Henk ten Pierick wrote: > Bruce, > > 1. Best wishes. > > 2. It has taken some time but here is the schematic. > > Henk > Henk
The circuit diagram helps a lot. 1) Since the crystal current is 1mA the RF voltage across R2 (and the RF at the input of the HC04) is 220mV rms (622mV pp). This is a little low as HC04's are a little noisy a higher input voltage slew rate should help. Ideally about 1.75vrms or so (just avoids clipping by the HC04 input protection diodes) should significantly reduce the noise contribution from the HC04. 2) Emitter follower Q4 has a relatively low collector base voltage which increases the collector base capacitance and associated phase noise. AC coupling the emitter follower would allow a much higher collector base voltage, and an inductor in series with the emitter resistor will allow the emitter to swing below ground. 3) Your AGC circuit seems to control the feedback voltage/current at the oscillator transistor base. An AGC in a crystal oscillator is typically used to control the crystal current, you circuit doesnt appear to do this directly. 4) The emitter follower will not drive a 50 ohm load without severe distortion unless the emitter current is increased. Its not just the output impedance (with zero input) that is important. 5) As Rick stated a common base buffer circuit would have better performance. A common base circuit has no difficulty when driving a 50 ohm load. A step down transformer between the common base collector and the load increases the RF voltage across the load. 6) A good buried zener has lower noise than the ADR445. The noise specs for the ADR445 are somewhat sparse in that the spectral distribution of the noise isnt given. One can always reduce the noise by averaging the output of several references 7) Unless you need a tuned circuit to supress oscillation at unwanted crystal modes, then using a circuit without one will provide better temperature stability. If the tuned circuit uses ferrites then they may increase the flicker phase noise. 8) If you do need a tuned circuit to suppress unwanted crystal modes then one (there are many) of the Driscoll crystal oscillators is a good solution. 9) The minimum ft of Q1 is a little low. I will create some circuit schematics for crystal oscillators that control the crystal current more directly and use a common base output buffer. Meanwhile look at: http://www.ko4bb.com/~bruce/LPND.html <http://www.ko4bb.com/%7Ebruce/LPND.html> This crystal oscillator circuit is a variant of the low noise oscillator proposed by Wenzel for use with fundamental crystals. It uses a common base buffer and demonstrates several low noise biasing techniques that can be used. The 2nd and third references above the oscillator schematic explain the mechanisms for generating AM and PM noise in a BJT RF amplifier. The derivation is quite mathematical (statistics and calculus) but the conclusions are relatively simple. To drive an HC04 the common base buffer can use a load consisting of an inductor shunted by a resistor to develop the drive. The shunt inductor reduces the dc gain (from base to collector) of the buffer stage and hence the low frequency noise voltage developed across the collector base capacitance. Such noise voltages modulate the output capacitance and hence the phase shift of the buffer, increasing the buffer phase noise. Nonlinearities such as hfe variation with current tend to increase the buffer output AM noise not the PM noise. Bruce _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
