[Premium-Rx] RA6830 phase noise fix

[Carcia, Francis A HS]

A While ago Brian Comer posted pictures of phase noise coming out of the
RA6830 synthesizer. I decided to fix it over the long weekend. This is
only a problem with RA6830s with the linear power supply. Well worth the
couple hours work. Frank

________________________________

From: Carcia, Francis A HS 
Sent: Monday, November 27, 2006 9:54 AM
To: 'ToddRoberts2001 at aol.com'
Cc: 'K2ors (k2ors at comcast.net)'
Subject: RE: 6830 phase noise fix


Hi Todd,
Well success! I mounted a 7808 in the bottom of the power supply behind
the slot for the ribbon connector. There is a series of cooling holes
just big enough for a 4-40 screw. I mounted a pair of 1 uf caps on the
regulator with 3- 8 inch wires heat shrinked over the leads.
Then cut the trace to the connector and wired the new regulator in. The
trace is on the top surface of the board so didn't even have to remove
it from the heat sink. I picked up ground near the cut. I did a number
of careful tests duplicating Brian Comer's spectrum analyzer set ups. My
results were pretty much the same as Brian's except for very close in.
My old HP141 didn't have the dynamic range to see the level of the noise
but surely less than when the ripple was present. I plan to modify the
power supply in the hot rod 6830 and measure phase noise ARRL method
since it has the filters to get accurate numbers. The ripple into the
synthesizer was under 10 mv (down from 1/2 volt peak to peak) even with
the other regulators under A6 and front panel hanging off the bus.
Then I plugged the module into a radio with a switcher and ran the same
tests with very similar results. 
There is plenty of room inside the power supply you just have to be
careful routing the leads as you close up the module. 
I figure the phase noise dropped close 15 dB for harmonics of 120 Hz
side bands which are gone now. 
Well worth the couple hours of work to get a linear as clean as the
switcher for phase noise. 
TNX for the file a good one. I'll post something in Premium later.
fc

________________________________

From: ToddRoberts2001 at aol.com [mailto:ToddRoberts2001 at aol.com] 
Sent: Sunday, November 26, 2006 2:38 PM
To: Carcia, Francis A HS
Subject: Re: 6830


In a message dated 11/20/2006 12:37:15 PM Eastern Standard Time,
francis.carcia at hs.utc.com writes:
Hi Todd,
I've convinced myself that the power supply ripple it the big hitter for
phase noise using the linear supply. Now to decide where to drop a 8
volt regulator. The nice thing is the unregulated 5 volts is a wire
across the bottom so it can be on the motherboard or inside the power
supply. 
I had the synthesizer up on an extender and put a big cap across the 5
volt unregulated while watching the phase noise and it dropped 10 dB
close in. I just need to take a look at another chassis with a switcher
to see how high the close in phase noise is compared to the linear. I
might also try a 120 Hz. trap later if there is still noise. 
After all you look at Brian's pictures with the radio running on battery
power and the noise is way down. fc
 
Hi Frank,
I have enclosed an article from Maxim on a buffered voltage follower
used to clean up noisy DC on a 5 Volt line for an extra quiet audio
preamplifier.  Not sure if you have seen anything similar to this but
thought it might give you some ideas on a method to get real clean 5VDC
for the 6830.
73 Todd
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