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.

TCXO Close up
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.

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.

6 comments

  1. Josh

    “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.”

    Does it have to be a 28.8 MHz oscillator? Could one a cheap, off-the-shelf TCXO with, say, a 26 MHz frequency? Software could be adjusted so that displayed frequencies would be correct.

    • Tehrasha

      I asked this same question a long time back, and never got a definitive answer. There was some speculation that there were internal chip clocking issues with the dongle that would be thrown off, but no actual testing was sited. But since the core rtl_* driver includes a variable for the crystal frequency, that implies that it -is- changeable. If I had a spare dongle, I would give it a try just to see what happens.

      • Tehrasha

        Too bad the NooElec page doesnt show the TCXO. The dongle shown only has the factory 28.8 crystal. I would like to see a pic with the end cap removed so I can see how the dongle is mounted. I am surprised that they would go thru the trouble of soldering on a TCXO and mounting in an aluminum hard case without also using a bulkhead SMA or similar for antenna connection.

        NooElec are a good company. I will likely get one of the Mini+2 when they are available.

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