SIGINT direct digital receiver

SIGINT direct digital receiver has the Analog-to-Digital Converter (ADC) directly connected to the incoming RF signal without any frequency translation. However this is contrary to the super heterodyne or direct conversion methods. They translate the RF signal to an Intermediate Frequency (IF) or the Base Band (BB) before digitizing.

How is Direct Sampling Realised?

Prior to the advancement of converter technologies in recent years, direct sampling architectures were not practical. Therefore of the limitations in converter sample rates and resolution. Semiconductor companies have been able to expand resolution at higher sampling frequencies. Using new techniques they reduced noise within the converter. With the availability of much higher-speed converters featuring increased resolution, you can directly convert RF input signals up to several gigahertz. This conversion rate enables digitization with very wide instantaneous bandwidth at the L- and S and even higher bands. [ Ka-band direct digital receiver ]

Instantaneous bandwidth

Instantaneous bandwidth is the frequency band that is continuously processable by the digitizer device. The sampling frequency determines the bandwidth. Half of the sampling frequency is often called the Nyquist-frequency. The maximum bandwidth of the processable signal should be less than this Nyquist-frequency. In accordance with the Nyquist-Shannon sampling theory

Input frequency range

However, the analogue front-end and the sample-and-hold circuit determines the input frequency range. If the limit imposed by these circuits is higher than the Nyquist-frequency, we have a chance to sample higher frequency signals as well. This is usually called under sampling or sub-Nyuist sampling.

UHF Input frequency test

The input frequency range of our receivers is more than few hundred MHz.  For instance we connected the 432 MHz output of the signal generator to the input of the SRM-3000 direct digital receiver. After tuning to the same frequency we got good look at the signal. We observed that the level is more than 20 dB less than in the HF band. In other words it means we need at least 20 dB pre amplification to maintain the sensitivity in the UHF band.

UHF SIGINT direct digital receiver

input level for VHF UHF SIGINT direct digital receiver

Sensitivity testing

The measurement was cross checked the sensitivity of the SRM-3000 receiver with FT-897 transceiver. We connected a signal with a -120 dBm output power generated by the lab RF signal generator shown above. The signal had a well audible sound level employing the Single Side Band (SSB) demodulator with some audio low-pass filtering on booth receiver.

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