Part Two: Airspy R2 - Questions and Answers

Question   1: There are three multiplexed ADC channels in the Airspy, is there a simple way to access the other two if I wanted to implement a custom DDC firmware? 
Answer     1: .By default channel 0 is used for the R820T2, but channel 1 and channel 2 are unconnected. You could hack the firmware source code and modify this line "#define DEFAULT_ADCHS_CHAN (0)" in "firmware/airspy_m4/airspy_m4.c" to use channel 1 or channel 2 instead of channel 0 which is connected to the output of the R820T2 tuner chip.
Of course the R820T2 would still be powered on so you should disable it, since you are not using it (use disable_r820t_power() anywhere where enable_r820t_power() is called) because it may add unwanted noise. Of course disabling the R820T2 chip, also means that you should delete/disable any calls to set registers in it, since you are not be using it. You should also delete/disable the frequency tuning from the firmware since the tuner is not used. And by the time you have done that it might be easier just to write your own custom firmware.

Extra info  1: Since the spyverter is now available it might be easier just to use it unless you actually need an ADC, that in theory could run at 80MSPS. The problem is that the USB bus will limit the data transfer to about 20MSPS. And there is not enough RAM in the LPC4370 to buffer more than about 2x16KiB (double buffering) or 32KiB of data, and there is not enough processing power within the CPU to reduce this data. So unless 32KiB of data sampled at 80MSPS is enough for your function this is probably a dead end for your idea. If you are still going to use the two ADC ports the good news is that there is ESD protection in the R2 board, the R0 board had no ESD protection on the ADC inputs.

Part One: Airspy NOS - Questions and Answers

Today (2014-10-29) the Airspy, was available for pre-order. And I thought that I would do the same thing that I did for the HackRF One. That I would try and keep a track of any questions that I see asked on IRC and the answers given.

I though that I would make a quick log of the problems, queries along with their corresponding solutions and suggestions. And add a few extra notes of my own, if I am able. I have removed all sources of questions and answers to protect peoples privacy (what little is left of it these days).

If you are currently scratching your head, because you have never heard of an Airspy before, then you should probably go to the Airspy website.

I'll probably sort these random questions and answers into some sort of logical order when there is enough of them, Hardware, Software, GNU Radio, RF fundamentals, .... but for now I'll just leave them mixed up higgledy-piggledy in the order that I saw them asked.

Question   1: "External clock input (10 MHz to 100 MHz via MCX connector) – Ideal for phase coherent radios” what is the expected voltage ? 
Answer     1: The Clock Input for the Airspy is described on page 5 of the of Si5351C datasheet.
It is 3.3v CMOS voltage levels so, low is below 0.8v (min -0.1v) and high is over 2.0v (max 3.6v). So either a square wave or sine wave that meets this criteria. Almost anything will drive it if the voltage is less than 3.6 V P-P and more than about 2.5 V P-P if it is applied to the CLKIN pin with a series capacitor, a 100k to ground, and a roughly 100k to 3.3 volts.
Extra info  1: So a 0v to 3v square/sine wave at 10MHz would tick all the boxes, and be a good clock source. The Airspy already has a pretty good crystal in it, a 1.5 ppm, low phase noise clock. It may not be perfect for some things but it is a good clock. 

Question   2: "Airspy NOS R0 03 Aug 2014" is written on the PCB silkscreen, what exactly does NOS stand for ?
Answer     2: Nest of Spies.

Question   3: What is the input impedance of the Airspy SMA antenna port ?
Answer     3: 50 ohms

Question   4: What is maximum current that the "4.5v Bias-Tee to pwoer LNA’s and up/down-converters" can use ?
Answer     4: USB 2.0 provides a maximum of 500mA at 5 volts. The Airspy uses less than 400mA at 5 volts (2W), so that would mean that at 4.5 volts in theory the maximum Bias-Tee current would be 111mA (500mW), but there are conversion losses so it is probably closer to 100mA. If more power than this is required then a micro USB Y cable could be used with an external power supply to provide more than 500mA at 5 volts. BiasT LNA's, typically, are at a maximum 250mA for 4.5V (1.125W). So Y cable is probably required, depending on the extra hardware that needs to be powered.
Extra info  4: From http://airspy.com/wp-content/uploads/2014/11/AirSpy_NOS_PinAssignment.jpg the LDO in the Airspy could provide 250mA at 4.5 volts if a Y cable is used to increase the input power. Provided the USB ports on the computer (or external power supply) can provide enough input power.

Question   5: What is the maximum safe input power before you start to damage the Airspy. For example people are breaking the pre-amp in their HackRF One (by enabling it) and pushing more than -5dBm into it (0.1257 volt RMS).
Answer     5: May be 10 dBm (0.7071 Volt RMS) would cause physical damage, for safety stay below 0dBm (0.2236 Volt RMS).

Question   6: What is the input voltage range for ADC IN1(P9) and ADC IN2(P8) ? The LPC4370 datasheet gives the 12-bit ADCHS input as 800mV peak-to-peak in a band from 0 V to 1.2 V. So what is the band, or is the DC offset removed by input circuit and all I need to worry about is the 0.8 volt peak-to-peak ?
Answer     6: 0.8v pp centered around 0.8v

Question   7: There are three gains in the Airspy IF(VGA), Mixer and LNA, what is the best way to use these for optimal results ?
Answer     7: Set the IF(VGA) to 5, the Mixer to 10 and optimize the LNA for the best SNR (the difference between the top of the signal and the noise floor). Then adjust the IF(VGA) so the noise floor stays at -80dBFS, then use the Mixer gain for fine tuning the gain.


Question   8: How do I convert an iq file created by the airspy_rx command into a cfile ?
Answer     8: If we assume that a sample type of "-t 2" (the default) was used with the airspy_rx capture command. Then the following gnuradio flow graph, (similar to the one used by the HackRF One) will convert a default Airspy capture file to a valid cfile.

Question   9: How can I check if my external clock has been selected by the Airspy ?
Answer     9: If you read a document called Si5315 AN619, the answer is hidden within it. You can use the following command "airspy_si5351c -n 15 -r" if bits 2 and 3 are 0 (0x00) then the internal TCXO is active, if bits 2 and 3 are 1 (0x0c) then the CLKIN is active. Or a different register could be used
"airspy_si5351c -n 0 -r". If bit 4 is 0 (0x01) it is locked to an external clock and if it is 1 (0x11) it is not locked to an external clock and is using the internal TCXO.