Scope For Design

One thing I've been needing for a while is a useful oscilloscope for digital and RF work. Having a limited budget and a fair idea of the project I needed to work on I decided to get a Chinese Digital Storage 'Scope with a 50MHz working bandwidth. Quoted as a 500 megasample per second device it should work at well over 50MHz. Ideal for the project in mind.

I ordered it from a Chinese dealer on Ebay and it arrived in about 7 days which was exceptionally good delivery speed through China Post. For a total delivered cost of $300 Au I was very happy. Now to see how well it performs.

I'd looked at a few reviews from various Tech Review sites and while some had commented on it being good value for money, others had remarked on the disappointing performance and low screen resolution. After unboxing and doing some preliminary tests I couldn't really fault it. Even the FFT (time to frequency) Math function worked as advertised.

The Storage Scope is made by Uni-Trend and is probably on the lowest level of quality that is available. Plenty of other Chinese manufacturers make much better quality and designed products but at a much higher price.

Project Skylight - RF design

So the main reason for getting a 50MHz Digital Storage Scope is the design and development of a Low Frequency (0-50MHz) Radio Astronomy Phased Array. Project Skylight has multiple antenna elements as active RF receivers using either an RTL-SDR software defined radio or other radio receiver and doing baseband sampling at the antenna with an analogue to digital converter, putting the raw RF samples into a data stream and, along with time stamping, send them down an Ethernet cable to a central data processing system.

To keep costs low the highest frequency of 50MHz was chosen for the performance of components at the price point. The sweet spot would probably be around 40MHz but the receiver design would be capable of 1MHz to 60MHz.

The design of the whole array allows for easy scaling by just adding Active Antennas. Each antenna has a power usage limit of 500mA and power supply is through the ethernet cable using a 'Power Over Ethernet' type system. The network router can connect up to 24 antennas with total maximum power of 12 Amps (24 x 0.5A).

Each receiver has a Low Pass Filter frontend with a 3dB rolloff at 56MHz. The filter is followed by a 30dB RF gain block using a FET amplifier stage to reduced overload and a MMIC amp with a 50 ohm output impedance. A mixer stage using an SAA612 Double Balanced Mixer is also fed from either a local oscillator using an AD9850 DDS Synthesiser or a buffered RF source fed down a spare twisted pair in the ethernet cable.

By using a common source RF local oscillator to all antenna the phase of the RF signal is the same for all antenna. With the inclusion of a locally generated (in each antenna) RF source each antenna can listen at a different frequency at the same time.

The individual antenna dipoles in the array are arranged roughly in a circle about 30 metres across. Distances between antenna should be unequal.

24.01.2016. 00:06