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Old 21st Mar 2018, 12:44 am   #1
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
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Default Mullard TDA1071 etc.

These Mullard Technical Communications issues turned out to be very scarce publications, but I have recently obtained a copy of the second of the two mentioned, containing the article “The TDA1071 IC in a.m./f.m. radio receivers.” Should anyone here want a scan of that article, please send me a PM. (It’s a 7.4 Meg .pdf but I can reformat.)

Separately I have also obtained a copy of the TDA1071 article “The TDA1071 radio IC in communications receivers” from the first of the two Mullard publications. This was done via a library service – which I’ll wish to use again - that required a declaration of “non-proliferation” as it were, so in this case I am not free to pass it on. (Obtaining one part inspired a harder search for the other!)

That 1976 article provided a good overview of the TDA1071, which I think it is reasonable to quote on a “fair use” basis:

“The TDA1071 (development number M715) is a unique integrated circuit for use in mains and portable radio equipment. The low current consumption and supply voltage, together with a very low peripheral component count, make the IC particularly suitable for portable battery-operated equipment.

“The integrated circuit incorporates an a.m. oscillator, an a.m. mixer with a.g.c., a four-stage differential amplifier and limiter, and a four-quadrant multiplier. The novelty of the IC lies in the combination of both f.m. and a.m. functions in the multiplier circuit, which acts as a quadrature detector (with squelch) on f.m., and as a synchronous demodulator on a.m.”

Actually, it was not the first IC to combine both AM and FM demodulation in a four-quadrant multiplier. The Plessey SL624C multimode demodulator of a few years earlier had done that. As well as AM and FM demodulation, the SL624C could also be configured as a self-oscillating SSB demodulator, using signal feedback to the limiter input to create the oscillator. But for communications applications, Plessey suggested using separate ICs for each required demodulation function rather than trying to switch a single IC.

In turn the idea for Plessey SL624C may well have been derived from the Sprague ULN2111A IC. The latter was nominally an FM IF subsystem incorporating an amplifier/limiter and a quadrature demodulator. It was by no means the first IC-based FM sub-system, the RCA CA3014 having taken that honour. Nor was it the first with a quadrature demodulator, Fairchild having been a bit earlier with a relatively simple form. But it does seem to have been the first to use a six-transistor tree multiplier for quadrature demodulation, to which Sprague adduced specific benefits in its 1967 paper. And it certainly set the general pattern for future FM IF subsystems, many of which added ancillary functions. Even RCA, which had a long history of FM demodulator developments and had most recently its differential peak FM demodulator as used in the CA3065 for example, eventually capitulated and adopted the transistor-tree quadrature form in its landmark CA3089 of 1971 (and which was I think about the last major project done by its noted engineer Jack Avins).

At the very end of the 1967 Sprague paper, it was stated:

“The use of full-wave coincidence gates has proven to be a convenient solution for providing the basic functions of an FM detector and is well-matched to the capabilities of monolithic circuitry. Furthermore, the same gating arrangement can be used as a high-performance synchronous
demodulator or as a double-balanced mixer.”

The other possibilities, including low-distortion synchronous demodulation, were elaborated upon in a following 1968 paper. But Sprague had set the ball rolling, and some of those FM IF subsystems, particularly those aimed more at TV sound applications, were subsequently used as AM demodulators in communications receivers. The block schematic for the Plessey SL624C looked very much like that for a TV sound IC IF subsystem that also incorporated an electronic AF volume control and an AF driver stage. Plessey did claim a benefit for synchronous AM demodulation, thus: “A major advantage of the SL624C as AM detector is that unlike an envelope detector, it does not give an output on broad band IF noise when used in a typical receiver following a block filter and a broadband IF amplifier.”

The National LM373C multi-mode (AM, FM and SSB) communications IF strip IC, with quadrature FM demodulator, should also be mentioned in this context. Although not primarily intended for synchronous AM demodulation, it was noted that could be used for such, again following the precedent set by Sprague. As best I can determine, the LM373C preceded the SL624C.

Returning to the TDA1071, an interesting feature was that the AGC function was done entirely in the mixer, and not at all in the IF amplifier. That seems contrary to the conventional wisdom as applied to discrete circuitry, where the application of AGC bias to valve mixers in other than simple receivers was avoided or at least moderated, and was almost always avoided in transistor mixers. But evidently IC structures allowed the AGC’ing of mixers without the usual drawbacks. The mixer stage could also be used as an additional IF amplifier pre-stage, AGC’d if needed, where it was not required for its primary function, and the AGC bias was available for an external RF amplifier.

The 1976 article showed examples of double-conversion FM and AM receivers using the TDA1071, in which the on-board mixer did the second conversion. It also showed examples of single-conversion FM and AM receivers, in which the mixer served as an IF pre-amplifier, AGC’d in the AM case. SSB could be handled by using a TCA240 multiplier IF fed from the IF output of the TDA1071.

I’d say that Eddystone pretty much followed the Mullard examples when it used the TDA1071 in its 1570 and 1590 communications receivers.

The 1570 had three TDA1071 ICs. One was used as the 10.7 MHz FM broadcast IF strip, with the mixer as IF pre-amplifier. Another was used as the 455 kHz AM IF strip, with mixer as AGC’d IF preamplifier, preceded by a mosfet front end (RF amplifier and mixer). A TCA240 was used in in a side circuit for SSB demodulation. And a third was used, in FM mode, as the 455 kHz AFC detector. The 1590 lacked the broadcast FM facility, but I think it had more elaborate SSB IF filters. In both receivers manual IF gain control was included, obtained by varying the bias fed to one of the mixer ports whilst grounding the AGC port. This facility was not mentioned in the Mullard articles.

And it looks as if Roberts followed the basics of the 1977 article in respect of its RM30 and RM33 AM-FM receivers. In the FM mode, when the mixer was not used as an IF preamplifier, supply to the mixer and oscillator part of the TDA1071 could be switched off, so reducing current consumption, a benefit in battery-operated equipment.

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Last edited by Station X; 25th Mar 2018 at 9:45 am. Reason: Threads merged.
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