around 140
to 150 mA @ 12.5 Volts. Below is an image of the complete
interconnected QRSS receiver system comprising all three of the modules
Theory states that a simple direct
conversion receiver will suffer a -3dB penalty due to the unwanted
audio "image" but in practice this penalty is far from obvious. Indeed
the receiver currently outperforms any other receiver here at M0AYF for
QRSS reception. The receivers frequency stability (a very important
factor in the reception of QRSS signals) has exceeded
expectations. I plotted the change of frequency with respect to
time starting with a cold crystal oven at switch-on. As a reference I
used a signal phase locked to the 60 kHz MSF frequency standard located
here in the UK. The graph appears below.
The graph is not particularly accurate
since very few points have been plotted but it shows that the frequency
drifts only a few Hz in the first few minutes after switch-on from a
cold start. After around 40 minutes the crystal oven temperature
stabilizes and the frequency remains substantially constant thereafter.
The maximum "drift" in frequency from switch-on is around 8 Hz in a
positive direction, after about 40 minutes the "drift" is below
measurable limits here at M0AYF.
While I have no way to measure sensitivity or dynamic range of the
receiver I can confirm that in use I can hear atmospheric noise which
suggests the receiver is sensitive enough. I can also confirm that so
far no signal overload problems have been encountered and all traces of
A.M. breakthrough have been removed thanks to the crystal bandpass
filter which hides many of the problems caused due to limitations in
the SA602. Below are a two screen captures from Argo
of several QRSS signals received using the DC-RX system described above.
If you find this receiver design
interesting then you may also be interested in the recently
completed advanced 30 Mtr QRSS RX designed by Andy (G4OEP) which
has many interesting features including the use of a crystal lattice
filter as a signal frequency bandpass filter and a "H-Mode" mixer. A
link to this excellent design appears below.
Advanced 30 Mtr QRSS receiver (lower third of web-page) designed by Andy (G4OEP)
that’s about it, thank you for reading this and please
send any questions, comments or "heckles" etc to the e-mail address