New BPFs

Trolling around the web I found a new configuration for BPFs. A couple of coils and three capacitors make a rather good response. Here's the design, simulated in LTSpice, for 20m and 40m.

These could be good for both RX and TX, the skirts are narrow and the central loss is zero.

Low Pass filter, SWR and TXRX switch

I have completed the PCB for the LowPass FIlter, SWR and TXRX switching for the Concept SDR system. The SWR part is based on the design and kit from Kits and Parts. Note that it uses FT37-43 cores which have a much higher Al than the T37-6 of the LPF, the inductance of the 10t windings is 44uH to make the broadband transformers. Have a look at the kit also to see how they are mounted and wired.

This is the schematic:

And here is the board, the PA input is top right. The links on the board at the left allow you to connect directly to the G0 output of the VFO (for example for direct WSPR transmissions) or the TX shield output (TX shield yet to be made...):



This is the stack of Arduino Uno, VFO & RTC, SDR RX and the new LPF & SWR & TXRX switch shields:

This is the LPF_SWR_40M shield mounted on a VFO_RTC_IQ shield to form a WSPR beacon:



Note the wire link bottom left to connect the VFO G0 output to the LPF input.

How about an FPGA Arduino compatible SDR?

I have been fascinated recently with a new PCB that has been announced - the FleaFPGA:



I am dreaming of an SDR based on the FPGA, but I have no idea how to go about it. If there is anyone out there interested to investigate this let me know, antonywatts at me dot com.

Unfortunately all the development software runs on Windows 7-8, and I have only Macs, so that would present a problem!

Desktop WSPR

Just to see if my software works, I am sending WSPR messages across my desktop! I cannot send them out into the ether as I do not have a LPF on the transmitter/VFO and there are tons of harmonics at the moment. Anyway the VFO output is only a few milliwatts, so I don't think I would get far with my small stick aerial.

Here's the set up. The transmitter is an Arduino Uno with the VFO shield (described previously) mounted on top, I have a 6" piece of wire as an antenna on output "G0". The receiver is also an Arduino Uno with another VFO shield and the SDR shield on top. The antenna is a, very inefficient, Wonder-Wand.



iMac running both HDSDR receiver software, Soundflower to send the audio across to WSPR software. HDSDR is running under Wine on the Mac as it is a Windows program, WSPR is native to OSX.



This is the transmitter LCD display. The code being sent, hardcoded into the Arduino sketch, is shown at the top (Call, Locator, Power). The Interval between sends is 2 min shown bottom left, this can be changed by pushing the encoder button to 4, 6, or 8 min. The frequency of the transmission is set to 7040.100, but can be tuned in 10Hz steps by the encoder.


And the SDR receiver display. This show the time at the top, the SDR centre frequency and the 40m band plan for this frequency at the bottom.



The Screen shots of HDSDR and WSPR. HDSDR is tuned to 7038.600 to have an output of 1500Hz from the received WSPR signal. This is fed to the WSPR program and should display at "100" on the scale. As you can see either the transmitter or SDR are about 50Hz off frequency - neither has been calibrated.





Fun and it works, which at least shows the software on both the TX and SDR Arduino is OK.

New Concept SDR User Manual

Here is a user manual for the Concept SDR, CONCEPT MANUAL

Work in progess:

- A Direct Conversion RX, PCB assembled, some solder blobs to cure...

- A LPF + SWR + TXRX Switch, built and working, see future posting

- A TX, PCB to be designed

More new later...