type. Fig- 2 A shows I lie essential part of the triode-tube detector and Kig- 2B shows its transistor counterpart.4 Since the BFO drives the emitter, we again must deliver rather a large power level from the BFO (into a low impedance) to make the detector work. In fact, it is the rectified BFO voltage that biases this product-detector. When the BFO is turned off, (he operating point changes considerably, and it is then possible to use the detector for AM. This product detector (the transistor version) is identical to most transistor mixers, and like them it cannot take large input signals.
The input-output characteristic of the circuit in Fig, 2B is shown in Fig. 5* If we define maximum useful output as the point on the input-output curve where the output departs from a linear relationship by 1 db> this circuit is only useful to 0.077 volts (rms) input, The curves were taken with the carrier-BFO difference frequency set at 2 kc. Che input signal level was not increased above 0,12 v rms, because at about 0,13 v rms the BFO is pulled" (injection ocked) to the signal frequency. The use of an emitter-follower between BFO and product detector here would have allowed more complete measurements.
There remain, in the collection of tube-type product-detector circuits we may use, two which would seem to have no obvious solidstate counterparts. They are the product-de-tec tor using a pentagrid mixer, and that using a beam-switching tube. Each of these circuits has the advantage ot having a pair of independent, high-impedance input ports, either of which will control the detector output current, This means that we get, ¿¿t once, signal— BlrO isolation, and small BFO input power requirement But, unfortunately, the solidstate equivalent of the 6AS6, fiBEfi, or 7360 hasn't been readily available; so no product-detectors along this line have been used in amateur circles to this author's knowledge.
Now, finally, at a price any ham can afford, a device is available to provide a solid-state product-detector with similar advantages to those of the pentagrid or beam-deflection tube types, I he Siliconix U89, an industrial version of the 3N89 field-effect tetrode, is available for about $5.75, less than the cost of two 7360 Tubes. It is the fact that this FET has two gates, that are mutual 1\ independent, that makes it so useful.
In Fig. 3 is shown the product detector using a U89. The second gate (G*) is driven by the BFO and its emitter-follower. The emitter-follower gives B1 O isolation and also, provides the proper bias level to the second gate, In Fig. 5 is also shown the input-output curve of this detector. One will note that the output only deviates by 1 db from a linear relation, i» *
over an operating range of more than 65 db. About 0,5 v rms is the maximum signal input level, before linearity is lost.
In order for one to be able to make comparisons of the solid-state product-detectors shown above, a conventional 6BE6 product detector was constructed and measured- The
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