Software Defined Radio - RTL SDR

RTL.SDR hardware - connections for usb and antennas

SDR# software - tuned to VHF FM broadcast station on 105.6MHz

For several years usb 'dongles' for DVB-T reception have been available. The dongle consists of a tuner and down-converter. The frequency coverage of the tuner depends on the dongle variant, and typically from VHF to SHF. A common chip set is R820T tuner, RTL2832 down-converter and ADC ( analogue to digital converter, 8bit ). In-phase and Quadrature ( I - Q ) digital samples of the signal are sent at speeds up to 3.2MSamples/S over a USB2 connection for digital signal processing by a computer running appropriate software.
Using  a similar architecture, although a little different from the dongles in size and appearance, is the RTL SDR ( Software Defined Radio ), sometimes called RTL Downconverter. I recently bought one of this type online for GBP27 (  + free shipping + 10 week wait ) which has the new R820T2 tuner. This particularly interested me as the tuning range is continuous ( no gaps ) from 100KHz to 1.7GHz, ( LF-SHF ); so includes the amateur LF ( 136KHz ) and MF ( 475KHz ) bands. I plan to use the RTL SDR as a spectrum analyser to check some of the equipment I've built for these bands. I downloaded SDRSharp ( SDR# ) software, selected source RTL-SDR ( USB ), connected the supplied antenna and casually tuned and listened. The dynamic range seems to be about 60-65dB; not an impressive figure but should be enough for my purposes.
Further information is available from RTL-SDR.com and I downloaded SDR# from www.sdrsharp.com
There are also many video tutorials at www.youtube.com , e.g., Getting started with SDR# and an RTL SDR tuner - YouTube
An alternative software package which I haven't tried yet is 'SDR Console'.

Class C transmit amplifier for the 630m MF band

I continue to try different amplifiers in order to improve my station for weak signal digital modes, such as JT9 and WSPR, on the 475KHz, 630m band.

unboxed prototype amplifier with space left for antenna relay

The amplifier posted here is a medium power, and physically small, two IRF640N mosfet class C design. I designed the amplifier so that the 13degC/W heatsinks would provide adequate cooling for up to 50 watts output power when run from a 54Vdc power supply, while still retaining the compact size. With 2.5W drive power at the input, I measured 33W output power, ( gain 11.2dB ).  This was deliberate because at that power level, the optimum mosfet drain load is almost 50 Ohm; so I avoided the need for any output matching circuit. It will be interesting to see what I can achieve on digital modes now.

antenna change-over circuit added (lower-right)

Using it for the first time yesterday evening, my WSPR beacon transmission was received at a record distance 1655kms in France, and I was rewarded with a JT9 digital mode contact with Scotland at a similar distance.

 finished amplifier in home-made enclosure

 

3 February 2023

I received a request from a radio amateur in the USA ( Jeff, callsign WB8RJY ) for the circuit diagram, as he wanted to construct the amplifier. A few days later Jeff sent me several pictures of his completed amplifier, and a summary of his WSPR activity using the amplifier detailing some very long distance reception reports.

amplifier built by WB8RJY based on my design

 

 

 

 

 

 

Test jig for DDS module and Dongle

AD9850 DDS module on breakout board with interface dongle

CH1 sine wave output, CH2 comparator output +DUT 25%

This post is about using those cheap DDS, ( Direct Digital Synthesizer ), modules based on the AD9850 or AD9851 DDS chips, and found on auction sites for only a few dollars. I have been programming and using them for several years, going back as far as August 2013 ( see post 27/08/2013 ). I am now using the interface dongle, ( USB-SPI v1.00 ), and software from www.spectecs.com which makes the modules extremely quick and easy to use; the tuning word is created and uploaded to the DDS over USB with just a few mouse clicks.
The modules are ideal as a signal source for testing amplifiers etc or embedded in projects which is what I have done recently with the phasing exciter ( 02/11/2017 ) using an AD9851 DDS module on that occasion.
I made a simple jig, ( break-out board ), for speed and convenience when connecting the module for use as a stand-alone signal source. The SPI and output connectors are extended out to pin-strip headers. I also provided LEDs for testing the two GPOs from the dongle.
The module has two sine wave outputs, ( OUT1 & OUT2 ), two square wave outputs ( VO_P & VO_N ). Sine output OUT2 is present on the disconnected black jumper. Referring to the top picture, I've set the jumpers as follows:
J1 green, connected = serial data upload, ( disconnected = parallel data upload ).
J2 blue, connected = enable square wave output, ( disconnected = no square wave output ).
J3 yellow, connected = digital to analogue converter ( DAC ) full scale current set internally, ( disconnected = DAC current set externally ).
I uploaded a tuning word ( hex 010624DD ) for the DDS to generate an output signal at 500KHz. The duty cycle of the square wave is adjusted using the trim-pot next to J2. I found the range of adjustment to be about 12%-87%. The lower picture shows the signal on OUT1 ( 1V peak to peak ), and VO_P ( 5V peak to peak ), which I adjusted for 25% duty cycle.
Although the top picture features the AD9850 DDS module, the AD9851 module is identical in appearance apart from the chip type of course.