[time-nuts] Trimble Thunderbolt question, splitting its output.
David C. Partridge
david.partridge at perdrix.co.uk
Sat Mar 28 21:38:08 EDT 2015
Yes of course, you are correct!
I'm not sure how to avoid the change in input impedance with the clipper involved unless I were to increase R1 to a very much higher value.
I had intended to re-work the input clock shaper to reduce jitter if the proposed re-spin of the board had gone ahead, but as there wasn't enough interest so that's not about to happen ...
Regards,
David Partridge
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Charles Steinmetz
Sent: 28 March 2015 19:56
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Trimble Thunderbolt question, splitting its output.
Dave wrote:
>??? How do you get to that 45 ohm figure. The input has a pair of 100R
>in parallel, so I can't see how it can be 45R input impedance.
Also in parallel with the 2x 100 ohm resistors (R2 and R3) is 10k
(R5) in series with 100nF, plus 475 ohms (R1) to a pair of (parallel) back-to-back diodes in series with a 100nF capacitor (C5). Both 100nF capacitors essentially sit at the half-supply bias voltage (there is a very small AC component on C3, ~1mV, which can be ignored). So, for peak input voltages of less than a diode drop, the input impedance is ~49.75 ohms (100||100|10k). But for peak voltages greater than a diode drop, the input impedance is 100||100||475||10k, or about 45 ohms. (There is a slushy transition zone of ~100mV as the diodes turn on.)
The TBolt puts out nominally +13dBm, or 1Vrms (2.8v peak-to-peak, 1.4v peak) into 50 ohms.
So, the TBolt sees a nonlinear load of ~50 ohms for the first ~600mV (plus and minus) of voltage excursion, then ~45 ohms from 600mV to 1.4v (plus and minus).
I'd be inclined to change R2 and R3 to 113 ohms for use with sources that put out +4dBm or more, although the practical effect in most cases is probably minor. Note, however, that if one feeds the divider and another instrument using a simple BNC "T", the nonlinearity of the divider's input impedance will raise the distortion floor of the sine wave seen by the other instrument to ~ -40dBc even if the source is perfectly pure.
Best regards,
Charles
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