[1000mp] INRAD Bonus mod VS MkV EDSP CW demodulation.

[lhlousek]

I just made some before and after comparisons of the INRAD "Bonus" mod which consists of adding a .047 mF cap across C3015 on the audio board.  I used an audio spectrum analyzer and my ears to make the comparison.

Prior to the mod, the radio exhibited an unacceptable amount of hiss without EDSP CW demod enabled.  After the mod, EDSP demod is still better but the RX is reasonable with it off too.  This is most evident listening to week sigs on quiet bands.

On my rig, this mod only had an effect only in CW mode and then only when the EDSP CW demod (menu 7-7) was turned off.  On SSB, or on CW with CW Demod on, there was no affect at all.  I did not look at any digital modes.


The audio board is located on the bottom of the rig and C3015 is a through-hole film cap on the top of and close to the center of the board.  I had some small clip leads that I was able to attached to C3015 and use to quickly switch between several values including .022, .033, and .047 uF.  All values substantially reduced high frequency hiss with .047 having the greatest affect, although not much different audibly or on the spectrum analyzer than .033.  I chose to go with .047 since it only affects CW and tacked it onto the bottom of the board.

On CW NOR (1800Hz INRADS) and with EDSP demod off, the passband is tilled downward toward higher frequency going down about 10dB at ~2400Hz compared to 800Hz.  This is not really a problem on CW.  There is also a noise floor that extends well beyond the passband cutoff.  This noise floor is the objectionable hiss.

EDSP CW demod has the effect of flattening the pass band so there is no drop-off toward the higher frequencies.  It has a sharp cutoff at ~1800Hz  that also takes out the noise floor above this frequency.  Of course, this cut-off is introduced in the baseband so signals above this cutoff, but still within the IF passband, will pump the AGC.  Adjusting the Width control to align the upper IF passband edge with the EDSP demod cutoff solves this problem. 

Adding the bonus mod has the affect of increasing the downward tilt of the passband so that it is more like 25 dB over the range 800 to 2400 Hz.  This still sounded fine listening to actual signals because it only attenuates the sigs that are far above a signal beat with your side tone.  It also greatly decreases the noise floor above 2400Hz but not to the same extent as EDSP demod.

I have a MS Word file with the spectrum analyzer screen shots.  I'll email it to anyone interested in seeing them.

Lou


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