[hpsdr] Horton LO: Further thoughts

[Chris Bartram]

I can see a few potential problems with Phil's proposal:

a/ Trying to make a decent phase-frequency detector in an FPGA isn't going to 
be that easy.

b/ you're handicapping yourselves by using a VHF VCO. To get the near octave 
tuning range required in a VHF VCO with low noise (even with pre-tuning) 
isn't easy, and you'll end-up with some critical analogue design problems. 

c/ The relatively large N-divider ratios will lead to worse close-in phase 
noise than might be possible.

If I were trying to design a synth. for an HF receiver, I'd be very tempted to 
start with a VCO operating in the 2GHz region, and to use that within a loop 
controlled by a commodity synthesiser chip such as those produced by National 
or AD. I'd then use the N-divider of a second synth. chip to divide the 
locked VCO output to HF. 

My reasons for suggesting the use of synthesiser chips rather than the FPGA 
approach include:

a/ the level of development of the PFDs - there's about three decades of 
engineering development residing in the current designs

b/ the relatively good phase noise of the dividers - see above...

c/ the potential to use a fractional/N part to get 'awkward' frequency      
spacings or to keep the reference frequency high, and consequently the N 
division ratio low.

The downside is that it needs, maybe, a single extra package costing a couple 
of Euros, although it might be possible to use one of the dual synthesiser 
parts.

Commodity VCO parts (using coaxial ceramic resonators) in the 2.4GHz region 
have phase noise in the region of -95dBc/Hz at 10kHz offset. Divided down to, 
say, 1.8MHz would give ~-160dBc at 10kHz (in practice that would probably be 
limited by the phase noise of the dividers) A simple VCO with a microstrip 
transmission line resonator printed on teflon/glass has given me about 
-105dBc/Hz/20kHz  at 1.872GHz in a VCO forming part of a 6cm transverter. The 
close-in phase noise at 5.7GHz, measured as spurious FM, was of the order of 
10Hz in the audio band. The 'note' was entirely T9 at 6cm.

Some of the more recent 'VCO-on-chip' parts from AD and National have VCO 
performance approaching that of the ceramic resonator VCOs.

As a matter of interest, by using ceramic coaxial resonator or circular or 
rectangular cavities at microwave frequencies as VCO resonators, it's 
possible to get much higher unloaded Q than is possible by conventional L/C 
or coaxial line techniques at VHF. With proper design, that can lead to 
higher loaded Q from oscillator resonators, and thus better phase noise 
performance. See Leeson's paper, and later work. The limiting factor is often 
the Q of varactors. Notwithstanding that, a move to microwave VCOs could 
offer two routes to improved phase noise at HF and VHF.

We'll probably be using a similar approach to that I've described, with a 
2.4GHz commodity VCO, in the uWSDR group front-ends for 70, 144, and 432MHz.

Vy 73

Chris
GW4DGU