It was caused by a spurious coupling via the 3V3 power supply.
I thought of a problem in the firmware of FX3 while the cause was hardware. I decoupled the R820T2 3V3 power supply using a separate LDO from the 5V USB bus to solve the problem in the prototype.
The R820T can be used to receive frequency band in the range 30MHz -1800MHz. The tuner uses a clock generated by Si5351A at 32.000MHz, the REGDIV bit of reg 4 is set to 1 to divide it by two internally to the nominal 16MHz. The IF output is selected at 5MHz ( I used up to 7.5MHz) and it’s sampled by the ADC at 64Msps and then decimated down to 8Msps or less. The R820T data sheet states that the standard IF filters are implemented for 6/7/8 MHz channel bandwidths.
The LNA, the mixer and the VGA gains can be set manually although their precise values are absent from the datasheet. They can also be set automatically via automatic gain control (AGC) in order to optimize the signal to noise ratio (SNR).
It uses the same USB3 Cypress Explorer Kit Board while the ADC is a LTC2206. Reginald Eisenblatt, gave a presentation on the BooyaSDR ( January 23, 2017, at Linaspace see http://www.amrad.org/ ). Some video at https://www.youtube.com/watch?v=uF6y0ETTJFA , notice the Waterfall with multiple rows!
The BooyaSDR application is very clever. It uses gcc compiler with pthreads and fftw library. The FX3 firmware is loaded at run time using the Cypress download protocol.
I decided to use the same software environment and compiler to test a version of ExtIO_sddc.dll for BBRF103 with pthreads lib.
The BBR103 has been updated to version 0.2 with the following patch. - RAND patch: a wire has been added to control the RAND pin of ADC using GPIO20 of FX3. This option allows the control and test of RAND feature of ADC (see pg 14, 24 of http://cds.linear.com/docs/en/datasheet/2217f.pdf). The wire connects RAND (U3-pin 63, R7,R9) to GPIO20 ( BGA K7 = PIN25 J6 FX3 SS kit).
The Firmware source can be found into the archive file of project under \Firmware directory
The 0.96 version is still a preliminary release with some bugs. It operates in HF mode only.
Problems remain in use of R820T2 tuner, and a post on this argument will follow.
The FFT output is sent to HDSDR with a selectable rate of 32 Msps or a decimated one at 16, 8, 4, 2 Msps.
Filtering and tuning is made using overlap and add with frame of 1024 as 768 +256 samples. The filter time responses are 257 sample long.
When 32Msps is used the local oscillator of HDSDR is fixed to 16 MHz at centre of spectrum and the fine tuning of HDSDR allows reception from 500 kHz to 31500 kHz, while with lower sample rates the local oscillator can be tuned with 125 kHz step while the fine tuning is made by HDSDR.
At this development stage ExtIO_sddc.dll has a GUI dialog with 4 tabs :
Status - reports ADC rate and I&Q rate.
BBRF103 - buttons :
LW-MW this is used to modify the FFT output filtering to receive the low frequency band. HF - standard HF setup . VHF - enables R820T2 ( it is disabled, to enable undefine _NO_TUNER_ in config.h and recompile) DITH - enables the ADC dither. RAND - enables the ADC randomize.
TRACE - enabled in debug mode to trace some signals to log files.
RF ADC stream: it requires BB103, ADC input ,default, RF virtual tone: it requires BB103, virtual tone, RF virtual sweep: it requires BB103, virtual sweep, IF virtual tone: NO hardware required, virtual tone , IF virtual sweep: NO hardware required, virtual sweep,
Hereafter a video recorded with a random wire antenna 5mt long on the balcony (in the city).