Integrating Software Defined Radio (SDR) and Standard Transceiver
Chavdar Levkov LZ1AQ
SDR it should be integrated into a standard environment – transceiver (TRX), power amplifier (PA) and the antennas. In this way the SDR will find much wider practical acceptance in ham radio community.
The schematics of one possible solution is shown here: SDR_TRX_splitter.pdf
The simplest solution is used: the antenna input/output of
the transceiver (TRX) and the antenna input of the direct conversion receiver
(DC RX) are tied together in rx mode. In
TX mode the DC RX input is grounded. A
small fast relay (FRT5 telecom relay; switch time < 2 ms) is controlled by a
signal which is standard output control signal of most commercial transceivers
for PA control. Return to RX mode is
delayed with 10 ms for safety reasons. The connection to T-connector of
Special care was taken for the safety and noise immunity of the total system. The IC 40106 consists of six Schmidt trigger input invertors. The splitter box is powered from the same TRX 12V power supply. Ground loops are avoided with common mode balun transformers in control and power cables of the box. Additionally the DC RX input is insulated with a broadband transformer. There is an input attenuator 3.7 dB which smoothes to some extent the influence of the input impedances of both radios adding strong resistive component. This is important when one of the receivers is on another band and its input filters add reactive component which is not good for the other radio. (I have measured the DC RX input impedance in receiving bandwidth - around 70 ohms mostly resistive). But more important is it’s “fuse” function. If for some reason the relay is not switched to ground in TX mode, the total transmitter power is applied to the attenuator and the 0.125 w resistors simply blow away thus saving the DC RX input circuit. (It has happened 2 times since I forgot to plug the control cable!). Additionally the broadband transformer went into saturation and limits the voltage that can enter the DC RX. This system is foolproof.
Using this small box (Fig.1)
there is no need to make any changes in existing station – just a coaxial
T-connector to plug there
Audio switch box
The audio switch box (Fig.2) permits to listen to main transceiver, to the SDR output or both (split into left and right earphones). I have headphone audio amplifier in DC RX so there is another switch which permits to listen to pure DC RX audio without any DSP processing. There is also “RX mute” switch which disconnects the DC RX input from TRX antenna in the case where we do not want to load the main transceiver input.
Using these boxes I do not have any problems with RF pickup or other noise. Moreover I am using only open wire feeders for all my antennas (this is my other hobby) with home made automatic antenna tuner so the level of the surrounding RF field was not so low. The T-connector is inserted between TRX and PA so a maximum of 100 V RF voltage is applied there.
On the air tests
The SDR should have to compete with the mainstream equipment and to prove its virtues in real environment so I decided to make a heavy test - the CQ WW 160m CW DX Contest 2008, QTH= Europe (some people call it EU Zoo contest.)
TRX: Icom 756pro, PA: 350 w, home made, Antenna: full sized offset center fed dipole for 160m at 15 m height, Automatic antenna tuner, home made.
Home made DC RX is used with double balanced Tayloe mixer with CD4052 analogue multiplexer. The oscillator is 28MHz VFO divided by 16 and D-trigger IQ splitter. The audio pre-amplifier is TS462 with 4nV/Hz1/2 noise density.
I have measured this radio on 14 MHz band (with Audigy SE 24 bit card) with 16MHz low-pass input filter and obtained:
MDS -129 dBm @ 500Hz BW, IP2 = 62 dBm, IP3 >= 15 dBm, Sound card saturation @ -26 dBm. (59+50dB)
It is expected this radio to have similar or better performance on 1.8 MHz band. It must be noted that the oscillator phase noise on this band will be very low. See the schematics of the radio SDR_sch.pdf. Additionally I added a band pass input filter (1.8MHz; 700 KHz bandwidth).
A description of this radio and how it is measured can be found on my home page www.lz1aq.signacor.com but the articles are in Bulgarian language. I plan to translate them into English if the free time permits.
Notebook Dell 2 x 1.8GHz AMD core. 2 GB RAM. Sound card is SoundBlaster Live! 24 bit USB external. The computer has internal Sigmatel High definition audio sound which appears to be useless for SDR since it was not possible to reach any I/Q balance by unknown reasons.
Windows XP. Rockie 3.2 ( http://www.dxatlas.com/ ) and Winrad 1.3.( http://www.weaksignals.com/ ). ASIO drivers are not used. The sampling rate was 96 KHz @ 16 bit resolution since this USB card uses USB1.1 and the bandwidth is insufficient for 24bits/96KHz resolution. Running Rockie or Winrad occupied between 15-20% of the CPU resources at 96 KHz s/rate.
QTH is KN12KD near
I tuned the VFO at 1854 KHz and entered this value in the software. I had now 96 KHz bandwidth from 1806 to 1902 KHz. (at the band edges some digital aliasing occurs. I do not know the reason for that but these few KHZ at the band edges were not very usable). This bandwidth was suitable since most of the activity took place from 1810 to 1890 KHz. I used the main TRX for basic work and simultaneously watched at the waterfall display for weak stations or pile-ups. If I found a perspective spectral line I clicked on it and listened to the station. If I needed this station I tuned the TRX manually on this frequency (as in very old days). I took the exact frequency from the SDR display. Then I switched the audio box to TRX, listened there and made a call. This was not very convenient method but the idea was to test the equipment and SDR together, not to win the trophy.
Most of the time the band was totally occupied. In 80 KHz bandwidth I counted almost 240 spectral lines on a frozen waterfall display. That means that on every 350 Hz and less there was a transmitting station!! I had continuously almost 30 to 40 booming stations with KW power (distance up to 300 Km) with signals sometimes above 59+30dB. In 20 km diameter I did not have a powerful station.
Comments and Suggestions
Remark: The results from selectivity tests are valid only for CW mode.
A simple hardware is developed in order to use simultaneously SDR and conventional radios in fast and convenient way without any modification of the existing equipment. Using two senses: eyes and ears give certain advantages especially in the uttermost conditions. Extremely narrow band filters of the SDR can be used easily. The practical very close-in selectivity and dynamic range in CW mode is better compared to a hi-end transceiver. The SDR hardware and the sound card used in this experiment were very cheap and common. Minor additions to existing software which benefit simultaneous usage of SDR and standard transceiver are proposed.
73, Chavdar LZ1AQ