# Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review
## 3 State of the Art of Sky Wave Sounding
### Whisper
#### 3.4.3 Measurement Setup  
WHISPER sounder is mainly composed of a wideband direct sampling HF transceiver, which is a software defined radio capable of digitize the whole HF band and downconvert to basedand and demodulate the channel of interest. It is designed as a digital HF radio modem on which an HF channel sounder can be implemented. The flexibility of this platform is based on the fact that its functionality is defined through the download of application software and Programmable Logic Device (PLD) configuration. The main parts and features of this software radio platform are the following:  
* A PCI interface allow the transceiver to be installed on a host PC.
* A digital Transceiver Bus Arbitration based on a CPLD from Altera. It manages the interface to the local bus side of the PCI interface device, manages the bus interface to the DSP sub-system and allows software configuration of a processing FPGA.
* A Digital Signal Processing (DSP) sub-system. It is mainly composed of a dual Super Harvard Architecture (SHARC) processor (ADSP-21060) from Analog Devices module with a 2 Mbyte \(\times\) 48 bit wide shared SRAM. This module undertakes all tasks related to the digital transceiver application and benefits from specific capabilities of SHARC processors: internal architecture able to process 80 million floating operation points per second (MFLOPS), high speed link port interfaces, high speed serial ports and optimised multi-processor cluster computing.
* A 100k gates FPGA is used to perform high speed or time critical processing in hardware and also allows data path to be configured as required for any particular application.
* 4-channel digital HF receiver and 4 channel digital HF transmitter exciter.
* Digital interfaces including synchronous and asynchronous serial data interfaces  
The transceiver includes a very important part, that is, a mixed analog and digital front-end which is responsible for the feasibility of a direct sampling HF receiver (see block diagram in Figure 7). Its main parts are:  
* Front-End Protection and Antialiasing filter. It is a 13 th order double stage elliptical low pass filter (LPF) with a 28 MHz cutoff frequency and with 120 dB selectivity well beyond 500 MHz.
* An RF switch allows the receiver to be switched between a number of sources.
* A digitally controlled attenuator (AT65-0223) and a fixed 20.5 dB gain RF amplifier(SNA-586) with a noise figure of 5.5 dB and an input \(-1\) dB compression point of +19 dBm.
* A 5 th order Chebychev harmonic LPF with a 30 MHz cutoff frequency reduces the level of internally generated out-of-band signals reaching the ADC.
* A 14b ADC from Analog Devices (AD6644) sampling at 62.208 Msps combined with a narrowband dither source reaches a Spurious Free Dynamic Range (SFDR) higher than 110 dB. It allows a Blocking Dynamic Range (BDR) of 114 dB inside a 3 kHz bandwidth.
* A Digital Down Converter (DDC), from Graychip GC4014 DDC, composed of a digital mixer and several decimation filters selects and downconverts the channel of interest.  
This front-end architecture allows to reach the following receiver characteristics: sensitivity of \(-113\) dBm, input 3 rd order interception point of +19 dBm, input 2 nd order interception point of +27 dBm, blocking dynamic range within a 3 kHz bandwidth of +110 dBm and a noise figure of +17 dB.  
Figure 7: Block Diagram of WHISPER RF Front-end Direct Sampling HF Receiver