[hpsdr] More Mercury

[Phil Harman]

I've been able to interface the LT2208 demo board to the Xylo and hence to 
PowerSDR. The screen shot at

http://www.hamsdr.com/dnld.aspx?id=328

show a +10dBm signal at 25MHz directly into the LT2208.  With the image 
amplitude and phase controls at 0 and 0 respectively there is no visible 
image.

The FPGA interface to the LT2208 is very crude at the moment. I've using a 
fixed NCO at 25MHz followed by a CIC filter decimating by 2048 since this 
gives ~48kHz from the 100MHz clock. You can see the sinx/x shape of the 
noise floor, the CIC needs following with a compensating FIR to flatten 
this.

The CIC is also very simple, this is the design file from MatLab

// Discrete-Time FIR Multirate Filter (real)
// -----------------------------------------
// Filter Structure        : Cascaded Integrator-Comb Decimator
// Decimation Factor       : 2048
// Differential Delay      : 1
// Number of Sections      : 3

I've chosen the input frequency on the screen shot to get rid of all of the 
spurs - there are still lots of them but I guess a better CIC and FIR will 
clean a lot of them up.  The bandscope vertical scale is wrong since I am 
sending 16 bits from the LT2208 and PowerSDR is expecting 24. Still it 
lools like we are getting the full 16 bits from the LT2208 since you can see 
about 96dB of DR.  What is intersting is that with no signal input the 
output from the LT2208 is 0, the LSB never moves, so the internal noise 
looks very good.

We are going to need a decent pre-amp for the high bands but even so the 
initial results look quite good.

Must sleep!!

73's Phil...VK6APH




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