The RTL2832 Noise Figure is not so good (on the internet I read about 7dB), so one way to increase the receiver performance is to place a LNA as near as possible to the antenna.
I searched on the internet, and I’ve found the LNA4ALL a very good solution. It also has an internal bias tee in order to be powered directly on the coax cable.
Nevertheless, it’s not very cheap and the shipment expenses are too high, then I decided to try first a SPF1589Z LNA for few euros from ebay, then decide if throw it away or keep it.
It arrived in about three weeks from China, then I wanted to test it on a Vector Network Analyzer, in order to compare the real gain with the datasheet.
Once the LNA has been powered at 5V, the current absorption is 90mA, perfectly aligned with the datasheet range.
Here below, the S21, S11 and S22 plots are reported between 50MHz and 3GHz (the LNA reaches 4GHz but the VNA not).
I palced these 4 markers:
When I downloaded the datahseet, I saw that the SPF5189Z has two possible configurazion:
- Optimized matching at 900MHz
- Optimized matching at 1900MHz
From ebay description and board P/N, it’s not possible to understand the frequency optimization.
Here below, I reported a comparison the datasheet (green line) and the measurement (red line) data, for both configurations. The frequency range of the datasheet is smaller than my measurements, sto the comparison is only on the datasheet frequencies and not wide-band.
The gain seems to be 1dB less than the datasheet on both configurations, while the input matching shows better results on 900MHz range.
At the end, the LNA is quite wide-band and is a compromise between the 900MHz and 1900MHz configurations.
In the next days, I will install the LNA near the antenna and I’ll let you know the improvements on the receiver.