EMRFD Message Archive 1525
Message Date From Subject 1525 2008-03-28 06:17:37 ea3ghs a minimalist software-defined radio for 15m Hello,
This is my first mail in this group.
Thanks every body. I enjoy a lot reading it.
And thanks to W7ZOI, KK7B,... I read and studied
the book. It's full of nice ideas.
Well, this is my small contribution to the EMRFD community:
a minimalist software-defined radio for 15m band.
Below there is a description of it. The schematics can be
found in the files area or in this link:
73/dx from Lleida, Catalonia
ea3ghs at qrp.cat
My interest in shortwave communications has been centered in digital
modes. Here we discuss a small crystal controlled double-sideband
receiver with digital intermediate frequency, covering from 21066 to
This circuit has been designed as simple as possible. Let's see how
It is a Colppits oscillator running at 7030 kHz. This xtal
is very easy to find in any QRP club. The output of oscillator is send
to a multiplier.
A transistor working in on-off mode generates a current
with a lot of harmonics. With a high-Q LC network we extract the third
harmonic: 21090 kHz. We connect the multiplier to the 220 ohm mixer
input port with a capacitive transformer. I measured 1'5Vpp here
It is the receiver's core. The signal from de local oscillator
VLO is injected in the source of the FET. The signal from the antenna
VRF is injected in the gate. The transistor generates a drenator
current proportional to the difference squared, ID=k*(VLO-VRF)^2
appearing harmonics currents in FRF+FLO, 2FRF, 2FLO and, the most
important for us, FRF-FLO.
The PSK31 traffic is centered in 21070 kHz. We extract de
difference signal from the drenator current with a LC filter centered
in the lower sideband 21090-21070=20 kHz. This signal is send to the
computer (or to an audio amplifier). The bandwidth here is not very
well defined. The sound card input impedance is not lower than 1 kohm.
Attention, you will listen signals and noise simultaneously at
21090+20=21110 kHz from the opposite sideband.
The preselector is a simple tuned circuit. At 21 MHz, I
measured a Qu=70 in my 1.5uH inductor, ie an equivalent parallel
resistance of RP=Qu*XL=70*2*pi*21M*1.5u=14 kohm. With a capacitive
transformer, we adapt the 50ohm of the antenna to 14kohm of the LC
filter. With SPICE, we estimated a medium-wave signals rejection of 70
dB. In on-air night tests I have not observed out-of-band signals
(autorectification).This validate the balance between performance and
Use the microphone input of the computer (max input level:
100mVpp). Digital signal processing can be made with a nice software
from M0KGK. This program has a direct output to RTTY/PSK/... decoders
like WinPSK and MULTIPSK. Try Canadian Rocky, too.
Connect the antenna to the receiver, read the noise floor
level. Disconnect the antenna and read the noise level again.
If the level is the same (here is the same) the total receiver gain is
not sufficient. The noise that you view in the screen is internal
receiver noise, not atmospheric noise.
What antenna level I need to obtain 100 mVpp in the output (+0
dBFS/10kohm)?. in my case, I need 110mVpp (50ohm,-15dBm). The receiver
has a power gain of -23dB. Fortunately, the sound card has a high
input range and we can detect signals 100dB smaller: 1.5uV. Then, we
can not hear signals under "S2".
Few hours after I assembled this receiver, I was able to
receive almost all central European countries. Unfortunately, we are
in minimum solar cycle and the propagation is unstable. QSB can be
viewed in PSK31 spectrograms. Few operators make fast contacts in
PSK63 between propagation minimums (QSB). Some special stations were
listened: PC650DAM from Holland, II2SFI "Spirit Flame of Italy" and
TM7EO from France.
1530 2008-03-28 11:52:14 Wes Hayward Re: a minimalist software-defined radio for 15m Hello Eduardo and group,
Many thanks for the interesting, yet very simple circuit. It is
always exciting to hear those first signals from a new receiver, on
in this case, to see the PSK31 signals on screen. It's wonderful
to see such results from a very simple circuit.
One thought: If you use a third overtone oscillator circuit, you
can cause the 7030 crystal to operate on the 3rd overtone at 21090
and would not need a multiplier. This might lead to further
While on the subject of straight forward radios, I want to point the
group to the web site of F6FEO. http://f6feo.homebuilder.free.fr/
I got an email from Michel this morning and he told me that he now
has details on the web for several versions of his transceiver,
the "Santerre." There is some really interesting stuff on his
site. There is a lot in the schematics alone, making it very
useful, even for those of us who don't read French. But Michel has
also added a "translation" tab to each part, which brings up a Google
1540 2008-03-31 09:18:02 ea3ghs Re: a minimalist software-defined radio for 15m > Hello Eduardo and group,Wes, thanks for your words.
> Many thanks for the interesting, yet very simple circuit. It is
> always exciting to hear those first signals from a new receiver, on
> in this case, to see the PSK31 signals on screen. It's wonderful
> to see such results from a very simple circuit.
My near friends do not understand me and the philosophy under this
kind of receivers, hi hi.
> One thought: If you use a third overtone oscillator circuit, youI have been thinking why I have a separated oscillator and mixer...
> can cause the 7030 crystal to operate on the 3rd overtone at 21090
> and would not need a multiplier. This might lead to further
i) for didactical reasons. Itâs easier to understand
ii) To generate high level second, fourth, fifth,... order harmonics
without change the schema
There is just an example:
xtal mult center purpose
7030 2 14060 20m rtty,psk,...
7030 4 28120 10m rtty, psk,... (1)
5068 2 10136 30m 10140 QRSS receiver
10000 5 50000 6m cw/beacon receiver
10000 7 70000 4m cw/beacon receiver
9041 3 27125 channel fourteen (citizen band ;-)
9041 2 18082 17m receiver
5000- 5 24980 12m receiver (2)
(1) View QUAD28 receiver in:
(2) Use a âsuperâ VXO oscillator
All can be made changing only time constants in the circuit.
To recalculate the capacitive-transformer you can use this tool:
http://usuarios.lycos.es/ea3ghs/ctrafo.php (sorry, in spanish)
Another companion PSK31 transmitter could be this