[Synchrodyne]

Something to note is that in general, the articles and other sources that discussed the introduction of jfets and mosfets to VHF TV and FM front ends seldom placed lower noise as the primary reason for the move from bipolar to unipolar devices. Rather it was the reduction of spurious responses and improved ability to handle large signals that were the key issues. Not that noise was never mentioned, but implicit was that good bipolar devices in general were seen as being quiet enough, but mostly much inferior to valves when it came to spurious responses and signal handling. The use of fets took this aspect of performance back to valve level, with no noise penalty as compared with bipolar devices, and sometimes with even lower noise. The latter initially seemed to be treated as a bonus, although later probably exploited for the highest performance FM front ends.

When it comes to noise figures, Ambit quoted this parameter for most of the FM front ends included in its catalogues. The Ambit FM front ends covered a range of performance levels and used a variety of devices types. The noise figures are mostly in the range 5.5 to 8 dB, with one top-end model down at 4 dB. I have attached a composite excerpt from Catalogues 1 through 3.

Noise figures were seldom quoted for complete FM tuners though. Typically there was a single-point signal-to-noise ratio sensitivity, and sometimes quieting curves. I guess that these numbers were obtained using a signal generator, thus essentially devoid of any galactic noise input. For the early solid-state era, if comparably-based numbers could be found, an interesting comparison might be had amongst the Revox A76 (dual-gate mosfet), Sony ST-5000 (bipolar), Sony ST-5000F (jfet), B&O Beomaster 5000 early (bipolar pnp) and Beomaster 5000 late (jfet).

The noise figure was quoted for the Radford FMT-2, at 5.5 dB. This had the D&W 341AFC front end, bipolar, four-gang, with the bandpass at the input, presumably to offer maximum protection to the RF amplifier. The Radford FMT-3 used the D&W 341AFC/FET/2 front end, essentially the dual-gate mosfet version of the 341AFC, and retaining the bandpass at the input. (More common mosfet practice, as with the Revox A76, seems to have been to place the bandpass at the interstage.) Unfortunately, I can’t find the noise figure for the FMT3. The late FMT2 was optionally available with a fet-based front end, presumably the 341AFC/FET/2. The rationale for this option was given as: “The F.E.T. model is being made available primarily for foreign markets where conditions demand the highest performance in respect of cross-modulation, image rejection and adjacent channel selectivity.” Nothing was said about noise, which inclines me to think that it was not much different to that of the standard model with bipolar front end. Presumably the mosfets had less adverse effect (at higher signal levels) on the performance of their associated tuned circuits, as well as lower production of spurs.

In the communications arena, Pye did not quote noise figures for its R401 (AM) and R402 (FM) base station receivers, which had dual-gate mosfet RF amplifiers and mixers. What it said was: “Helical resonators and field effect transistors in the RF section give the receiver high selectivity with good signal-to-noise performance and outstanding protection against blocking and cross-modulation.” That suggests that low noise level was not the primary reason for the choice of mosfet devices.

Noise factors quoted by Eddystone for its 2nd and 3rd generation solid-state VHF communications receivers were:

Eddystone 1990R (VHF to 500 MHz, dual-gate mosfet RF amplifier) Typically 4 dB, not worse than 10 dB at any frequency.
Eddystone 1990S (UHF 440 to 1000 MHz, bipolar 1st RF amplifier, dual-gate mosfet 2nd RF amplifier) Typically 10 dB.
Eddystone 1995 (VHF & UHF, 20 to 1100 MHz, front-end devices unknown) Typically 10 dB.


Cheers,