[Synchrodyne]

I have since found some more information on the previously-mentioned RCA UHF TV tuner that perhaps throws some light on the very recent discussion.

It was said to have used a new mosfet device for its RF amplifier. So that gives us an approximate date (1976) for the advent of UHF-capable mosfet devices that were economic for TV receiver uses, so answering one of the early questions in this thread. That also appears to be about the time that mosfets were first used at UHF in commercial VHF-UHF communications receivers, but I’ll cover that in a separate post.

The use of the mosfet RF stage gave a cross-modulation performance improvement of around 12 dB as compared with the previous bipolar type tuner. Here I need to correct my earlier assertion that US UHF TV tuner practice jumped direct from the simple type with no RF amplifier and a diode mixer to the more complex type with a mosfet RF amplifier and diode mixer. Not so, evidently there was an intermediate stage in which a grounded base bipolar RF amplifier preceded the diode mixer. Reading between the lines of the available RCA information, the need for an RF amplifier arose with the introduction of varactor tuning, which was lossy as compared with variable capacitor tuning. Thus the RF amplifier was required to offset varactor losses.

Looking at the timeline, it would appear that the use of mosfets in VHF TV tuners (1968-69) just predated the arrival of varactor tuning (1970 or so), confirmed by the fact that the references to early such tuners showed them as having turret-type tuning. Then varactor tuning required the addition of RF amplifiers, initially bipolar, to UHF TV tuners. As this happened after the advent of mosfets in VHF TV tuners, one assumes that bipolar RF amplifiers were more-or-less a forced choice, and perhaps regarded as interim until UHF-capable mosfets became available. RCA had been a leader in mosfet technology, making the case for initially single-gate then dual-gate small-signal devices (whereas TI seemed to have leaned towards jfets before moving on to dual-gate mosfets), so I’d take its 1976 introduction of a mosfet-based UHF TV tuner as being a very early example of the art, although perhaps the Japanese makers got there at around the same time.

RCA also noted that the use of a mosfet RF amplifier in the UHF tuner made it compatible with the VHF tuner for agc purposes.

I imagine that varactor losses would vary by type, but RCA quoted 4 dB per circuit at the bottom end of the UHF band for those used in its UHF tuner. So that is 12 dB total loss for the three pre-mixer tuned circuits. Presumably the loss was maximum at the low end of the band. The RF amplifier mosfet had a gain of 15 dB at the top end of the UHF band, and “considerably better” (whatever that meant) at the lower end. So the RF amplifier covered the varactor loss with not very much to spare, and I should think there was not enough net gain to allow the use of (overcome the noise of) an active mixer such as a dual-gate mosfet, hence the retention of the diode type per established American practice. The choice of a grounded base bipolar pre-IF amplifier following the diode mixer was chosen to provide the diode mixer with a relatively constant impedance load, and the gain was adjusted to allow the same RF agc delay for the UHF and VHF cases. Working back from this, presumably agc considerations made it preferable for the IF signal not to be routed through the VHF RF amplifier, but rather to the VHF mixer via an IF pre-amplifier.

The corresponding RCA VHF TV tuner had its mixer (dual-gate mosfet) performance adjusted to minimize the so-called channel 6 color beat, which seemed to be a general bellwether of VHF TV mixer linearity performance, somewhat like the half-IF response for FM tuners. The mechanism was that a three-way mix (f1 + f2 – f3) of the channel 6 vision (83.25 MHz) and sound (87.75 MHz) carriers and the local oscillator (129 MHz) produced an in-IF band 42 MHz spur, which then demodulated against the colour subcarrier as a 170 kHz beat. I suppose that it was one of those circumstantial artefacts. The American TV channel frequencies were established before the advent of colour, and the migration to the 45.75 MHz standard vision IF frequency (upon which UHF channel allocations were based) from the previously used numbers around 25 MHz happened in the very early 1950s, just before NTSC-II defined the colour standards. And it seemed not to have been a major problem with valved VHF tuners, rearing its head with the arrival of bipolar transistor tuners.

All of this speaks to G6Tanuki’s comment about cross-modulation, intermodulation, etc., performance being very important in typical US reception conditions.

Once established as both VHF TV and UHF TV RF amplifiers, dual-gate mosfets seemed to stay prominent in that role throughout most of the remaining analogue era. But the mixer function in both cases was usurped by bipolar IC double-balanced multiplier devices, although I do not have timeline information for their early use. I suppose that such devices could be very linear (their primary mixing function excepted) if kept below overload level, and that could be done with an agc’d mosfet amplifier coupled with adequate selectivity. In US VHF TV tuner practice, at least 50 dB RF gain reduction by agc was the desideratum.

Actually, there were HF and MF precedents for having a mosfet RF amplifier feeding into a bipolar IC mixer. The Marconi Apollo marine receiver of the late 1960s had a 40673 mosfet RF amplifier with bandpass input, coupled to an SL640C IC mixer by a single-tuned interstage and an emitter follower. The high-quality (including PLL demodulation) AM side (MF-only) of the Sansui TU-X1 “supertuner” of the later 1970s had a 3SK41 mosfet RF amplifier followed by an LM1496N IC mixer. The earlier TU-S77AMX and TU-D99AMX tuners with multisystem AM stereo capability used a 2SK192 jfet AM RF amplifier with tuned input, more I think as a buffer to feed the LA1245 AM IC, which likely included a double-balanced bipolar mixer.

Cheers,