Tuesday, May 12, 2015

A modern GNSS front-end

Earlier this year I had the occasion and privilege to be trying out a new front-end produced by NTLab, the NT1036. I thought it would be interesting to share this with the GNSS crowd.
The kit arrived composed by two separate boards: a control board and the actual chip evaluation board, as well as a CD with the software and detailed data-sheet. The controller board connects seamlessly to the evaluation one by means of a single flat cable with RJ12 ends. Although the suggested supply voltage is 3.0V pm5%, it was very convenient to use the same cable to power the board with 3.3V. Also, having a single common supply avoids currents on the control lines. In the end the chip worked fine in this configuration so I assume it was a safe choice to take.
The chip has many unique characteristics that make it suitable for a modern GNSS receiver. The ones of greatest interest to me are the following:
  • Four independent input channels
  • Two wideband VCO banks, on high and low RNSS bands, which can be routed with great flexibility amongst the four mixers, in particular allowing:
    • GPS/Glonass L1+L2 or L1+L5
    • GPS/Beidou L1+L2 or L1+L5
    • All 4 channels on either L1 or L2/L5
  • 3.0V supply voltage and low power dissipation (ideal for USB-powered devices)
  • Analog or digital output options for IF (real-only, which I like best) and clock lines.
  • Small, easy to assemble package
Obviously, the killer applications for this kind of chip are well contained antenna arrays and multi-frequency multi-constellation hardware and software receivers.
Having a lot of testing equipment at hand one could really crack a nut like this one. However, with limited hardware at hand I decided to use my SdrNav40 board and slightly modify its firmware so to ignore the 4 on-board RF channels and capture instead the evaluation kit outputs and clock.
Figure 1: Test setup, with 4 way power splitter and SdrNav40 powering the antenna.
Figure 2: Closeup of the test setup
Two tests were particularly useful for me: GPS/Glonass L1+L2 and four channels on L1. The first should lift any doubt on the potential fields of application of the chip. The second should solve my curiosity on phase behaviour of common LO (Local Oscillator) MI (Multiple-Input) front-ends.
The GUI to control the configuration of the NT1036 is incredibly rich and professional: low hanging fruit for a curious engineer.
Figure 3: NT1036 configuration tool: general settings tab, where the synthesizers can be programmed
Figure 4: NT1036 configuration tool: channels 1 and 2 tab
Figure 5: NT1036 configuration tool: main chip blocks tab
For GPS/Glonass reception the tuner offers a default configuration with the two VCO banks tuning in the middle between GPS L1 and Glonass G1 (and similarly for L2/G2), thus having GPS in high-side mixing and Glonass in low-side mixing. Configurable IF filter banks select one or the other. The distance between the centre frequencies (being about 26 MHz on 20 MHz for high and low RNSS respectively) suggests a L1 plan in which a FS of about 52 MHz puts both carriers around FS/4 for ease of down-conversion. Setting FS to 53MHz (derived from an integer PLL) allows achieving GPS L1 on 14.58 MHz. Plots that everyone likes follow.
Figure 6: PSD of samples acquired in high-injection mode on L1 at about 50Msps
Figure 7: Histogram and time series of the signal acquired with NT1036 (sign and magnitude output)
Figure 8: Results of GPS satellites acquisition.

I have in mind to continue my tests on the chip, subject to time which is always very little!
Till next time...

1 comment:

Twins' Farm said...

Hi Michele

I have an AgLeader GPS 1600 receiver and I would like to use RTKlib to connect to GNSS station and work using RTK signal. I don't know how I have to configurate the rtknavi. Could you help me, please?

Thank you very much
Best regards.
JoaquĆ­n Tribaldos