Although reversible converters diodes have been used in Amateur designs [1-3], they have not yet received wide distribution. The reason is probably purely psychological: everyone knows that the ultimate sensitivity of the receiving channel in this case is limited due to the losses in the passive converters. Nowadays, however, when operating in an overloaded Amateur LW bands determining parameter receiver sensitivity becomes, and the real selectivity. It primarily depends on characteristics such transformative (and input) of the cascades, as. the dynamic range, the lack of blocking purse hindrance, etc. off The ring converters on modern silicon diodes these characteristics are on average 20...25 dB higher than that of simple transducers on lamps or transistors [4].
Loss arising on account of smaller transmission coefficient of passive diode Converter. compared to active, you can compensate by increasing the gain in the subsequent linear cascades (the if amplifier, detector, low frequency amplifier). We emphasize that in the case of active devices (tubes, transistors) loss in real selectivity cannot be offset by any filters in the circuits of the if and LF [5].
Despite the fact that total losses in passive selectively transforming the path of the transceiver with dual frequency conversion (two diode mixer, FSS and EMFs) $ 35...40 dB voltage, LW at all ranges you can achieve the sensitivity of the receiving channel is not worse than 2...3 mV. However, at frequencies above 10 MHz in such a device it is necessary to apply the RF amplifier. So it is not too deteriorated real receiver selectivity, it is desirable to perform push-pull circuit powerful transistors.
As an example in Fig. 1 is a schematic diagram of a passive selectively-Converter tract used by the author in-band (14, 21, 28 MHz) solid-state transceiver.
(click to enlarge)
Signal circuit L1C1, tunable within the three ranges of the capacitor C1, is connected with the transmitter. diodes made on the V1 - V4. Diode Converter, in turn, is associated with a tunable FSS (elements L2 - L5, C2 - C6, C29 1,C29 2) with an overlap of 6...of 6.8 MHz and a bandwidth of about 30 kHz. The second Converter diodes V5-V8, similar to the first loaded on Electromechanical filter Z1. Smooth local oscillator transistors V11-V13 covers an area of 5.5...of 6.3 MHz. In band quartz oscillator, performed on the transistor V10 uses a switchable crystal resonators B1 - OT.
As can be seen from the figure, from point A to point B tract represents a single whole, without switching in the cascades and in the circuits of the signal processing as. when working on reception. and on transfer.
The remaining stages of the transceiver, not shown in the figure, the model, with minimal noise levels. They must have the following coefficients of transmission voltage: RF amplifier is about 20 dB, FC - 80 dB. LF - not less than 60 dB, the detector is about 20 dB, the amplifier DSB - not less than 40 dB (with a margin of ALC). In order to simplify the figure does not show some auxiliary circuits (smooth detuning of the local oscillator, wire filter, switching linear cascades).
The transformers T1-T4 are executed on cores from ferrite MN (size CHH). Winding - three wire. The windings T1 and T2 each contain 27 turns, and T3 and T4 - 30 turns of wire sew-2 0,18 (are three wires). Coils L3 and L4 have 6 turns of wire sew-2 to 0.6, and the coil connection L2 to L5, one coil of the same wire. These coils are wound on the ferrite core SOFC (size CHH). Coil L1 contains 9 turns of wire sew-2 to 0.8 with a branch of the first circuit and performed on the ferrite core WC (size KHZ). Transformer T5 contains H turns of wire sew-2 0,2 on the ferrite core MN (size CHH). The number of turns of the coil L7 connection is 1/5 1/8...of the number of turns of the coil L6. The inductance L6 - 1.5 μg.
It is wound on the frame with a diameter of 8 mm (podstrochnik - SCR-1) wire sew-1 0,42. The number of turns - 12, the winding length of b mm. Coil L8 performed on Teflon frame with a diameter of 20 and a length of 35 mm. It contains 17 turns of silver coated copper wire with a diameter of 0.5 mm, a discharge, from the 4th round. Winding length is 17 mm. This coil is placed in the screen of brass (diameter and height of the screen 36 mm). Its inductance without screen is 4.7 mg, and the screen is 3.6 µg.
R1 resistor - non-inductive, str or CP3-1B. Variable capacitor from radio "Ocean" (used only part of the range of capacitance). In the circuit modulating the local oscillator and the contours of the FSS applied capacitors CSR, the Capacitors C1 and C20 - air-dielectric, and the rest - C50-6, KLS, km, KD, CT.
Preliminary setup of the tract is convenient to make Pataskala in the following order. The outputs of the local oscillators is disconnected from the transducers and load them with resistors 50 and 70 Ohms. The selection of modes of transistors V10, V12. V13 and capacitor 27 and the number of turns in the coil L7 is mounted on the load resistors necessary high-frequency voltage (see figure). The shape of the voltage should be sinusoidal, without limitation, that it is important to obtain a good noise parameters of converters. At the same stage set ceiling GPA in frequency and produce a preliminary setup of the FSS and the pairing of its contours. The coil connection L2 and L5 must be disconnected from the windings, connection of transformers T2 and T3 and loaded with resistors 50.. .70 Ohms.
Then, restore the outlet connection of the GPA to the midpoint of the winding of the transformer TK, as well as coil winding L5 connection TK. To the coil L2 is connected by a resistor 50... 70 Ohms and a signal voltage of 5...7 In the frequency 501 502...kHz (if the EMF from the upper sideband). Engine. resistor R1 is set to the middle position. Selecting capacitors C7-C9, clear the filter resistors Z1 and Converter.
After that, the resistor, which is loaded on the coil L2, connect the measuring device, pair the correct setting circuits FSS and GPA and finally set the voltage GPA at the midpoint of the winding of TK.
Restoring the connection of the output of the crystal oscillator to the midpoint of the winding of the transformer T1 coil T1 connection is disconnected from the coil L1, load it to the resistor 50. ..70 Ohms and finally heterodyne set the voltage at the midpoint of the winding T1. Then reconnect the winding connection T1 with L1 and configure the circuit L1C1.
The voltage at point a is, depending on the quality of the filter Z1, 25...40 mV RMS., when the voltage signal at point B to about 3 V RMS. In operation of the device should not exceed the specified value of voltage at the point B, so. this will lead K. malfunctioning of the Converter.
In conclusion, checks the settings of the tract consisting of the entire channel of the transceiver in the "Transfer". Resistor R1 balance Converter in the "Reception", ensuring minimal noise at the output of the amplifier WOOFER.
Operated author transceiver has the following main parameters of the receiving channel in SSB: blocking (relative to the level of 10 mV when the detuning b kHz) - 300 mV, the rejection (in the range of 28 MHz) - 55 dB, sensitivity at signal-to-noise ratio at the output. tract 10 dB is not worse than 2 µv (range 28 MHz).
Literature
Author: Vladimir Vasiliev (UA4HAN); Publication: N. Bolshakov, rf.atnn.ru