New front end for use of the Red Pitaya in SDR applications

The Red Pitaya SDR board is based on the Xilinx Zync SOC and has 14 bit external A/D converters. However, for SDR usage on the HF bands from 0.1-30 MHz (and for that matter up to 50 MHz) the Red Pitaya is a bit “deaf” in the stock configuration. I have made a broadband amplifier that has a fairly high gain and very good IIP3 properties. Below I have posed some pictures of the prototype amplifier.

20160424_153046-1This is the prototype amplifier. I inserted a ferrite ring on the input lead to roll off the VHF / UHF sensitivity to reduce problems with nearby broadcasters etc. There is a also a PI network attenuator on the ouput and I have inserted a couple of beads in that as well to roll of the outpu response when frequency increases. The other components in the lower part is a input pi attenuator I used when I did some VNA frequency response measurements. This as well as the RCA plus is not used (RCA plugs are surprisingly good for low level RF signal routing in the HF bands and nice to use in the lab).  I used a more professional attenuator with a large attenuation range and flat response to determine the proper attenuation level after the preamp into the Red Pitaya. Reducing gain after the first amplifier has very little effect on the noise figure. Reducing it before the first amplifier directly adds to the noise figure. I added some protection diodes over the input to reduce the risk of strong RF signals or static voltage build up damaging the input. Below I am measuring the response of the attenuator with the DG8SAQ VNA. It was flat from 0-1,3 GHz.






Testing of a HF current transformer with a vector network analyzer

I wanted to check the load that a ferrite core with a secondary winding presents to the common mode current carrying conductor (the outside of a coax) when set up as a current transformer. The coax runs thru center, the secondary winding could be one or several turns loaded by a R+jø load.  (+jø load but the windings of the secondary will give some +jx component).

I did the following three S11 measurements with the VNA (M1, M2, M3):

M1) No secondary winding is present but the single turn is running thru the core

M2) A shorted two turn secondary and an open two turn secondary (winding is pulsed in and out off to be able to better detect a difference). The time domain is captured by the slow scan and sampling rate of the VNA.

M3) Only the primary winding is attached to the calibrated S11 measuring plane. (to measure the self inductance of the single turn in itself. This is an air-core measurements.)

Above is a similar test setup. The core is an “unknown” Amidon toroid core. The red conductor is the simulated coax common mode current path (a one turn loop). The green conductor is the secondary winding. The load could made by paralleling several resistors in series with the secondary (the blob on the left side, right picture). Note that the measurements below were done with a one turn loop and a short – no resistor. The measurement device is a VNA from DG8SAQ calibrated by O S L  references in the S11 measuring plane (the SMA in the end of the coax from the TX port).  
The left picture shows measurement 1) The mid picture shows measurement 2) the right picture shows measurement 3)

Edited: What is interesting to see is that the image to the left shows that this core has some resistive loss as can be seen on the blue trace. The Q is quite low. When there is a secondary winding present, the loss is shorted out but the inductancechanges since the inductance of the secondary is reflected into the S11 measurement plane. On the right image it can be seen that there is no resistive loss and a linear inductive reactance caused by the air core inductor (no appreciable drop off or frequency dependent effects, in the measuring frequency range)

What this tells me is that this core setup probably is not too well suited as a high current measuring setup for frequencies above 160m because it will affect the measuring circuit too much. Not in terms of the R element but because of the +jX element. The R is low to the RF current passing thru the core (in the common mode), but the +jX element will present a reactance to the RF current and thereby giving you lower current than should be expected without the core.  My analysis says that another core type should be selected or that a frequency compensation technique should be used. Alternatively that a lower turns ratio should be tried. However looking at the plot to the left, it can be seen that below 30 Mc*s^-1 the +jX component is too high. Perhaps this core has a too large permeability and that a lower permeability core should be used. The primary turn with secondary loading should give a low reactance on the primary. The voltage given over Rt would then be lower but the gain of the detector could be adjusted. (I may be wrong). Please comment if you have comments or suggestions.