RF Amplifier

The value of R1 depends on where it is located in the receiver circuit. When it is used as the RF amplifier for the VFO, 4 mhz oscillator amplifier/buffer, and the BFO amplifier/buffer, the value is 100k. This loads the oscillators very lightly and promotes frequency stability.

In the 455 khz IF strip, R1 is 2.2k to promote stability of the amplifiers and the best match for the mosfets between the stages.

When used as the first mixer RF amplifier, the value of R1 is 100 ohm. However, one fine tuning tool of this receiver is the gate 1 resistor of this first mixer amplifier. Changing the gate 1 resistor not only changes the impedance to the amplifier, but it also varies the impedance that the mixer works with through the reflection of the impedance of G1 through the broadband output transformer of the mixer. By raising the output impedance (R1 = 100k), the gain and sensitivity of the mixer and amplifier is increased, at the expense of dynamic range and IMD. The overall gain for receiver is higher which consequently drives a stronger signal through the crystal filter, which widens the effective bandwidth of the crystal filter. This is an ideal situation for when the band you're operating in is quiet and you're trying to dig out the weak ones.

With low impedance levels (R1 = 100 ohm), dynamic range and IMD of the mixer is much better, at the expense of sensitivity and gain of the mixer and amplifier. This results in lower signal levels at the crystal filter, which tightens the effective bandwidth. This optimizes the receiver for contesting and improving reception and readability of signals during times of strong band openings.

By making the Gate 1 resistor variable, the receiver can be fine tuned for almost any band opening or operating desires. Three things are varied: the dynamic range of the mixer, the bandwidth of the crystal filter, and the action of the AGC in the 455 khz IF strip. Because of its critical location between the mixer and 1st amplifier, you have to put the adjustment pot on the circuit board.

Audio level is different, but the receiver has adequate audio gain to compensate for any changes. You can literally put the noise level of the band anywhere you want. If all the signals are in the noise, raise the amplification of the receiver so that you can pick them out of the noise and read them! If you're in a contest with strong signals blasting all over the place, tune for super dynamic range and selectivity! In this receiver, they go hand-in-hand.

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Last Update: 01/20/2000
Web Author: David White, WN5Y