[hpsdr] HPSDR Signal Overload ( In the digital processing???)

[Phil Harman]

Hi Mike,

Thanks for sending me your screen captures.  As far as I  can see there is
a few dB increase in noise floor between the two images.

We need to determine if this is due to a hardware fault with Mercury, a
bug in the DDC FPGA code or the PC software, or external signals.

I suggest you first check your Mercury board. Do this as follows.

- calibrate Mercury on say 20m using a known signal source.
- connect a 50 ohm load directly to the antenna socket on Mercury. I use
one of old 50 ohm BNC Ethernet terminators.
- select preamp on, CW, 500Hz bandwidth and Average on the S Meter.
- Note they S Meter reading. If your Mercury board is working correctly
this should be in the region of -133dBm.

When you next see the increase in noise floor place the 50 ohm load on the
antenna socket and check the S Meter reading on the same frequency that
you originally did the calibration.  If you see the same noise floor then
the analogue part of Mercury would appear to be OK.

I have seen the effect you report, where a single in or out of band signal
increases the noise floor, when the PowerSDR Noise Blanker is in circuit.

This generates intermodulation products that increases the noise floor.

I have also seen the noise floor increase due to wide band noise from
local appliances e.g. SMPSU, air conditioning etc.

Anyway let's start by checking the analogue portions of the Mercury receiver.

73 Phil...VK6APH



> ***** High Performance Software Defined Radio Discussion List *****
>
> When I first saw this problem and did a couple of screen captures the
> band noise was quite high.  I turned preamp off prior to the screen
> captures.  I also verified that the adc overload light was not lighting.
>
> The problem was being caused by a single transmission from an additional
> station within my 192 kHz bandwidth out of the 61.44 MHz total band
> width.  It doesn't make sense that one station within my greatly
> processed, filtered, and decimated display window would be adding enough
> energy to the total 61.44 Mhz spectrum so that adc overload produced the
> results I described.  Further, I have an analog LP filter that is down
> 80 dB at 41 MHz.  This could be the result of an overflow in the digital
> signal processing.
>
> Since my last post to the forum I have seen the same conditions when the
> band noise was substantially lower than during the original observation.
>
> I sent an email to Phil Harmon and Jeremy McDermond earlier today that
> had my screen captures attached.  Let me know if you are interested in
> them.
>
> Mike Fager, K7SR
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