Re: a simple spur fix

Jerry Gaffke
 

A quick recap, as I recall the spur fix thing thing thus far:

Allison noted that the LO's had too low of an injection level, the mixers were overloaded with signal,
the 45mhz IF amp was not amping quite enough, there was coupling from the power amp into the IF's
due to poor board layout, and that most multiband HF rigs would have a filter for each band of use at L1,2,3,4.
All of this contributing to spurs, and suggesting a significant redesign was needed.
 
Raj then found that the spurs were reduced by inserting a 45mhz crystal filter in series with C22.
Alternate placements and some impedance matching schemes were proposed, in part to bring the transmit power
back up to what it was.  Resonant freq of any new 45mhz filter would have to match the old 45mhz filter at Y1.
This fix seemed to bring spurs down to where they about met regs in most cases, but still borderline.

Then Raj found he got better results by replacing L5 and L7 with a particular brand of 1210 sized shielded surface mount inductor.
Then found that the same treatment to L1,2,3,4 made it even better.
So forget the 45mhz crystal filter.

Now Farhan says forget all the above, he's doing better still with a couple series LC traps
at TP13 and C202 for 90mhz and 12mhz respectively.
And the discussion continues from there.

By all means, jump in and try any of the above if so inclined
and you have the instrumentation to measure spurs. 
Otherwise, sit on your hands a bit longer.
Allison is right, a complete redesign of about half the uBitx is not a bad idea.
But Farhan and Raj appear close to finding a spur fix for v3 and v4 uBitx boards that is very easy and cheap.


Spurs are at frequencies more or less unrelated to the operating frequency.
On a stock rig, in some cases they can be stronger than allowed by regulations, though at QRP levels
are reasonably low compared to the typical 100W hf transceiver.

Harmonics are a separate issue, solved by either adding external low pass filters
or hacking the relays and traces around the existing transmit LPF's.

Jerry, KE7ER
Jerry, KE7ER



On Sat, Nov 10, 2018 at 10:09 AM, <behlmer@...> wrote:

I remember a relatively simple fix using a 45 MHz crystal across 2 points with a ground on the center wire. Has anyone done this?  What were the connection points?  Thanks. Steve. KE7GO 

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