Last change 18.11.2010


Integrating Software Defined Radio (SDR) and a Standard Transceiver. Revision 2

Chavdar Levkov LZ1AQ   Sofia, Bulgaria


In a previous article with the same name ( I mentioned that it is necessary to integrate the SDR with a standard transceiver (TRX) in order to benefit from both devices. A simple hardware was developed in order to use simultaneously SDR and conventional radios in convenient way without any modification of the existing equipment.

Here the idea and technical solutions presented in the first article are developed further on.

There are several major issues:

  1. TRX and SDR must use the same antenna
  2. Any TRX modifications are not allowed
  3. Switching the earphones to both radios
  4. Mutual protection of TRX and SDR in transmit mode (tx-mode)
  5. Side tone monitoring
  6. Avoiding ground loops
  7. Reducing the leakage of the SDR heterodyne oscillator into common antenna
  8. Synchronizing the receiving frequencies of both radios


The schematic diagram is given on Fig.1 and also here  SDR_TRX_sch.pdf  .





RF path:  the antenna terminal of the transceiver (TRX) and the antenna input of the SDR receiver are tied together by antenna switch in rx-mode.  In tx-mode the SDR input is grounded. No additional relay contact junctions are inserted in TRX-PA-ANT path.

 The antenna switch is controlled by a –SEND signal which is standard output control signal of most commercial transceivers for PA control.  Usually it is a relay contact which connects to ground in tx-mode. The main difference compared to previous v. 1 is that the antenna relay now diconnects the SDR input when there is no power to antenna box. A diode D1 can be inserted in PA control line to decouple the control circuits. Return to rx-mode is delayed with 10 ms for safety reasons. There is also manual control (Sw4) of the antenna relay and SDR can be disconnected from the antenna permanently. The control Led1 is on when the SDR is connected to the TRX antenna.

Audio path:  The outputs from the sound card and the transceiver are switched to the headphones with two relays. These relays are controlled both manually (Sw1,2,3) and by the – SEND signal. In tx-mode the headphones are always connected to the TRX and the side tone signal is from the transceiver thus avoiding the confusion of time delayed signal from SDR. The control Led2 , 3 are indicating which audio path is active.



Power supply: The device is powered from the same TRX 12V power supply.

The same antenna: The connection of SDR to the antenna is through coaxial T-connector. The connection is made very short to keep to minimum the stray capacitance from the connector to the relay contact. This capacitance plays role in tx-mode when the SDR input is disconnected and might degrade the transmitting SWR.

There is an input attenuator 3.7 dB which smoothes to some extent the mutual 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.

The receiver sensitivity is reduced due to parallel connection of the two receiver inputs. Theoretically the input signal level will be reduced with 3.5 dB if both receivers and antenna have equal 50 ohm impedances. The attenuator reduces additionally 3.7 dB and the signal at the DC RX input will be 7.2 dB less. Another source of losses is the increased SWR=2 of the feeder in rx-mode. Calculations for 50 m RG213 antenna feeder show additional losses of 0.06 dB for 1.8 MHz rising to 0.25 dB for 14 MHz compared to SWR=1 case.  On the low bands (1.8-7 MHz) this degradation of sensitivity is not significant. The atmospheric and man made noise are much higher than the receiver noise level even in rural QTH. For higher bands this solution might be not acceptable and the input attenuator should be made with lower attenuation or omitted at all. (but then the “fuse” function will be lost).

            Additional preamplifier: I am using a simple preamplifier which is described in “Preamplifier and Band pass filter for SDR from  1.8  to 18 MHz” . The loading of the antenna is lower. Simulations with CAD show attenuations from 0.5 dB at 18 MHz increasing to 5 dB at 1.8 MHz. The attenuator might be simplified to a single resistor as is shown on the circuit diagram. Also the heterodyne oscillator leakage is reduced at least with 20 dB compared to direct SDR mixer connection. The losses in the attenuator are compensated by the increased input gain and practically no degradation of sensitivity of both receivers can be reached.

Mutual Protection of the TRX and SDR in tx-mode:  If for some reason the antenna relay is not switched to ground in tx-mode, the total transceiver power (not the PA)  is applied to the attenuator and the 0.125 mW resistors simply blow away thus saving the DC RX input circuit and TRX.  These resistors have a “fuse” function. (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.

Noise immunity: Special care has been taken for noise immunity of the total system. HF and LF ground loops are avoided with common mode balun transformers in control and power cables. Note that the common terminal of the headphones is also swiched by the relay. The same function has the broadband transformer T1 to decouple the SDR ground which is connected to PC ground through the sound card. Schmidt trigger input invertors (40106) are used in control circuit.



The antenna relay is a fast communication relay FRT5 (2 ms, 12V,12mA,  DPDT, non latching type).  It is cheap one but any similar relay will do the job. (These types of relays are produced by several manufacturers.)   Only 1 pole is used. The same relays are used in audio path. Only one 3-pole relay can be used there.

The wide band transformer T1 is wound on ferrite toroidal core with mu=1000, 10x6x4mm, 5 turns with two twisted wires. The balun chokes are wounded on the same core with 10-15 turns of twisted pair.  RCA audio connectors are used in the audio path.



Physically the device is separated in two boxes – antenna switch and switch box.  In previous design the relay and the control circuits were in the same box but it is better to separate the antenna relay alone. In antenna switch box there is only the relay and wideband transformer. A small screened box made from PCB material with copper foil is used. The audio switch box is plastic and contains all other parts.  The box is glued with plaster pads (“UHU tac patafix” sold here in stationer’s shops) at the right side of the TRX as seen on Fig.1.


 Fig. 1


Synchronizing the receiving frequencies of both radios

Usually I am listening and looking on the SDR. When I find interesting station on the band I manually change the TRX frequency of my Icom 756pro to the frequency displayed on SDR but this way is very old fashioned and inconvenient.  Most of the modern TRX have serial computer control (CAT) and it is very convenient to have some automatic way to pass the receiving frequency of SDR to the standard TRX. 

Fortunately in Winrad program (Alberto, I2PHD there is a DLL mechanism open to other programmers to do that.

I am applying here a DLL file   ExtIO_Icom_v##.dll  written by my son Chavdar jr. (LZ1ABC, ).  The program works only with Icom transceivers. The transceiver must have CAT control board connected to one of the existing COM ports of the PC.


Installation procedure: The ExtIO_Icom_vxx.dll file must be copied into Winrad working directory.  Start Winrad.  If in the folder there are other EXTIO_xx.dll files a dialog window is opened. Choose there ExtIO_Icom_vxx.dll file.  When first started a new configuration text file is automatically created named ComSettings.cfg.  COM 1 and ICOM756pro (address 92 decimal) are default. If the COM1 port is not available a message appears “Can not open COMx” since the particular hardware setup parameters are not set. Ignore and close the message and then exit Winrad in normal way.  Go to Winrad working folder and open with Notepad editor the just created ComSettings.cfg file.  It looks like:


92 //RadioAddres(decimal)

Com1 //ComPortName

9600 //BaudRate

8 //ByteSize






Comm settings for ExtIO_ICOM.dll. Leave the //comments untouched.


There should be no intervals between the comments otherwise the program will not work correctly.

Also between the values and the comments empty space should exists.



Now enter the address of the Icom radio and other communication parameters and save changes on exit.  Start Winrad again and activate the DLL in “Show options->Select Input menu”. Enter the mixer oscillator frequency (LO).  Now any change in Winrad receiving frequency will be passed to Icom radio. In CW mode choose upper sideband CW in Icom radio (CW-R in Icm756pro) since Winrad is using USB CW mode. Set the same “cw pitch” frequency in Winrad and in the TRX. When my Icom756pro is in “split mode” any frequency change in SDR will have effect on TRX transmitting frequency. This is very useful for DX pileups. With the SDR you can  listen and see the frequencies of calling stations and with TRX to listen the DX station.

In this version of the hardware I have abandoned the mode where the outputs of two radios are directed to separate ears – e.g. to listen with left ear to SDR and right ear to TRX. It is confusing due to the SDR time delay and is useless.

This program is in its preliminary version. Future plans are to implement this frequency control to transceivers of other producers. It is a good idea to control with one DLL the frequency of the transceiver and the SDR heterodyne frequency (e.g. programmable PLL or DDS).   There should also be a DLL control window where the synchronization function might be switched temporarily off in order to move around transmitting frequency as in transceiver RIT function. This window should have “Always on top” mode since Winrad occupies the total desktop screen and task switching is not straight forward. ( I have found in the Net numerous programs that can be used to implement “Always on top” function. I tested  “Power Menu”  and it works quite well.)

Other open source programs like Linrad (Leif, SM5BSZ and Power SDR ( )  are waiting for someone to do this modification to benefit the SDR community. The Rockie program for now lacks this feature but we can do nothing. It is the author (Alex, VE3NEA,  ) decision whether to put such a function in Rockie. The same holds true for KGKSDR (Duncan, M0KGK, ).


Simultaneous TRX control from Winrad and other control programs

The Winrad program takes control over the TRX CAT interface which is closed for other programs. I am using a freeware program “VSPE - Free virtual serial port emulator” from  to avoid this limitation.  This program creates something very useful – so called “Splitter”. The Splitter creates a virtual Com port which is connected by software to the physical Com port used by the CAT interface. The peculiarity of this virtual port is that it can be used simultaneously by numerous Windows applications which send or receive data through this CAT interface. There is no conflict between them. The setup of the Splitter is described in the VSPE Help after the installation of VSPE. In Setup set “Redirect modem registers” if RTS or DTR lines are used for TRX control. In present time I am using USB to COM port adaptor on my notebook where a single CAT interface is connected. Winrad, N1MM logger and CwType programs are running simultaneously on the same COM port.


December 2008,       

73 and Happy New Year,    Chavdar LZ1AQ



Revision 18.10.2010:  Schematics v.3.3 has a bug. Sw4 is now moved to the base of T1. Led1 is moved in antenna box.

Revision 18.11.2010:  Schematics v.7.0 has a bug. +12V   at T2  is now moved to proper place.  Thanks to Goeran  SM4YPT.