[hpsdr] Fixed SAW Oscillators

[Chris Bartram]

I'm afraid I don't think a fixed SAW oscillator is the universal panacea! Like 
Francis, I've had a certain amount of experience in that area, in my case as 
a user. Waving hands, SAW oscillators are about an order of magnitude better 
than good L/C oscillators in terms of phase noise and stability, but even 
phase-locked to a good 10MHz reference, the noise beyond the loop bandwidth 
won't be improved. 

As cliche-man said 'there ain't no free lunch'! 

The best way of making a very good 1GHz reference still seems to be to based 
on a very low noise VHF crystal oscillator phase-locked to a low frequency 
(10MHz) reference. It may be possible, to use an 'inverted-mesa' crystal at 
500MHz or even 1GHz, if you can get a good crystal at a realistic price, but 
I think I'd still bet on a fifth overtone crystal at 125MHz.  

Multiplying 125MHz to 1GHz will inevitably increase the phase noise sidebands 
on the crystal oscillator by about 18dB. There's no way around that. In 
practice, you'd also need to add a noise contribution from the multiplier 
chain. One reasonably quiet way of making a multiplier is to use a chain of 
resistive mode transistor multipliers, or balanced diode (or JFET or HEMT) 
doublers. Don't try to use a typical amateur tuned BJT multiplier chain, as 
many designs I've seen look like potential noise generators as they don't 
control parametric effects.

If it's designed well, a low-noise VCO based on a quarter-wave ceramic coaxial 
resonator could be locked to the 125MHz oscillator using a sampling phase 
detector (or a suitable harmonic mixer...). I'm still working on that. A 
phase/frequency detector isn't suitable and a divider chain would have its 
own noise contribution. If the loop bandwidth is kept reasonably large and 
the oscillator stays within the lock-in range of the loop, that could make a 
relatively simple and compact 1GHz reference.

Vy 73

Chris
GW4DGU

 1204078630.0