[HBR] GC-HBR Project -- Was 'Navy guy ...'

[N2EY at aol.com]

In a message dated 6/28/06 7:35:11 AM Eastern Daylight Time, 
waltah at earthlink.net writes:


> I said:
> > The plan is to take the 120 crystals of the BC-1335 WW-II portable
> > FM set, 5675-8650 kcs (25 kcs spacing) and multiply the
> > frequencies x10 to give 120 frequencies 56.75-86.50 Mcs.  

whoa - hold it right there!

Multiplying by 10 requires a quintupler and a doubler. That quintupler isn't 
going to be very efficient. Worse, there's going to be the challenge of 
filtering out the 4th and 6th harmonics, which will not be very far away, 
percentagewise. And it's percentagewise that counts!

For example, if the 6000 xtal is quintupled, we have harmonics at 24,000 and 
36,000 that we don't want, yet we want the 30,000 It's doable, but not easy, 
and you'll need to switch the tuned circuits as well as the xtals.

> > Subtract 49.2 Mcs yielding 7.55-37.30 Mcs by 250 kcs steps.  

What bandswitch has 120 positions?

> > Adding a tunable oscillator 1.704-1.454 Mcs to a fixed IF of 4.096
> > Mcs (the ladder filter with 4.096 microprocessor clock crystals)
> > gives a tunable IF of 5.800-5.550 Mcs.   The difference between
> > that and the synthesized band crystal frequency is the tuning
> > range for the band:  for the lowest band, it's 7.550 Mcs - 5.550-
> > 5.800 Mcs = 2.0 - 1.75 Mcs. 
> 
> Today's quiz question is "how many times can the same person 
> look at the same numbers and miss a problem?"  
> 
> Answer:  A bunch.   
> 
> The synthesizer starts with those fundamental crystals 5675-8650 
> kcs; after multiplication and subtraction of 49.2 Mcs, the result is 
> mixed with the signal from the antenna.   However, I overlooked the 
> fact that the lower end of the range of crystal frequencies overlaps 
> the tunable IF, 5550-5800 kcs.   So the five bands using crystals 
> below 5800 kcs (1.75-3.00 Mcs) would all contain a birdie from the 
> frequency of the crystal for that band.   
> 

And it will be loud!

> There will be two or three three tuned circuits to attenuate these 
> signals, but they'll be audible.  The band 3.000-3.250 would have a 
> band-edge marker at the low end, where the 5800 kcs crystal for 
> this band is picked up by the high end of the IF tuning range.  
> 
> If you want to look on the positive side, these frequencies supply 
> some calibration points for the LO.   Not enough to do the whole 
> job though -- they're all at the low end of the band.  So I probably 
> can't get away with claiming this was just a clever piece of design 
> work.
> 

"It's a feature"


> You could move the band crystal range up 150 kcs to start at 5825 
> kcs -- or just skip the lowest frequency crystals in the set and 
> change the 49.2 Mcs subtraction frequency to start the coverage 
> where you like.  Neither idea was really terribly attractive.  The 
> receiver is planned around a set of crystals that I already have. If 
> starting with purchased ones, you would go with overtone units 
> ($20 x 120 = $2400 ... Ah don' think so ...) and avoid the whole 
> issue, as well as getting rid of the need to hetrodyne to get the 1st 
> mixer frequency.   And I'm too obsessive to really like throwing 
> away part of the set.
> 
> I also considered moving the tunable IF by moving the LO down 
> from 1.454-1.704 Mcs to 1.304-1.554 Mcs, giving an IF 5.400-5.650 
> Mcs.   The fourth harmonic of the LO would then cover 5.216-6.216 
> Mcs so it would sweep entirely through the tunable IF range, going 
> in the same direction.   The LO and 2nd mixer (which gets the 
> tunable IF as the input) are in the same box.   This would be a very 
> strong spurious signal tuning slowly through the same spot on 
> every band -- a definite loser.
> 

All those conversions...

> Or you could keep the LO range the same but move the fixed IF 
> down to somewhere in the upper end of the 75 meter band.  You'd 
> have the BFO in a ham band and the least bit of feedback would 
> make the IF passband shape go west when tuned near that 
> frequency ... The idea was dead before I even looked at available 
> clock crystal frequencies.
> 
> Obsessive or not, skipping the first six crystals in the set seems 
> the best approach.   This means starting with the 5825 kcs 
> (channel 276) crystal and changing the subtraction frequency to 
> keep the lowest synthesizer output frequency at 7550 kcs so the 
> tunable IF remains 5550-5800, This requires an overtone crystal at 
> 50.7 Mcs, but I have that crystal -- it was used in the PRC-6 handy-
> talky.   You lose the coverage above 30.50 Mcs, but that's not a 
> big deal.   
> 
> The harmonics of the LO (1.454-1.704) also must be considered.  
> None of them are in the fixed IF, tunable IF, or the lower ham 
> bands but the 9th harmonic goes through the 20M band. It's going 
> in the reverse direction (higher LO frequencies -> lower dial 
> frequencies) so it'll be going fast.   
> 
> Some other (non-ham) bands will be hit by LO harmonics.   Since 
> the LO comes after the front end, these harmonics should be close 
> to inaudible if the LO/2nd mixer box is closed and well-filtered.   
> Also the layout there (and tuning cap design) makes it easy to 
> return all the grounds to the tuning cap frame, thus minimizing 
> chassis currents.  
> 
> With 30 Mcs of total coverage it might seem logical to start lower 
> than 1.75 Mcs -- say down at 500 Kcs.   However doing so would 
> require tuning the front end range in four rather than three bands 
> because it's tough to get more than a 3:1 tuning range with 
> conventional tuning caps.  In addition, extending the tuning range 
> down would make it overlap the LO range.  You'd pick up another 
> (probable) birdie per band from the LO and there could be trouble 
> with more complicated spurious responses caused by strong BC 
> signals near the LO frequency.  
> 

You really don't want to go there!

> It could be done on a 'take what you get' basis, just by using the 
> six omitted crystals and adding the fourth range to the bandswitch. 
> Given the strength of typical BC band signals, that probably would 
> be useful.  However the goal was to keep this project fairly easy to 
> build.  Adding BC band coverage probably exceeds my level of 
> ambition.   
> 
> Commercial grade general coverage receivers beat these sorts of 
> problems with complexity -- adding or taking away a conversion on 
> some bands, for example -- or by making other types of 
> compromises such as using single conversion with phase-locked 
> oscillator.   In a practical homebrew project, I think it's better to 
> accept that there will be some garbage and try to leave it in places 
> you won't often visit.  
> 
> It's still a paper receiver (plus what I hope is now a complete 
> bucket of parts) but I think I'm closing in on the design.   
> 


Some ideas:

1) The complex LO system can be big trouble. I learned the hard way why 
premixer designs are so rare in ham gear: the LO has to be *really* clean to avoid 
spurs. In a transmitter, if we have spurs 60 dB down, it's no big deal. But in 
a receiver, if we are trying to listen to a -130 dBm signal, and we have a 
spur that's 60 dB down in the LO, and the spur is in the wrong place, we may 
hear a -70 dBm signal that really isn't there. In the HF ham bands, -70 dBm 
signals aren't rare when the band is open and you have a decent antenna.

There's a circuit in a QST article about 1961 that allows fundamental-mode 
FT-243s to operate in overtone mode. (It's in the article by W1ICP that uses a 
single 6U8 and a 3500 kc xtal as a converter for  40, 20 and 15 to 80 meters, 
without having to get a costly xtal for 10.5 or 17.5 Mc. Being able to overtone 
those xtals might have some uses. 

73 de Jim, N2EY