Paraceiver is a novel regenerative receiver that's based on a coupled-resonator parametric down-converter. The use of a ceramic resonator in the idler is a new development. A pair of 1N4002 rectifiers are used as varactors. The prototype 40m receiver tunes from 7016 to 7057kHz with excellent stability. The measured MDS is -150dBW, or 0.22uVrms.
A child on a playground swing uses his or her legs to pump the amplitude of a single-resonator (pendulum) system. They understand that it's important to maintain a proper phase relationship between their leg-pumping motion and the position of the swing. This is known as degenerate parametric amplification. If we were to transform this system into a radio circuit, we'd be faced with the task of maintaining a proper phase relationship between our pump and the incoming signal.
You might try it by clicking here. Click the little hand just below the word "Stop"on the page that appears. Now you may use your mouse to click on the support-rope in order to manually pump the pendulum. To increase the signal amplitude one must pull the rope just as the pendulum passes through vertical. Release the rope when the pendulum comes to a temporary halt on either side of center. Notice that you're pumping at twice the signal frequency. Reversing the pump-phase dampens the pendulum motion. Also notice that it's impossible to pump a "still" pendulum into transverse oscillation. An initial transverse disturbance (provided by either signal or noise) is necessary.
As we generally don't know the phase of the incoming signal, we can't pump our reactance at the phase required to increase the signal amplitude. However, it is possible to successfully pump two coupled-resonators using an arbitrary pump phase, so long as we pump at the sum of the natural frequencies of the two resonators. In non-degenerate parametric amplifiers the coupled-resonators automatically maintain the necessary phase relationship. Looking at my circuit diagram (available at the top of this page)
T1, T2, and C1 to C3 constitute a 40m bandpass filter.
L1, VC1 and C4 form a 7MHz, series-tuned, resonator.
X1 and VC2 constitute series-tuned resonator at 4MHz.
The 11MHz crystal-controlled "pump" (U1), splitter (T3), and the varactors (D1, D2), together produce a time-varying shunt-capacitive reactance.
Thus, the parametric amplifier/mixer amounts to a pair of series-tuned resonators connected in parallel with a time-varying capacitive reactance. Indeed, it's nearly identical to an electromechanical model that may be seen by clicking here.
Receiver tuning is accomplished by pulling ("VXOing") the frequency of the ceramic resonator using a variable series capacitance (VC2). Given a fixed pump frequency of 11MHz, if the ceramic resonator's natural frequency were dropped to 3.95MHz, for example, the LC tank-circuit resonant frequency must rise to 7.05MHz in order to maintain the required non-degenerate frequency relationship. In practice, the LC resonator only needs to be re-peaked a few times across the frequency-span covered by this receiver.
To begin operation, the pump amplitude is increased by advancing VR1, while simultaneously sweeping VC1 back and forth until the onset of parametric oscillation is detected (those familiar with regenerative receivers will recognize this from the onset of noise in the headset). It's now possible to tune across the band using VC2, re-peaking VC1 as required. If VR1 is set for the threshold of oscillation at the lower end of the CW band, readjustment shouldn't be required until the top-third of the receiver tuning range has been reached.
The JFET "infinite impedance" detector drives a pair of headphones via one-stage of transistor audio amplification.
I've bread-boarded Paraceivers for the 10, 20 and 40m CW bands thus far. The 10m version uses a 40MHz "canned oscillator" in conjunction with a 12MHz ceramic resonator. My first 20m receiver used a 20MHz oscillator with a 6MHz ceramic resonator. Although this worked well during the daylight hours, the choice of a 6MHz idler frequency resulted in excessive 49Meter shortwave broadcast interference, come late afternoon. The interference was greatly attenuated by the use of a 16MHz clock and a 2MHz ceramic resonator. However, the lower idler frequency required a greater magnitude of pumped-reactance. In this case, it was necessary to exchange the 1N4002's for a pair of "real" varactor diodes; I used MV1650's.
Please click here to listen to a short recording taken at the headphone terminals of my 40m Paraceiver.
Of course, it's possible to replace the "canned oscillator" module with a crystal-controlled oscillator built from discrete components. Indeed, a vacuum tube version of the circuit shown in my schematic might be constructed using a single tube (triode crystal-controlled oscillator/pentode grid-leak detector).
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