Tagged: thermal drift

October 2, 2014

Review of the USA TCXO Modified RTL-SDR Dongle

A few days ago we posted about a new US source for TCXO modified RTL-SDR dongles. We received a sample from the company that does the modifications and below we present a review of the product.

The modified TCXO dongle is based on a standard full sized R820T RTL-SDR PCB board. Inside we can see that the standard 28.8 MHz oscillator has been removed and in its place is a 28.8 MHz TCXO oscillator. The old oscillator has been removed and the new oscillator is carefully soldered in its place. The soldering job appears to be nice and tidy.

Next we tested the TCXO dongle against a control RTL-SDR which had a standard non-TCXO oscillator. First we measured the PPM offset at room temperature against a known ATIS signal. The TCXO dongle had an offset that was somewhere around 0.5 PPM. The control RTL-SDR had an offset of 60 PPM.

As the dongle heats up from use, the oscillator will experience thermal drift, causing the frequency offset to change. The TCXO should be immune to this problem due to it’s temperature compensation circuitry. To test the TCXOs temperature compensation capabilities we placed both dongles in a freezer for 30 minutes and then took them out and recorded their start and end PPM offsets after 30 minutes of operation. To simulate a warm environment the dongles were also placed under a warm tungsten light during operation.

The control RTL-SDR started with an PPM offset of 58 PPM and ended with an offset of 72 PPM, giving a total drift of 14 PPM. The TCXO RTL-SDR dongle started with a PPM offset of ~0.5 PPM and ended with an offset of ~0.5 PPM, giving a total drift of 0 PPM.

Below we have recorded animated GIFs of the drift observed in both dongles. The first GIF shows the control RTL-SDR. Note how the frequency offset oscillates at first and then slowly drifts away in one direction. The TCXO dongle exhibited no drift whatsoever.

STD_Dongle_Crop_Faster_comp — Control RTL-SDR Drift over 15 Minutes (Sped up)

USTCXO_comp — TCXO Dongle Drift over 15 Minutes (Sped up)

The results show that the US TCXO has a very low overall PPM offset and is very stable over temperature changes. If you want one of these dongles they can be found for sale over on Ebay for $65 USD.

In the previous post some commenter’s raised the concern that this product was overpriced at $65 USD. However, we believe this price is reasonable. The reason the cost is much higher than a standard RTL-SDR is that it is very difficult to obtain TCXO oscillators with 28.8 MHz clocks. The company selling these needed to have the TCXOs custom made from the factory with a large minimum order quantity of 1000 pieces. Then after adding the labour, quality control and selling costs the profit margins become quite small.

September 23, 2014

New USA Source for Modded TCXO RTL-SDR Dongles

Previously on our blog we posted about RTL-SDR dongles for sale in Japan that had been modified by replacing the low quality 28.8 MHz oscillator with a quality low ppm and high stability temperature controlled oscillator (TCXO).

Now there is a new USA based source for modified TCXO dongles over on Ebay. The new dongles are modified with a 1 ppm high stability TCXO and sell for $65 USD. The manufacturer of these modded dongles has been kind enough to send us a sample and we will have a review of the product up in a few days.

The oscillator on the RTL-SDR is prone to thermal drift, which means that as the dongle heats up from use, the frequency you are tuned to may change over time. A TCXO compensates for differences in temperature and thus keeps the frequency stable as the temperature changes.

December 18, 2013

Review of the TCXO Modified RTL-SDR Dongle

A few days ago we had a post about a modified RTL-SDR dongle for sale which has the low quality 28.8 MHz oscillator replaced with a high quality 28.8 MHz temperature controlled oscillator.

Nobu Saitou, the creator of these dongles has sent us a sample to review. On the inside of the dongle the 28.8 MHz crystal has been removed, and replaced with a 28.8 MHz temperature controlled oscillator. The desoldering of the old oscillator and soldering of the new TCXO appears to be neatly and professionally done.

After plugging in the dongle and firing up SDR#, we tuned to a known trunking control channel at 152.850 MHz and measured the frequency offset. It turns out that with the TCXO no frequency correction was required at all. I believe that from Nobu’s blog post, this oscillator can have a max deviation of +-2 PPM, which is incredibly small.

TCXO Dongle with Zero PPM correction required

We compared this result to a standard dongle with the original oscillator and found the frequency offset required to be 44 PPM.

As the dongle heats up from use, the oscillator will experience thermal drift, causing the frequency offset to change. The TCXO should be immune to this problem due to it’s temperature compensation circuitry. To test the temperature compensation, we cooled both a TCXO dongle and a standard dongle down in a refrigerator first to simulate cool climate conditions. We then measured the change in PPM offset after 30 minutes of dongle operation. As expected, the TCXO had almost zero drift after 30 minutes (<<1 PPM), whereas the standard dongle had a drift of about 6-7 PPM (approx. 1 KHz drift).

TCXO Oscillator: Frequency drift after 30 minutes

Standard Oscillator: Frequency drift after 30 minutes

The results of this simple test show that the TCXO used in these modified dongles is an accurate and stable frequency source as was expected. If you want one of these dongles they are for sale at the creators Amazon Store (direct link to TCXO product here). Currently it seems that Saitou’s products cannot be sent abroad outside of Japan, but he recommends this agent service for ordering internationally.

Edit: The TCXO can now be bought internationally from 1090mhz.com

December 12, 2013

Modified RTL-SDR with Temperature Controlled Oscillator (TCXO)

Nobu Saito, a Japanese RTL-SDR hardware developer has come out with a modified RTL-SDR which has a +-2PPM 28.8MHz Temperature Controller Oscillator (TCXO) instead of the standard 28.8 MHz stock oscillator. (Note this link is machine translated to English from Japanese).

Normally 28.8 MHz TCXO’s are difficult to find, but Nobu was able to find a source in Japan, and he is now selling on his Amazon Store (with international shipping) modified dongles.

The oscillator on the RTL-SDR is prone to thermal drift, which means that as the dongle heats up from use, the frequency you are tuned to may change over time. A TCXO compensates for differences in temperature, and thus keeps the frequency stable as the temperature changes. This is extremely useful for applications such as receiving GPS, Beacons, APT, HFDL, and for radio astronomy.

Nobu sells the modified dongle with TCXO on this page. We expect to receive a sample of his product soon and will write a review when we receive it.

Edit: Now available at the 1090 MHz webstore.