A TCXO is a temperature controlled oscillator. Its advantage over a standard oscillator like the one used in a normal RTL-SDR is that its frequency will not drift as the temperature of the dongle changes.
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.
TCXO close up
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.
Control RTL-SDR Drift over 15 Minutes (Sped up)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.
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.
