[Radio Wrangler]

That made interesting reading!

I've still got the H2900 structure in my head, so looking at the downloaded article on the H2450 made for interesting comparisons and good example of how technologies moved quite a bit at that time.

Both receivers were up-converters of course. H2900 to a helical filter around 70MHz, whose width was sufficient to allow the synthesiser to be split with discrete crystal oscillators for 2MHz steps, and finer steps done with the second local oscillator. (Any error in the crystal oscillators being passed to the 2nd LO to offset it to correct the tuned frequency.

The H2450 is a lot more mainstream to today's eyes. It has a narrow crystal filter at the first IF of 68.6MHz chosen so that a simple 2nd LO of 70MHz does the mix-down to the 1.4MHz final IF. I still have a pair of these filters, and a large collection of 1.4MHz filters for all sorts of modes.... never known to pass a good crystal filter at a radio rally! All steps of the frequency control being implemented at the first LO by a multiloop PLL synthesiser.

The H2540 is more sophisticated than the H2900 in avoiding a wide first IF and getting a narrow filter earlier in the structure. This should help with intermod and overload characteristics. The H2900 is more sophisticated than the H2540 in having a servomotor-tuned narrow preselector, while the H2540 has only sub-octave switched filters, and optional at that.

From an intermod point of view, the H2900 is likely to be better with widely spaced interferers, but the H2540 should win with closely spaced interferers due to the difference in width between a tuneable RF preselector and a narrow, crystal, first IF filter. Assuming some progress in the detail of mixer design happened in the period between the sets. The H2900 mixer was not very advanced. Racal was ahead of the game at that time and I assume the Rafuse patent would have still been in force.

The H2900 may be the more sophisticated one in terms of phase noise with that switched bank of crystal first LOs.

The H2900 also is dual-diversity in one box. H2540 users would simply buy two boxes and get a degree of redundancy as a bonus, losing a number of single points of failure which would take out both diversity channels at once.

So, if it came to a shoot-out between H2900 and H2540 each one would beat the other in different parameters, I expect. The H2540 must have been a better fit to the market. I was told that very few H2900s were sold, that Interpol was the main user.

In the comparison table in the H2540 article the H2900 looks to tick all the boxes, except one: Merchant marine. I doubt if they could have afforded one!

These receivers cover an interesting transition phase in receiver architecture and the H2540 shows things starting to converge on what became a standard approach until the onset of digital conversion at IF.

David