1525	2008-03-28 06:17:37	ea3ghs	a minimalist software-defined radio for 15m
Message	Date	From	Subject
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: http://ea3ghs.googlepages.com/TRIPPI15-DSB-DetectorForSDR.doc 73/dx from Lleida, Catalonia eduardo 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 21114 kHz. This circuit has been designed as simple as possible. Let's see how works: oscillator 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. 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 (+1dBm/220ohm). mixer 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. IF filter 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. RF filter 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=QuXL=702pi21M1.5u=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 simplicity. digital IF 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. test 1 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. test 2 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". operation 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 simplification. 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 translati
1540	2008-03-31 09:18:02	ea3ghs	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. Wes, thanks for your words. 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, 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 > simplification. I have been thinking why I have a separated oscillator and mixer... 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: http://ea3ghs.googlepages.com/trippi15-quad28.pdf (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 http://kd1jv.qrpradio.com/PSKTX/SIMPLEPSKTX.HTM 73 Eduardo EA3GHS