TADD-1 ECO (Was: Re: [time-freq] RE: time-freq Digest, Vol 11, Issue 4)

[John Ackermann N8UR]

OK, I made the following changes:

R4 changed from 10k to 1k

R6 removed

R1 and R8 both changed from 15k to 47k

and it made a dramatic difference -- more than I was expecting.  The
higher impedance of the bias network must have some other effect that I
haven't worked through yet.

Now, with +7.6dBm input at 10MHz, the maximum output is +13.7dBm (all
measurements were made with JP3 on and a 50 ohm load on the output).
The output with the gain pot set to minimum is +6.9dBm.  That makes the
gain range -0.7 to +6.1 dB.  Much better than before!

However, at this input level with the gain pot set wide open we run into
U1's clipping point.  Looking at the output on the scope, with a 50 ohm
load, the maximum output is 2.96 volts peak-peak but there is definite
clipping.  Backing the pot off about 45 degrees yields a nice sine wave
and drops the output level by about 1.5dB.

As another experiment, I attached a 1db step attenuator to the input.
It took 3 to 4db of attenuation to make the waveform look nice and clean
at maximum gain; I used 4dB for the test so that's +3.6dBm.  At that
input level, the output with minimum gain is +2.8dBm and with maximum
gain is +11.3dBm.  So, with an input signal just below the clipping
level the gain range is -0.8 to +7.7 dB.

The message here is that with the modification for greater gain, you
will need to watch the input level if you want to avoid clipping and the
resulting odd-harmonic distortion.  The maximum input for no clipping at
maximum gain is about +3dBm at 10MHz.  If your drive is above that level
and you care about the waveform, it's worth looking at the output on a
scope as you adjust the gain setting (unless you are running the gain at
a fairly low setting).

I'll update the TADD-1 assembly manual with this new information, and
all future kits will include these changed resistor values.

John