Monday, March 24, 2014

R820T with 28.8 MHz TCXO

I recently looked around for tools to use as low cost spectrum scanners, being the objective frequency range 400 MHz to 1.7 GHz (incidentally, DVB-T and GPS).
Of course rtl-sdr is an attractive option so I dusted off some dongles I had bought 6 months ago in China and played again with them, coming to the conclusion that I really like it especially after its main limitation is overcome :)

The 28.8 MHz crystal is quite poor. I asked Takuji for a TCXO but he said he emptied his stock rapidly. Of course a replacement is nowhere to be found on the big distribution (Digikey, Mouser, Farnell, RS, etc..), so I went to an old time acquaintance at Golledge and, despite having to order 100 pieces, my request was fulfilled. After all, dongles look quite good with the new crystal:
Figure 1: RTL-SDR with 28.8 MHz TCXO (Golledge GTXO-92)

I measured the frequency deviation with my simple GPS software receiver and I happy to report that it is within spec, bounded to 2ppm. By the way, I tried using other GNSS software receivers and will write about my experience in another post soon.

On the frequency plan side, the R820T combined with the RTL2832U is great for GPS. Most people would use it with an active antenna, where the LNA solves the problem of losses due to the impedance mismatch (50 against 75 ohm) and the noise figure of the tuner (3.5 dB according to datasheet).
The frequency plan with an IF of 3.57 MHz solves elegantly the problem of LO feedthrough and I/Q unbalance typical of ZIF tuner. The IF is recovered automatically in the digital domain by the demodulator so it does not appear in the recorded file. 8bit I/Q recording at 2.048 Msps is more than sufficient for GPS and I also tracked Galileo E1B/C with it (despite some obvious power loss due to the narrow filter band). In my tests, I used a Dafang technology DF5225 survey antenna and the signal time plot shows that 5 bits are actually exercised. I powered the antenna with 3.3V from a Skytraq Venus8 (Ducat10 with S1216F8) through an all-by-one DC blocked passive 4-way splitter/combiner (6 dB unavoidable loss) from ETL-systems.

Figure 2, 3 and 4: Power spectrum, histogram, and time series at L1.

I posted three GPS files here:
https://app.box.com/s/wxizs3p7zu8x2jmbnzod
https://app.box.com/s/xvfabkqfkmehg5osa3ra
https://app.box.com/s/dqrel15mwj73xijflkma

Since someone asked for it, here are the tracking results of Galileo E19 plotted after the fact with Matlab:





and Galileo E20:






More to come later,
Michele

17 comments:

Dan GNSS said...

What is the ppm of TCXO you ordered from
Gollegdge?

Michele Bavaro said...

Hello,

here it is.

28.80MHz GTXO-92T/GS SM TCXO
TCXO, no VC, ±1.5ppm, -30+75°C


Cheers,
Mic

Dan GNSS said...

Hi Michele,

Thanks, Can you tracking the GPS signal using the new TCXO?

Dan

Michele Bavaro said...

Dan,

Tracking GPS is possible even with the standard oscillator.. although you will track the crystal more than the receiver dynamics.
With this TCXOs, I am hoping to be able to provide a usable carrier phase (for differential applications).

Cheers,
Michele

Dave Smith said...

What sample freq did you set on Galileo E1B? Lost any data?

Dave

Michele Bavaro said...

Dave,

2.048Msps, as usual. It is sub-optimal, so what? Most people worry too much about idealities, when the reality is: "yes you can track Galileo like that".

By the way, you can see for yourself with the files I attached.

Cheers,
Mic

Dave Smith said...

You are right. I tried first file acq Galileo PRN 19 and 20

Would you please confirm you had 19 and 20 (maybe more) included in the first data file?

Thanks,
Dave

克·单 said...

Hi Michele,

Is there any TCXO other than 28.8MHz
will work with this rtl-sdr?

Michele Bavaro said...

Yes Dave I did, post is updated with Galileo E1C tracking of PRN19 and PRN20.

The R820T works fine with many standard frequencies and probably AirSpy has something else than 28.8MHz.
However, from the datasheet it seems like the RTL2832U prefers to have a 28.8MHz oscillator. I don't see a reason why it should not work with -say- a much more popular 26.0MHz, but I am too old to start reverse-engineerning the demodulator.

Cheers,
Mic

Dave Smith said...

Michele,

Is this dongle can be used for carrier tracking DGPS after updated TCXO?

R,
Dave

YO8TLC said...

Hello Michele,

Since you had to order tcxo in a quite big quantity, can you help others in this matter?
I would happily buy several, if you can waste some time to do the shipping. I'm interested in narrow band applications where stability is a big issue. Thanks!

Best wishes,
Cezar

Michele Bavaro said...

Cezar,

As I wrote, I simply asked Golledge.
They told me the min quantity was 100 so I had to order that amount and wait two months. I don't see how I can help you further?

Bests,
Michele

mt said...

Michele he wants to know if you are willing to sell some of the 100 that you had to order to us. I'd also be interested in buying 10 of them from you if the price is right..

Michele Bavaro said...

Hi all,

I have no more TCXOs left in stock.. apologies.

Bests,
Michele

Alfredo said...

Hi Michele,
good stuff. Is there a particular brand of dongle you can recommend, perhaps one factory-fitted with an already acceptable TCXO?
Thanks,
-Alfredo

Hugh said...

You can buy the same dongle with a
similar upgraded TXCO from

http://h2204566.stratoserver.net/SmartStore.NET/de/usb-tv-dongles-for-ads-b

Alfredo said...

Thanks much Hugh.
-A