Radio Receiver Intermediate Frequencies

[Synchrodyne]

The lowest radio receiver IF that I have found to date is 18 kHz, used in the various Racal SSB and ISB outboard adaptors (RA63, RA98, RA121, etc.) intended for use with the RA17 series of receivers.

These took their feed from the 100 kHz final IF of the associated RA17 family receiver. Another conversion was necessary in order to facilitate the inclusion of an on-board AFC loop for pilot carrier transmissions, thus obviating the need for arranging for AFC control of one of the receiver oscillators.

So why 18 kHz? Here’s an attempt at post facto rationalization: Racal used multi-section LC low-pass and high-pass filters for sideband separation, and I imagine that these were easier to implement and had steeper slopes (on a per kHz basis, not a per octave basis) at lower frequencies. So the goal may have been to have a final IF that was as low as reasonably possible. Given that the input bandpass was 94 to 106 kHz, that band, and in fact somewhat beyond it, would need to be free of any IF harmonics. So 18 kHz, with 5th and 6th harmonics at 90 and 108 kHz respectively, was about as low as was workable. The 118 kHz oscillator frequency was also clear of any IF harmonics.

Also, for greater sensitivity, the AFC discriminator worked on the 5th harmonic of the 18 kHz IF, namely 90 kHz, which was another reason for the harmonics to be well clear of 100 kHz and the passband.

Cheers,

Steve

[Synchrodyne]

Some more on the 1.4 MHz IF number mentioned in earlier posts:

Redifon also used it for its fixed-tuned, single-conversion R499 receiver. This was announced in WW 1968 June (page 184), so it preceded the R550 and R551. Apparently it used essentially the same circuitry as those two, except that it had a crystal-controlled oscillator (up to 10 thereof) and fixed-tuned RF coilpacks. The latter must have provided sufficient RF selectivity that single-conversion provided adequate image rejection.

Conceivably in selecting 1.4 MHz, Redifon did look back to the Rees Mace precedent, and perhaps the original Rees Mace and Admiralty work-up was still available. One supposes that there was also some kind of industry informal agreement or understanding, perhaps also including the GPO, that 1.4 MHz was an appropriate number for marine receivers designed around the new SSB requirements. It seems unlikely that Cathodeon and Plessey would have developed standard ranges of 1.4 MHz IF filters without there being reasonable certainty about the market.

And although having marine origins, 1.4 MHz then moved into the wider professional receiver field, with the Racal RA1772 being an important marker.


Cheers,

Steve

[turretslug]

It occurred to me that the designers of Eddystone's 910 receiver (late '50's/early '60's) had also come to the conclusion that the 1.4MHz region represented a good place to be for a marine receiver IF. This set could be viewed as a development and "professionalisation" of their 750 theme, apparently specifically aimed at the marine market and incorporating the 750's basic topology and much similar circuitry. Instead of fixed 1.62MHz first IF and 1.535MHz second LO for 85kHz IF, +-50kHz interpolation tuning was now adopted around 1.4MHz first IF with 1.265-1.365MHz second LO. The subsequent 830 receiver could be seen as a refinement of the 910 theme, now with +-100kHz interpolation IF centred on 1.35MHz, rather than 1.4MHz- perhaps this was to ensure adequate clearance between the consequent highest possible 1st IF excursion of 1.45MHz and the 1.5MHz lowest signal frequency of dual-conversion operation.

[Synchrodyne]

Thanks; that’s an interesting connection. So the Eddystone 910 was another arrow that pointed to the eventual standardization (de facto if not de jure) of the 1.4 MHz IF for marine receivers.

The lineage may be extended from the Eddystone 830 to the EC958 solid-state receiver. This was triple-conversion (on the HF bands above 1.6 MHz) with a 1st IF of 1335 kHz nominal, but tuneable down to 1225 kHz to facilitate high-stability operation above 1.6 MHz, for which a narrow-bandwidth drift-cancelling loop was used. It could be that Eddystone started with the 1350 kHz precedent of the 830, and adjusted it slightly to suit the high-stability circuitry. The Eddystone paper on this receiver did not go into detail, essentially stating just the main requirement which was for a 1st IF below 1.6 MHz.

The EC958 was designed with marine main receiver requirements in mind, including the new SSB rules, although the base version did not meet those in full. The marine version was the EC958/5 (Marconi Nebula).

The EC958 2nd and 3rd IFs were 250 kHz and 100 kHz respectively, the latter being a well-established standard. 250 kHz might have been an ad hoc number chosen to suit drift-cancelling loop implementation. But there is one datapoint that suggests that it [250 kHz] might have been used for aviation HF SSB receivers. In WW 1958 October there was an article entitled “Single Sideband Aircraft Communication”. This included a diagram of receiver upper sideband and carrier extraction filters, with 250 kHz carrier frequency (page attached). There was no commentary on the IF itself, so whether it was representative of a production receiver or just a “for instance” is unknown.


Cheers,

Steve