[time-nuts] NIST frequency doubler

[Dr Bruce Griffiths]

Said
To give some idea of the transformer ratios required a simulation using 
pSpice shows that:

When using 2 x 2N4393's connected in parallel (limited choice of JFETs 
available) for each JFET in the NIST circuit, an  input transformer with 
a 1:2 turns ratio with centre-tapped secondary and an output with a 2:1 
turns ratio is about optimum. Input is fed from a 5MHz  2Vrms source 
with a 50 ohm source impedance (develops 1V rms in a 50 ohm load) source 
resistor is 100 ohms. Input transformer primary magnetising inductance 
was 50uH. Output transformer secondary magnetising inductance was 50uH.
Output is 1V rms into a 50 ohm load. Output transformer primary dc 
current is ~ 20mA.
 
To allow adjustment for individual FET characteristics the input 
transformer secondary centre tap can be grounded and a capacitively 
bypassed resistor connected in series with the source of each JFET. The 
resistor values are then selected/adjusted to achieve the desired output 
and minimise the fundamental component in the output. Trimpots can be 
used, initially adjust the trimpots to the required value whilst the 
specified input is applied, then measure each trimpot's resistance 
setting and replace it with a fixed resistor.

Since JFETS have a relatively large spread in characteristics individual 
adjustments are required.

Bruce