L'amplification de signaux sur 2,4GHz
The amplification line
Now, I propose to focus on the transmission and amplification equipment, individually.
First element, the SDR transmitter
As indicated in the introduction, I only deal here with the case of an SDR transmitter. The power considerations obviously remain the same for a conventional up-converter transmitter.
The LimeSDR is an open-source SDR receiver and transmitter USB stick, to be connected to a microcomputer, and covering frequencies from 10MHz to 3.5GHz (for the LimeSDR mini and LimeNET-Micro version) or from 100kHz to 3.8GHz (for the LimeSDR version) and operates in full duplex (simultaneous reception and transmission). Its maximum output power measured at the output of LimeSDR mini is 3 dBm (2mW).
Other SDR transmitters / receivers are possible, such as Adalm Pluto from Analog Device (Amazon Link). A modification to improve stabilization is required for narrowband transmission (SSB). I invite you to refer to this modification of the Adalm Pluto of Lucien F1TE’s french article which concerns the replacement of the cristal by a TCXO whose temperature derivation is compensated. Another alternative is to provide a stable 40 MHz reference signal using a GPSDO (GPS disciplinated oscillator) generator
These SDR transmitters can be used for narrowband transmission but also as a broadband transmitter for DATV digital television (Maybe the subject of another article later ?).
Before mentioning filters, I would like to recall some points of the regulations concerning the amateur band 2300 – 2450 Mhz. The amateur-satellite service may operate in the bands 435-438 MHz, 1260-1270 MHz, 2400- 2450 MHz, provided that this does not result in harmful interference to other user services. In these bands, the amateur service has a secondary status and any harmful interference caused by satellite emissions must be immediately eliminated (RR provision S5-282). As a reminder, the maximum transmitter output power is 120W (50.8dBm).
On the 2400 MHz band part, we will find public uses such as bluetooth and wireless networks Wifi 802.11b/g/n. It is therefore important that spurious emissions are properly rejected at low levels, and at least to those expected by the regulations [ APPENDIX 3 of the Radio Regulations for ground satellite stations: 43 + 10 log (P) or 60 dBc minimum]
It is therefore advisable not to drive the amplifier circuits into their non-linear operating area and to complete the stations, especially SDR, with a 2400 MHz band pass filter after the transmitter.
- A cheap filter exists on the Asian reseller market: the 2400 NMRF FBP-2400, with significant losses (4dBm) on 2400 Mhz and its Return Loss -20 dBm (SWR 1.2) Links. Links Aliexpress ou ebay.
- A second one is available in Canada, and sold by GPIO Labs: 2450 GPIO LABS. These losses are less significant (2.4dBm) for the same RL than the previous one. Link to Ebay.
- There are other more professional filters where losses are reduced:ID-Elektronik in Germany
- For do-it-yourselfers, there is a solution that consists in building a resonant cap “Pipe-cap filter”
On the diagram, after the transmitter, you will find a pre-amplification stage for the signal. In this diagram, it is a CN0417 board from Analog Device that is equipped with a two-stage, 1W, two-stage ADL5606 wideband amplifier (see ADL5606 datasheet), which operates over a frequency range from 1800 MHz to 2700 MHz. It integrates an in-line band-pass filter and will provide a 30dBm level output with the LimeSDR mini thanks to its real gain of 20 dB on 2400 MHz. It is powered by a micro-USB plug. Its maximum input level is 18 dBm.
Second possibility. The SPF5189Z is a 5V powered preamp that offers 8 to 11 dBm of gain. You will find a detailed measurement report on the Matthias DD1US website. My measurements give me 11 dBm of gain on 2400 MHz. According to the SPF5189z datasheet, the maximum acceptable input is 27 dBm. You can therefore completely cascade two preamplifiers, one after the other. The Chinese reseller market offers them for less than 5 EUR (link SPF5189Z Ebay, link SPF5189Z Aliexpress).