Paramithiotti, Luciano Paolo S wrote:
http://www.timeok.it/files/5_to10_mhz_advanced_doubler.pdf
Best regards, Luciano
Luciano P. S. Paramithiotti
The claimed 50 ohm input impedance is incorrect except perhaps for small
signals.
For large signals the input impedance will be much lower essentially the
input transformer equivalent series resistance plus the transformed
large signal emitter resistance.
The key to low phase noise is low base to collector dc gain and local RF
feedback (largely due to the source impedance in this case).
The circuit is essentially the BJT equivalent of the NIST common gate
JFET doubler.
Common emitter variants are also useful especially since the input
impedance can easily be 50 ohms if a suitable termination resistor is used.
As pointed out by NIST decades ago, using traps for the unwanted
harmonics improves the phase stability and phase noise over the
alternative of using a high Q bandpass filter.
NIST have recently explored the possibility of using an all digital
implementation of the DMTD at least for clock comparisons.
The mixers are replaced by undersampling ADCs to produce an aliased beat
frequency signals from which phase differences can be extracted by DSP
techniques:
http://tf.boulder.nist.gov/general/pdf/2442.pdf
Bruce
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