"gb" ) writes:


I see you used a 9 MHz IF.
Any particular reason that you used separate (LO) oscillators for each band
(rather than a single oscillator with switchable crystals) ?

Surely because it works out to be simpler.

People often make the mistake that extra circuitry is bad because it
complicates things, but the reality is often that extra circuitry simplifies
things.

Back in the days of tubes, their bulk and filament current (or slow
startup if you switch the filament) meant that one did give thought
before adding another tube. It probably was simpler to switch crystals
than switch oscillators, and compared to the cost of tubes and sockets,
the switch was relatively cheap.

Once transistors came along, their size, low current and instant on
meant the game changed. If adding another transistor in an IF stage
meant that all the stages were running at a lower level, better to
do. It added little to the cost, and pretty much nothing to the current
drain.

If you switch crystals, you limit layout. You need that switch near
the crystals (and in some cases may need two poles per position), or
you need to use relays (I've seen it done in mods to tube equipment)
or diode switches. Those diodes can often cause some problems, depending
on choice and useage. Switching crystal oscillators means none of this
applies, and the cost of the transistor and passive components is
nothing compared to the cost of the crystal.

This is a theme of some of Hayward's work. Various times, he's described
transceivers, and not only is he using separate IF strips for receive
and transmit, but he's got a separate IF filter for each function. His
reasoning is that it makes the chain cleaner by not having the switching.
Given the cost of commercial filters, I'm not sure that's the best choice,
but certainly there is a tradeoff. If adding a few dollars for an extra
RF stage rather than switching tuned circuits requires a cheaper switch,
or makes it easier to do the switching, it may be worth spending those
few dollars rather than fuss too much.

Jerry Vogt (I"ve spelled that wrong) who seemed to be connected
with Hamtronics (at least, he wrote about their stuff for Ham Radio magazine)
pointed out in an article about preamps that a manufacturer needs to
cut costs. They skimp on parts because any single part saved is
multiplied by the number of units built. But a hobbyist building a single
unit doesn't have that multiplier. If they toss in an extra bypass
capacitor here, or build each stage into a metal box for shielding,
the cost is right there. They don't have to worry about 10,000 more
capacitors, or whatever.

Michael VE2BVW