FA-SDR-TRX part #3 – the Transmitter


In the third article we will have a deeper look into the TX path of the FA-SDR-TRX. All components will be discussed, including the optional 1 Watt amplifier.




Earlier Posts I wrote:

FA-SDR-TRX, and allband & low budget SDR TRX

First pictures of FA-SDR-TRX

FA-SDR-TRX – part #1 – a Systems View

FA-SDR-TRX – part #2 – the Receiver

Block diagram

An important design goal of the FA-SDR-TRX was to make maximum reuse of all available components. Beside the Local Oscillator, the Bandpassfilter and the Buffer Amp are shared between the receiving and transmitting path. In the transmitting path, RF signal is generated in the  TX-Mixer / Quadrature Sampling Exciter (QSE). This signal needs to be filtered from unwanted mixing products. Afterwards the Buffer Amp is used to amplify the signal to 10mW. If desired, this signal can be boosted up to 1 Watt with an optional amplifier.


Tx Mixer

FA-SDR-TRX-tx-mixerThe IQ Signal is generated with the PC. The (stereo) IQ signal is fed from the LineOut jack into the PCB. Before the signals reach the mixer, the amplitudes of I and Q can be adjusted with a potentiometer. The following op-amps ensure the phase conditions (0°, 180° / 90°, 270°) needed for by sampling mixer.

Excellent linearity and low distortion are crucial requirements for the op-amps. Here, Linear Technologies LT1498 was found appropriate. Unbalances can be minimized with an equalizing current. The better the balance is, the better the carrier rejection (e.g. of an SSB signal) will be. With hardware, a perfect balance can only be achieved on a single frequency. As a rule of thumb it is recommended to use a frequency between 10…15MHz. This will result in acceptable results for all amateur bands under the premise that low tolerance compents are used and the PCB is assembled symmetrically! Small unsymmetries can of course be eliminated by software, but the intention is to avoid unsymmetries as much as possible.

The Tx mixer also uses a 74LVC4066 switching IC. The LO signal is supplied by the Generator component (see below). Like in the receiver, a 74ACT74 Flipflop is used to drive the 74LVC4066. After the mixer, the two DSB signals are combined in a broadband transformer (Mini Circuits T4-1-X65) to the SSB Signal. At this stage, the signal strength is about 1mWatt.


Si570 Signal generatorThe Generator is a shared component between the receiver and the transmitter. The LO Signal is generated by a Si570 VCXO. The FA-SDR-TRX uses the FA-SY module as a COTS (commercial of the shelf) product. Frequency and PTT signals are controlled via USB. Depending on the state of the PTT signal, the BPF and AMP are switched into the RX or TX path.

A nice little detail in the FA-SDR-TRX design is, that the PTT Signal is also used to ensure that RX-Mixer and TX-Mixer are don’t operate at the same time. During reception there is no need for the the TX-Mixer, therefore it is disabled. During Transmission of course vice versa.

A CW key can be connected to the FA-SDR-TRX. This signal is used to directly key the FA-SY Generator.

I appreciate, that Harald, DL2EWN also considered EMC aspects in the FA-SDR-TRX. Ground loops are successfully avoided by using either transformers or opto-couplers for all external signals.


FA-SDR-TRX-bandpass-filterLeaving the TX-mixer, the transmitting signal is fed into the Bandpassfilter. The BPF “cleans” the TX signal from unwanted mixing products. Good filtering at this stage is crucial. The design of the Bandpassfilter was already covered in depth in the FA-SDR-TRX receiver description.

Buffer Amp

FA-SDR-TRX-buffer-ampThe Buffer Amp is a broadband (0…5000MHz), 13dB fixed gain MMIC (SGA5289). In the TX path it is used to amplify the TX signal to about 10mW. The following illustration shows the 10mW output spectrum of a two-tone signal on 10m. Please note that the intermodulation products of 3rd order are suppressed by 56dBc (-62dB PEP). With these excellent characteristics, the signal is ready to be put on the air or boosted in the following optional amplifier.

FA-SDR mit Presel. 10mW PEP 10m

Measurements have been done with a RF-Space SDR-IQ receiver and additional attenuators to avoid receiver overloading.

Optional 1 Watt Amplifier

FA-SDR-TRX-tx-amplifierEven if it is possible to make contacts with 10mW, it’s quite hard. A few dB Gain could help to produce signal which can be seriously used on the bands. Having this in mind, Harald DL2EWN developed an optional amplifier with these design goals:

  1. Output Power 1 Watt
  2. High linearity
  3. Easy assembly
  4. Reproducible results
  5. Easy available and inexpensive components
  6. Optional, but easy integrateble into main FA-SDR-TRX PCB

After my last conversation with Harald, it seems that all design goals could be met!

The Amp uses two RD00HHS1 Mitsubishi MOSFET transistors in push-pull mode. According to the diagrams in the datasheet, a single transistor provides +28dBm (630mW) within the specificated parameters. Measurements confirmed that intermodulation products are at least -40dB PEP throughout 1,8MHz…30MHz. The following diagrams show the output spectrum of a two tone signal with 0,25W, 0,5W and 1Watt power.


FA-SDR-TRX amplifier with 0,25Watt output power:

Neue 1Watt-PA 0,25Watt PEP

FA-SDR-TRX amplifier with 0,5Watt output power:

Neue 1Watt-PA 0,5Watt PEP

FA-SDR-TRX amplifier with 1Watt output power:

Neue 1Watt-PA 1Watt-PEP


Most parts of the amp are SMD and will come premounted. Commercial transformers ensure good matching and low losses (Minicircuits T9-1 and ADT 1,5-1). The chosen MOSFET transistors (RD00HHS1) are inexpensive and easy to handle.

Already during the design of the main FA-SDR-TRX PCB the optional amplifier was considered as part of the whole system. The result is, that the amp can now be integrated within seconds. By plugging the amp into the main PCB and changing two jumpers, the amplifier is ready to use.

The price of the optional 1 Watt amplifier is still not defined will be approximately 30 Euro. Currently, the first batch of amplifiers should be available in February March 2010. They can be ordered directly from the publishers (Funkamateur) online shop.

About Tobias (DH1TW)

Self-confessed Starbucks addict. Loves to travel around the globe. Enjoys the technical preparations of Amateur-Radio contests as much as the contests themselves. Engineer by nature. Entrepreneur. For more, follow him @DH1TW


  1. Great Post! I personally really enjoy your article. This is a great website. I will make sure that I stop back again!.

  2. Jindra, OK4RM says:

    In none of the articles and Baumappen about FA-SDR there *nowhere* any mention how this tcvr will switch to send. Is this info simply assumed to be generally known???? I have fully assembled FA-SDR on my desk, but there is no way to transmit!!! This is mostly because all descriptions discuss only Rocky software nad PowerSDR. Both of these are craps that do not work. Rocky for some strange reason does not see the soundcard for I/Q processing (any other software in the world does, also non-SDR software), whereas PowerSDR on the other hand is unable to communicate with the on-board Si570 (strangely, however, it did obtain the Si570’s xtal frequency correctly).
    Is there any real software that works and can be used with FA-SDR transceiver? The only two that work are M0KGK decoder (but this is only Rx) and SDR-RADIO console (but for some strange reason it does not transmit either).
    Baumappe mentions “PTT-Taste” but there is of course no such thing in the whole documentation of FA-SDR. I am pretty pissed off that I threw out 250 euros for this – it is definitely a nice hardware, but unfortunately not fully functional as TRX…

  3. Vince N2AIE says:

    Thank you for a well done site. very educational, and love the quick tips..you should do more.
    Example of one would be a quick video on PSDR setup…maybe tab by tab..

    thanks again 73’s

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