The DC receiver has the advantage that, with an absolute minimum of components, one
can build a reveiver with a fine performance. In this case, the abbreviation DC does not mean " direct current" but Direct Conversion, refering to the principle this machine is based upon. In fact, this type of receiver is nothing else but a " super" having an Intermediate Frequency (IF) in the audible part of the frequency spectrum.
In general one can say: FMF = FANT - FOSC
For example: 2 KHz = 3580 KHz - 3578 KHz.
So, the difference between the antenna signal frequency and the oscillator signal frequency (both signals offered to a mixer) is a signal which can directly (without any additional detection circuitry) be heard. Because a numerous amount of components can be ignored (no dectection circuitry required), this type of receiver invites for homebrewing.
A DC receiver can be recognized by its characteristic sound, being produced when the oscillator is set 2-3 kHz above the missing carrier frequency of an SSB signal. Reception of AM signals is possible. However, a microscopic mistuning (from the carrier frequency of the received signal) immediately results in a loud tone !
Another advantage of the DC receiver is the fact that, besides receiving CW (Continuous Wave; morse - keyed carrier wave) and AM (Amplitude Modulated) signals also SSB (Single Side Band) signals becomes possible. Receiving SSB signals using the conventional method with a superheterodyne receiver (' super' ) is possible; however additional circuitry is required: a BFO (Beat Frequency Oscillator) and a product detector. DC receivers do not need those add-ons to convert SSB signals into normal speech. A sensitivity of approximately 0.3 m V (without any additional high frequency pre-amplification) with commonly used regular mixer devices is feasible.
With every advantage there comes a disadvantage. This type of receiver can be blamed of the fact that it has a limited dynamic range. HF signals with more than average amplitudes can easily generate unwanted mixing products (as produced by the mixer itself). Processing of the high fequency signal, just after the antenne stage, is a requirement. A signal divider (potentiometer) with a bandpass filter can do miracles. When a filter is added, the intruding strong broadcast signals ( ' short and medium wave') can be taken away. Now nothing can stop a fine reception of the proper shortwave signals.
Also, the sensitivity to pick up 50 and 100 Hz rumble is greater than compared to that of the ' super'. When carefully asembled (sufficient spacing between power supply transformer and receiver and proper shielding) this disadvantage can be neglected. In one case I could not cope with the 50/100 Hz rumble and noise. I have replaced the bridge rectifier by 4 seperate diodes; over each diode I soldered a 10 nF capacitor. The result was asthonishing. To obtain the best result one have to do some experiments now and then.
The design as shown by photo 1 is suitable of receiving CW-, AM- and SSB signals with frequencies within the 80 meter HAM radio band. Frequency range of this receiver exceeds 1300 KHz (3680-3810 KHz).
If you like to know more about DC receivers and design philosophies I can recoomend the book of Joseph J. Carr, ' The secrets of RF-technology. Beautiful book with very useful hints and tips.
Description and details
The receiver is designed around an NE602. This ic contains amongst others a double balanced mixer, an oscillator and a voltage regulator. The mixer is designed to handle signals up to a frequency of approximately 500 MHz (!) and the oscillator is capable of generating signals with frequencies till about 200 MHz. The low frequencies (appr. 3500 KHz) used here are no big deal for the NE602 whatsoever. The dynamic range of the NE602 is something which can be improved. A later version of this ic, the NE602AN, has improved dynamic range characteristics. Also available is the NE612. This ic, being pin-compatible with the NE602, has, like the NE602AN (which is difficult to obtain), an enlarged dynamic range.
The suppresion of unwanted harmonics by the mixer device inside the NE602 (Gilbert cell) takes care of the presence of the sum- and difference frequencies of the antenna and oscillator signals only on the balanced output of the ic (pin 4 and 5 ). In this case, only the difference signal is a useable audio signal and will be used for further LF amplification.
The input filter stage resonates at a frequency (exactly adjustable by the ferrite cores of the TOKO coils) of 3.7 MHz. The more the frequency of the antenna signal diverts from the frequency on which the filter resonates, the more the input signal is suppressed. In this way strong broadcast signals will sufficiently be blocked. The potentiometer in the input circuitry brings the amplitude of too large input signals to proper level, making sure the mixer device inside the NE602 is able to handle them. To enable the reception of weak stations, a one stage (selectable) RF amplifier forms part of the design. Thus an additional + 6 dB gain is obtained. The manufacturer of the NE602 (Signetics) advises to, if the power supply voltage is +9V, use a 1000 ohm resistor in the power supply feed line. As was done in this design.
Both varicaps BA125 enable the internal VFO to exactly be tuned by an adjustable voltage. For this design it was choosen to obtain this control voltage by a multiple turn potentiometer (Bourns). Precise tuning, an absolute must to listen to SSB transmissions, has been realized in this way. For an acceptable ' low-budget' solution, one might use two (regular) 1 turn potentiometers (e.g. 10 k + 470 ohm in series). Tuning becomes less comfortable, yet remains possible. A small trimmer capacitor is used to make the coarse frequency adjustment. Fine tuning is done with the multiple turn potentiometer. For LF amplification the LM386 has been selected. Depending on the type, the LM386 can deliver an LF output power of 250…750 mW. (The LM386N-1 makes about 325 mW where the LM386-4 achieves a huge 750 mW).
Following add-on circuitries have been mounted into the DC receiver housing:Power supply unit, signal strength meter, frequency indicator
Circuit 2 shows the S meter suitable for looking at the relative signal strength changes. The output of this circuit can be connected directly behind the 100 m F electrolytic capacitor ( LF output). The circuit was built up on a seperate VERO-board.
-The complete receiver has been built onto a VERO-board. The VFO frequency determining coil uses 30 turns (diameter of 0.35 mm copper wire) on an Amidon ring core T50-2. Wen using a different core, the number of turns should be determined by trial-and-error (frequency counter recommended!).
-TOKO KANK3333R coils are used in the input circuitry. This HF transformer is designed for the 3-4 MHz frequency range. Lots of alternatives can be used (e.g. KANK3334).
-A disadvantage of the circuit here described is the small amount of LF output power when receiving weak signals. I did some experiments using two LM386's in series to increase the output level. The results were very dissappointing. Because of the very high amplification factor a low-frequency oscillation can not be avoided. The amplification of the LM386 is adjustable; if a value of 10m F for the elco between contacts 1 and 8 is used, the amplification is 46 dB (200 x). When the elco is deleted the amplification is only 25 dB (about 20 x).