[time-freq] TADD-1 Modification

[John Ackermann N8UR]

Yes, that's the only change to the circuit.  There have been several
updates to the manual, but nothing that affects the unit itself.

John
----

Eric Lemmon said the following on 09/09/2006 11:35 AM:
> John,
> 
> Are the below-listed changes the only ones made since first release?  I
> bought the TADD-1 kit a while back, but haven't had the time to assemble it.
> When I start assembly- hopefully in the new few weeks- I'd like to
> incorporate all of the mods and bug fixes (if any) to date.
> 
> 73, Eric Lemmon WB6FLY
> 
> 
> -----Original Message-----
> From: time-freq-bounces at lists.tapr.org
> [mailto:time-freq-bounces at lists.tapr.org] On Behalf Of John Ackermann N8UR
> Sent: Saturday, September 09, 2006 8:02 AM
> To: TAPR time and frequency projects
> Subject: TADD-1 ECO (Was: Re: [time-freq] RE: time-freq Digest, Vol 11,
> Issue 4)
> 
> 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
> 
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