Jerry is right.
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However on design review the reason for that is interesting.
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Simple answer too much gain in the low level parts of the radio for the TX path.
This is not a requirement of the architecture of the transceiver.? ?Turns out that
loss total is about 18db (two mixers at about 7db each and the filters at about
4db each) so a total of 18+7=25db is exactly what we need. so each stage
at about 12.5db is enough, we have easily 35db or more.
This is the core of the issue, carrier leakage plus signal at 45mhz?
makes the 45mhz mixer very rich in spurs. unless we keep the levels down.
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The transmitter has much less need for gain to get clean signals.
If we have too much we end up with the first mixer the modulator leakage
as carrier.? we want as much or that mixer as we can get or about -17dbm?
as it works out.
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The total loss (mixers, filters) is about 18db, theres about 34db of gain!
To both mixers are being pushed hard with even 50mv(70mvPP) of audio
at the mixer not the mic input.? The balanced mod should be able to take?
200mv PP (140mV RMS) of audio and if run at that level its output is clean?
with headroom but the 12 and 45 mhz mixers are being run due to the existing?
gain at very high levels hence the spurs.? And we needed to be close that?
limit for full power resulting in high order spurs that seemed random and?
well as low order spurs.? I had to really lower the audio drive (well?
below 50mV)to drop the spurs and it resulted in carrier showing even with?
a reasonable BFO offset.?
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That resulted in an experiment. With TIA style amps the gain can be easily?
dialed down with feedback.? So with a few measurements in hand I dial the?
first amp (12mhz) down to? insure that and full modulator input the input
to the 12mhz to 45mhz? mixer is not more than -10DBM.? The 12mhz crystal?
filter has a loss of 4db so I need 12db of gain to hit -10dbm at the mixer.? ?
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Then I repeat that and its about 4db for the 45mhz filter with matching?
network so we need another 11-12 Db of gain to keep the 45mhz mixer?
happy at -10dbm in.? The result is -17dbm output and the spurs are?
not less than 53Db down and most are more like 60-70, much cleaner.
Also the carrier to signal ratio is improved by about 10db, also better.
Yes we also need more audio drive but only little and its easy to do and?
we get a bit more headroom this way.
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Here the annoying part, to get full power at 80m we are about ok.? At 10M
we are about 11db short of gain.? Maybe a tad more as the 30mhz filter?
eats a db or so.? So fixing the spurs by improving the gain distribution helps
clean things up but power is still wanting.
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We go from about -18- to -19dbm at the mixer output to +40dbm(10W)
We must have 58 to 59db of gain and a stock power amp chain does?
that only on 80m with a little margin about 3db.? As we go up the amp?
as it exists has less gain.
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So now we have killed the spurs mostly and improved the carrier leakage.
But the amps is back in line as "the project".
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First step is to flatten the gain to less than 3dB slope and then add gain
to get the power up.? Adding gain will be some form of added stage.
at the RV1 point.
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That is the long story of why we have spurs.? We just put too much?
into the mixers.??
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Allison
kj6etl
Allard PE1NWL