[HBR] Yet Another HBR Concept

[hbr@mailman.qth.net]

In a message dated 10/12/03 10:12:29 PM Eastern Daylight Time, 
waltah@earthlink.net writes:


> VFO tunes 8.7-9.2 Mcs.   11th harmonic of 1.75 Mcs is 19.25 Mcs.
> 19.25 - 8.7-9.2 Mcs = 10.55 - 10.05 Mcs.   With an IF of 6.55 Mcs 
> that tunes 3.5 - 4.0 Mcs.   6.5536 is available as a clock crystal and 
> the frequency seems to be clear of high power SWBC stations.   
> VFO is above IF so no problems there, all the harmonics of the 
> oscillators are well above the injection frequency ... does anyone see 
> any problems?

Some possibles:

6.55 IF is very near low end of 40 meters. IF feedthrough may be a real 
problem on that band.

With VFO at 9.2, premixer output is 10.55 for 80 meters. VFO feedthrough may 
be a problem, resulting in pickup of strong signals in ~2 MHz region.

Other bands need checking


> 
> The virtue of 8.7-9.2 Mcs as a VFO range is that's the FT-101 VFO 
> and I have another one of those units.   In the original both gangs are 
> used with a small coupling cap to tune a single ended oscillator but 
> for the HBR2K I used just one gang and had no serious trouble 
> getting it to track the dial; I could use both gangs for a push-pull 
> oscillator and still get tracking.   

Neat!

If a 5.2 MHz filter or some xtals could be dredged up, 80 and 20 could band 
image and the rest be premixered

> 
> This VFO and dial mechanism is ideal for a really high-quality 
> receiver because it's a unit oscillator and the dial is calibrated 
> directly in one kcs intervals; marked 0-500 and 500-1000 so all the 
> meaningful bands are covered.   The rest of the FT-101 doesn't work 
> too well for this new project because the push-pull front end may not 
> fit -- but I guess I'll take a look.  I do have another junker chassis and 
> a blank top plate.  Secondly, the mounting of the oscillator assembly 
> doesn't allow proper cooling for a vacuum tube design -- one 
> disadvantage I faced in the HBR2K.   Again, it's worth a look.
> 
> Those oscillators turn up on eBay every month or so; they go for $10-
> 25.   You have a good tuning cap (aluminum plates -- it could be 
> better) and a far better gear drive and dial mechanism than any of the 
> usual HB projects of the 60's.  Tuning rate about 15 kcs/knob 
> revolution with a single stage JB reduction drive between the knob 
> and the dial.  
> 
> > One thing to consider with the Hahnel oscillator: Locked oscillators 
> > of any kind can have significant phase noise. 
> 
> Well, yes ... sort of.
> 
> A phase locked oscillator stays on a certain frequency because it's 
> phase is regularly yanked over to be where it would be (in the cycle) 
> if it had been on the right frequency.   But those adjustments cause 
> distortion of the waveform.   'Distortion of the waveform' means 'other 
> frequencies' -- the yanking process generates noise.   That noise can 
> be serious -- and many of the early phase-locked-loop transceivers 
> not only had noisy receivers because of phase noise, but they 
> transmitted the $#!^ too.   
> 
> You can't get rid of PLL noise by filtering because it's worst right next 
> to the desired output frequency.   Any filter both narrow enough and 
> stable enough to help, would be ideal for use in an oscillator that 
> would make the PLL unnecessary!   
> 
> Various techniques can be used to make things better but most of 
> them apply only to PLLs -- not to a simple locked oscillator like the 
> Hahnel idea.   The one thing that does apply is to build a good stable 
> oscillator to start with, because the noise is in proportion to how 
> much correction is required.   The problem is considerably simplified 
> by the fact that we're not talking about a tunable oscillator -- just 
> about one that works on a small number of fixed frequencies 17.5 
> Mcs (160M), 19.25 Mcs (80), 22.75 Mcs (40) ... that we want to have 
> precisely right, rather than just within a few kcs.
> 
> Moreover, the noise of a locked oscillator is a better kind.   In a PLL, 
> the oscillator 'coasts,' gradually getting further out of phase until it is 
> pulled back; how often it is pulled is a design property of the PLL and 
> is related to questions of stability and response time of the locking 
> loop and there's noise in the 'how often,' as well.  In a locked 
> oscillator, OTOH, correction is applied every nth cycle.   For example 
> in the proposed design, 80 meters uses the 11th multiple of 1.75 
> Mcs so every 11th cycle, the phase of the 19.25 Mcs oscillator is 
> corrected by the signal from the 1.75 Mcs oscillator.   The 'noise' is 
> actually sidebands that appear at multiples of 1.75 Mcs on both 
> sides of 19.25 Mcs.   *These can be filtered*, although at the highest 
> multiples -- somewhere around 50 Mcs for 10 meters -- you need a 
> pretty narrow filter to get much help.
> 
> I need to look at the spurs for the other bands.   Other than that, it 
> looks okay.
> 

Might just be the answer

> > ZL2AMJ  solved this problem by using an inexpensive surplus xtal at
> > about 3500 kc. on its third and fifth overtones to get output on
> > about 10500 and 17500 without even-order harmonics or the
> > fundamental making spurs in the front end ...
> 
> Slight disadvantages of the method are that it further constrains the 
> problem -- wouldn't work with the conversion scheme I'm looking at, 
> for example, because I use an odd multiple of 1.75 Mcs.   Also, 
> overtones are not precise multiples of the fundamental -- they are 
> *mechanical* harmonics and if you look at what the crystal is doing, 
> the acoustic standing waves inside the thing have different end 
> conditions than those that terminate at the surfaces.   The 
> frequencies are typically within a few kcs of the multiple, I think -- 
> you can cover by recalibrating the VFO on each band, as would be 
> required with separate crystals.   It is, however, a system of 
> noteworthy simplicity -- one triode with various non-critical tuned 
> circuits to select the overtone is all it takes.
> 

That was the aim of the original. 


> One small virtue of the Hahnel scheme is that once the control 
> crystal is precisely on 1.75 Mcs, the calibration is identical on all 
> bands  -- just calibrate the VFO and you're done.   A disadvantage is 
> that the 1.75 Mcs crystal will put a spur (a marker!) at the lower edge 
> of each band.   It's weak on the 1MHBR, but in that one the crystal 
> oscillator is injected into the 2nd mixer -- in premixed form, it'll be 
> right up there with the RF stage.
> 
> Regarding the W5OMX receiver:
> 
> > Note that the premixer is used on all bands even though 80 and 20
> > can be tuned directly by the 5-5.5 VFO output. Why this was done is a
> > mystery, because it has to be a source of avoidable birdies and
> > spurs. 
> 
> Doing otherwise gives you the problem of matching gains and 
> assorted possiblities coming from having a different signal path on 
> those bands.   

Not really. WIth a single ended mixer as used in the OMX, you just turn off 
the het osc and the mixer becomes a buffer. If one is really fussy, one pole of 
the bandswitch can be used to cut in different cathode resistors to set the 
buffer gain. Same pole opens the cathode circuit of the het osc.

> 
> Thanks for the info on the W5OMX receiver ... now *there's* a classic 
> set you'd like to see turn up under a table at a fleamarket.   Maybe 
> even on eBay -- how many hams would recognize it? 

I would!

  The call 
> 
> appears to be unassigned at present.   
> 
> 


Wonder where 'OMX is today?

73 de Jim, N2EY


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