[Premium-Rx] Cubic HF-1030 followup

[VK3XYZ]

Hi all,

Further to my post on the HF-1030, it was suggested that the switchmode regs
I'm using may cause some unwanted RFI, something I didn't check initially.

Unfortunately, they do. The IC on the board operates at around 500kHz and
despite there being  reasonable capacitance at the regulator there is a
signal detected by the receiver at around 507kHz (~ -100dBm reported signal
strength for 1 off of the regs fitted) and at twice that frequency - around
15-20dB lower, at a guess.

It appears to be conducted emissions back through the 12V rail to other
modules. I can see about 5mV at the 12V rail on the backplane with the
oscilloscope in fft mode. Placing a low resistance 6.8uH inductor in the 12V
rail of the module has eliminated the spurious signal on the receiver and on
the oscilloscope (reduced it about 150uV). So I'll make that mod to both
modules and it should be good to go. When time permits I'll investigate
further & try improving the filtering on the 12V rail, might try and source
some solid electrolytic caps to replace the low esr ones I used, they should
have an esr about an order of magnitude lower than what I'm using now
(perhaps 2 orders - they claim milliohms ). 

 

Cheers

Greg