[eagle] Re: New 70 cm Receiver Test Results
juan-rivera at sbcglobal.net
Sat Jun 9 07:39:15 PDT 2007
My preliminary "sniff" last night didn't detect any noise near the Can-do
module. The overwhelming amount of noise was picked up when the probe was
near L21. I could also vary the frequency by adjusting the DC input voltage
to the radio, putting both the "sniffed" and the IF output spikes on 5.0 kHz
with the same input voltage. I'll look deeper later today but I believe the
IF spurs are coming from the receiver switcher and not the Can-do module.
The spec sheet for the max1776 goes to lengths to warn about how critical
the layout is due to high peak switching currents. It also discusses
radiated noise. See the section on PC board layout and grounding. If the
supply and the RF section share the same common ground to the Can-do module
I can easily see how switching noise could couple into the radio.
Since we have unused space at the back of the board I'd suggest isolating
the supply away from the RF section and separating the power and RF grounds.
The DC from the Can-do module could be brought back to the power supply via
a short section of coax or a twisted pair to keep the DC noise isolated from
the receiver ground completely.
I spent quite a while trying to isolate the microphonic source. I softly
dragged an insulated probe across every component on the board. Some areas
of the board seem to produce more noise than others but no single component
jumps out. John, if you're right and its several ceramic capacitors that
should be film that might explain it. I'll try to zero in on this later
From: Robert McGwier [mailto:rwmcgwier at gmail.com]
Sent: Saturday, June 09, 2007 7:04 AM
To: John B. Stephensen
Cc: juan-rivera at sbcglobal.net; eagle at amsat.org; David Smith; Dave Black
(Work); Dave Black (Home); Samsonoff at Mac. Com; Juan.Rivera (Work)
Subject: Re: [eagle] Re: New 70 cm Receiver Test Results
John B. Stephensen wrote:
> Since the noise is always symmetrical around the received signal, the
> switcher noise must be modulating the VCO control voltages. There are
> two switchers - one on the receiver PCB and one on the CAN-DO module. We
> must determine which is causing the problem or whether both are. Can you
> cut the trace on the PCB and vary the DC voltage to the receiver
> independently of the voltage to the CAN-Do module?
I agree with this switcher comment and did as soon as I saw the picture.
I really hope this is not the CAN-DO module as FLIGHT UNITS have been
DELIVERED for P3E. We need to have Stephen, et. al. check the noise
output of the switcher on the Can-Do module.
> I hope that you can tap each component and find the microphionic one.
> Good candidates for microphonic components are the capacitors in the PLL
> feedback loop. The capacitors in the integrator are film types so they
> shouldn't be microphonic but perhaps SMT parts aren't as good as leaded
> parts. The ceramic capactors in the feedback path might also cause
> problems and could be changed to film capacitors. Another
> possibility may be the 4.7 and 10 uF bypass capacitors on the outputs of
> the linear regulators. These are ceramic because of concerns about
> tantalums in space but they might have good piezoelectric properties and
> could be switched to 1206 size tantalum capacitors. There could also be
> a cold solder joint somewhere.
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TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
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